Copyright © 2014 by the author(s). Published here under license by the Resilience Alliance.
Downing, A. S., E. Van Nes, J. Balirwa, J. Beuving, P. Bwathondi, L. J. Chapman, I. J. M. Cornelissen, I. G. Cowx, K. Goudswaard,
R. E. Hecky, J. H. Janse, A. Janssen, L. Kaufman, M. A. Kishe-Machumu, J. Kolding, W. Ligtvoet, D. Mbabazi, M. Medard, O. C.
Mkumbo, E. Mlaponi, A. T. Munyaho, L. A. J. Nagelkerke, R. Ogutu-Ohwayo, W. O. Ojwang, H. K. Peter, D. Schindler, O.
Seehausen, D. Sharpe, G. M. Silsbe, L. Sitoki, R. Tumwebaze, D. Tweddle, K. E. Van de Wolfshaar, H. Van Dijk, E. Van Donk, J. C.
Van Rijssel, P. A. M. Van Zwieten, J. H. Wanink, F. Witte, and W. M. Mooij. 2014. Coupled human and natural system dynamics as
key to the sustainability of Lake Victoria’s ecosystem services. Ecology and Society 19(4): 31. http://dx.doi.org/10.5751/
ES-06965-190431
Research
Coupled human and natural system dynamics as key to the sustainability of
Lake Victoria’s ecosystem services
Andrea S. Downing 1,2,3, Egbert H. van Nes 2, John S. Balirwa 4, Joost Beuving 5, P.O.J. Bwathondi 6, Lauren J. Chapman 7, Ilse J. M.
Cornelissen 3,8, Iain G. Cowx 9, Kees P. C. Goudswaard 10, Robert E. Hecky 11, Jan H. Janse 3,12, Annette B. G. Janssen 2,3, Les Kaufman
13
, Mary A. Kishe-Machumu 14, Jeppe Kolding 15, Willem Ligtvoet 16, Dismas Mbabazi 4, Modesta Medard 17, Oliva C. Mkumbo 18,
Enock Mlaponi 19, Antony T. Munyaho 4, Leopold A. J. Nagelkerke 8, Richard Ogutu-Ohwayo 4, William O. Ojwang 20, Happy K. Peter
8
, Daniel E. Schindler 21, Ole Seehausen 22, Diana Sharpe 7,23, Greg M. Silsbe 24, Lewis Sitoki 25, Rhoda Tumwebaze 4, Denis Tweddle 26,
Karen E. van de Wolfshaar 27, Han van Dijk 17, Ellen van Donk 3, Jacco C. van Rijssel 22,28,29, Paul A. M. van Zwieten 8, Jan Wanink 28,30,
F. Witte 28,29 and Wolf M. Mooij 2,3
ABSTRACT. East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In
particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole
region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately
connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out
piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to
disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a
whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system
through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its
contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that
can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that
high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’
vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple
pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management
toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt
or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a
paving stone for future quantitative analyses of system dynamics at local scales.
Key Words: eutrophication; feedbacks; fisheries; Lake Victoria; model; multidisciplinary, social-ecological system; sustainability
INTRODUCTION
Nile perch (Lates niloticus) was introduced to Lake Victoria in
the 1950s and suddenly became dominant in the system in the
1980s (Goudswaard et al. 2008). Prior to this dramatic event, the
ecosystem of Lake Victoria harbored a large diversity of
haplochromine cichlids and a few native tilapia species destined
for local and regional markets (Pringle 2005a). The Nile perch
boom co-occurred with other major transformations to Lake
Victoria’s ecosystem, including a collapse of the native diversity
1
of haplochromines and the eutrophication of the lake’s waters
(Witte et al. 2007a, Hecky et al. 2010).
Despite the dramatic diversity loss and catastrophic ecosystem
changes that surrounded the Nile perch invasion, management
currently aims to maintain the system in its Nile perch-dominated
state, not to recover the former Nile perch-free, diverse ecosystem
—should that even be possible. Indeed, Nile perch has become
the product of an international export industry worth
approximately US$250 million yearly (Pringle 2005a, Kayanda et
Stockholm Resilience Centre, Stockholm University, Sweden, 2Aquatic Ecology and Water Quality Management group, Wageningen University,
Netherlands, 3Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, Netherlands, 4National Fisheries Resources Research Institute
(NaFIRRI), Jinja, Uganda, 5Department of Cultural Anthropology and Development Studies, Faculty of Social Sciences, Radboud University,
Nijmegen, Netherlands, 6University of Dar es Salaam, College of Natural and Applied Sciences, Department of Aquatic Sciences and Fisheries, Dar
es Salaam, Tanzania, 7Department of Biology, McGill University, Montreal, Canada, 8Aquaculture & Fisheries Group, Wageningen University,
Netherlands, 9Hull International Fisheries Institute, University of Hull, United Kingdom, 10Institute for Marine Resource and Ecosystem Studies
(IMARES), Wageningen University, Yerseke, Netherlands, 11Biology Department and Large Lakes Observatory, University of Minnesota-Duluth,
USA, 12Netherlands Environmental Assessment Agency (PBL), Bilthoven, Netherlands, 13Boston University Marine Program, Biology Department,
Boston University, USA, 14Tanzania Fisheries Research Institute (TAFIRI), Dar es Salaam, Tanzania, 15Department of Biology, University of
Bergen, Norway, 16Netherlands Environmental Assessment Agency (PBL), The Hague, Netherlands, 17Department of Sociology of Development
and Change. Social Science Group, Wageningen University, Netherlands, 18Lake Victoria Fisheries Organisation, Jinja, Uganda, 19Tanzania Fisheries
Research Institute (TAFIRI), Mwanza, Tanzania, 20Kenya Marine and Fisheries Research Institute (KMFRI), Kisumu, Kenya, 21Aquatic & Fishery
Sciences/Department of Biology, University of Washington, USA, 22Eawag, Kastanienbaum, Switzerland, 23Smithsonian Tropical Research
Institute, Panama City, Panama, 24Royal Netherlands Institute for Sea Research, Yerseke, Netherlands, 25The Technical University of Kenya,
Nairobi, Kenya, 26South African Institute for Aquatic Biodiversity, Grahamstown, South Africa, 27Institute for Marine Resource and Ecosystem
Studies (IMARES), Wageningen University, Ijmuiden, Netherlands, 28Institute of Biology, University of Leiden, Netherlands, 29Naturalis
Biodiversity Center, Leiden, Netherlands, 30Koeman en Bijkerk bv, Ecological Research and Consultancy, Haren, Netherlands
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
al. 2009). Many people migrated to the lake’s shores to benefit
from the fisheries, a migration that has led to important
transformations to the lake’s societies. Currently, an estimated 30
million people live in the lake’s basin and are shaping and being
shaped by its ecology and economy (Awange and Ong’ang'a
2006).
Management measures on Lake Victoria have focused on fisheries
and aim to manage a certain quality of catch (size structure of
Nile perch) rather than total fishing effort or stock biomass (van
der Knaap et al. 2002). Some of these methods are applied on the
lake itself; e.g., banning beach seines and increasing the minimum
mesh size of gillnets, or introducing fish slot sizes. In the case of
Nile perch, measures exist at the processing output level, where
there is a minimum length of fish that the factories are allowed
to process. Approaches to managing the fisheries assume that the
fisher communities, as well as fish stocks, are homogeneous
(Muhoozi 2002). Management measures are difficult to
implement and monitor given that Lake Victoria is the world’s
second largest freshwater lake by area (68,800 km2), has a highly
convoluted shoreline that is shared by three countries (Tanzania,
Uganda, and Kenya), and the fisheries are open access. Also, the
effectiveness of regulations is limited, in part because policies can
be immediately detrimental to the filleting factories that process
the fish for export markets (Johnson 2010). Resources available
to implement or enforce policies are scarce, and enforcement can
be subject to corruption.
Added to difficulties in choosing and implementing effective
management measures, stakeholders and scientists disagree about
the current state of the Nile perch fisheries and what the dominant
threats to these fisheries are (Witte et al. 2007a). Indeed, owners
of filleting factories complain that they are operating their
factories at decreasing capacity, overlooking the fact that many
of the factories were built at structural overcapacity (Abila and
Jansen 1997, Johnson 2010). Some scientists read overfishing in
the fishery survey data (Mkumbo et al. 2007, Mkumbo and
Mlaponi 2007, Kayanda et al. 2009), while others see a total stock
that fluctuates around a stable average despite increased fishing
effort. The stable stock under increased pressure rests on the logic
that increased productivity results from eutrophication (Kolding
et al. 2008).
The cumulative pressures influencing Lake Victoria are multiple,
intricately interconnected, and threatening to all its fisheries as
well as to the lake’s ability to carry out supporting services. Indeed,
the whole ecosystem is influenced by climate change (Hecky et al.
2010, Cózar et al. 2012) and has been degraded through
eutrophication (Dobiesz et al. 2010), which impoverishes the
system by driving an irreversible decline in biodiversity
(Seehausen et al. 1997a). Beyond a certain threshold,
eutrophication threatens to cause a more general collapse of
stocks, not only of Nile perch (Kolding et al. 2008). Additionally,
Nile perch is not the only exploited fish. For example, the native
pelagic cyprinid Rastrineobola argentea (known as dagaa in
Tanzania, omena in Kenya, and mukene in Uganda; herein
referred to as dagaa) has become an increasingly important
product for local societies (Wanink 1999). Furthermore, since the
explosion of Nile perch numbers in the lake, social and economic
drivers of fishing effort have changed. Risks and opportunities
that face fisher communities have evolved with increasing human
population sizes and market value (Beuving 2013). Nile perch is
destined predominantly to European markets that impose strict
hygiene standards and influence fish prices. Market demand and
processing costs influence factory owners, fish buyers, and trade
middlemen, who in turn affect the organization of fishing and
fisher communities and thus shape stock exploitation (Abila and
Jansen 1997, Ponte 2007, van der Knaap and Ligtvoet 2010).
Two lake-wide organizations—the Lake Victoria Fisheries
Organisation (LVFO) and the Lake Victoria Basin Commission
(LVBC)—have played the most important roles in coordinating
research on the lake’s resources. While originally conceived as an
instrument for holistic, ecosystem-based management, the LVFO
has a much stronger fisheries perspective. The LVBC has the
potential to address water quality and nonfisheries management
issues through its Lake Victoria Environment Management
Project Plan II (LVEMPII). So far, coordination of field efforts
and data sharing between the two organizations are poor. Fishery
science, and to some extent limnology, have undergone a measure
of regional integration under LVEMP activities, but on the whole,
an ecosystem’s view is still distinctly lacking. Dissemination of
results has also been limited at times, with LVFO investing greater
effort in communications than the LVBC.
In sum, changes in Lake Victoria are a complex product of social,
economic, and ecological processes. To comprehend the dynamics
of this complex system, to visualize the scenarios likely to result
from alternative management measures, and to manage human
activities that influence the lake in an intelligent and farsighted
manner, the lake and lake basin ecosystem must be understood
in their entirety. Many analyses of their parts—for example, water
quality, diversity, or fisheries—have been conducted in isolation.
Our aim is to connect the dots and study the interactions among
the parts of Lake Victoria’s system.
An important barrier to assembling the pieces of Lake Victoria
lies in quantifying the ecosystem’s components in a spatiotemporally representative way. As a first step in this direction, we
employed a form of qualitative modeling called loop analysis
(Levins 1974). We first built a feedback diagram of Lake Victoria’s
social-ecological system based on the components and
interactions that compose it, matching the model to Lake
Victoria’s past trends and changes, and investigated the effects of
quantitative and structural modifications to the system on its
behavior.
SOCIAL-ECOLOGICAL SYSTEM REPRESENTATION
Qualitative models
We first created a feedback diagram containing three different
types of objects: (1) the key elements of the system, (2)
connections between interacting key elements, and (3) the sign of
these interactions near equilibrium. If an increase in the value
(biomass, concentration, or economic value) of an element causes
an increase in a connected element, the sign is positive (+). If, on
the other hand, an increase in one causes a decrease in the other,
the sign is negative (-). We then carried out the loop analysis, where
we identified the feedback loops in the system; i.e., the pathway
of interactions that go from any element and back to it, through
other elements in the defined system. We also identified whether
these feedbacks are overall positive or negative by multiplying the
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
signs of the interactions along each pathway. A positive feedback
loop reflects a “reinforcing” process, where an increase in one
element causes it to increase further: positive feedback loops are
therefore destabilizing. A negative feedback loop, on the other
hand, is self-regulating: an increase in an element’s value leads it
to limit itself; a negative feedback loop is stabilizing. We obtained
several outputs from this qualitative analysis: (a) a description of
the system as a whole, (b) all the direct and indirect pathways
connecting different parts of the system, (c) the effect—positive
or negative—of the change in one element on its different
connections; i.e., chain reactions across the system, (d) the
multiple positive and negative controllers of each element, (e)
stabilizing and destabilizing feedbacks, and (f) a view of key
elements that can alter the sign of a feedback; i.e., those for which
connection signs can be changed; e.g., policies.
Determining which elements of a system are key is a subjective
process. To reduce bias caused by the oriented knowledge of any
single Lake Victoria scientist, scientists in the fields of aquatic
ecology, fisheries sciences, and social sciences, as well as
stakeholders, all with expertise in Lake Victoria’s system, were
brought together in a workshop in Dar es Salaam in May 2012.
To make the process more manageable, we first built two separate
diagrams: one for the ecological subsystem and one for the social
subsystem, including fisheries. We then combined the two
diagrams into a linked social-ecological system representation.
Four groups of scientists and stakeholders with mixed expertise
discussed and built the two subdiagrams. These groups thus
determined the elements and the interactions to enter the model
based on their understanding of underlying processes (e.g.,
eutrophication) and knowledge of the system (e.g., the types of
fishes landed with different nets), and discussed them until the
final diagrams satisfactorily represented the perspectives of the
different topics (social sciences, water quality, and fisheries). The
discussions continued online after the workshop and then
included even more participants, who have in turn become the
authors of this paper. In this way, the biases that remain are
inherent to research conducted on Lake Victoria thus far. This is
the first time such diverse perspectives on Lake Victoria’s system
have been assembled on an equal footing. Our representation of
the system as we know it can further point out where ignorance
remains.
To make the diagrams tractable, we excluded all self-effects and
assumed density dependence to be negative, reflecting a stable
system. Also, where interactions are known to be nonlinear, such
as the effect of eutrophication on productivity, we cut the process
down into its parts: the positive effect of eutrophication on
growth, and the positive effect of eutrophication on anoxia and
shading, which in turn is negative for growth. We emphasize that
the model is not quantitative, and we do not include the weight
of each interaction. In this way, our model can represent both
locations or time periods where growth processes prevail over
negative effects of deoxygenation, and vice versa, and thus
represent the full system irrespective of its current state. To make
our final linked diagram more parsimonious, we eliminated the
elements flanked only by unidirectional positive interactions, as
they add detail and complexity to the loops but do not alter their
signs—and therefore, have no effect on the overall dynamics of
the system.
Ecological subsystem
The nutrients of the system are represented in nitrogen (N) and
phosphorus (P) (Fig. 1) that contribute to an increase in
phytoplankton biomass, which is in turn consumed by
zooplankton and fish (Fig. 1: phytoplankton --> zooplankton
and haplochromines). The major source of nutrient input to the
lake is through atmospheric deposition, resulting from intensive
land use in the basin, through land clearing and burning
(Tamatamah et al. 2005, Hecky et al. 2006) (Fig. 1: from the local
and regional economy and society—herein abbreviated to
Soc&econ—to N&P). An increase in nutrients has a nonlinear
effect on the system. Indeed, while there is an initial phase of
increase in phytoplankton biomass, as nutrient input continues
increasing, phytoplankton biomass begins to limit light
availability and eventually reaches an upper limit depending on
light extinction and mixing depth (Silsbe et al. 2006). We try to
account for this strong nonlinearity by adding phytoplankton’s
self-shading property and contribution to detritus (Fig. 1:
phytoplankton --> shading and detritus). Detritus also increases
shading and oxygen consumption, and exists as a trophic pathway
for the shrimp Caridina nilotica (Fig. 1: detritus --> shading,
anoxia, and C. nilotica).
Fig. 1. Ecological interactions. Green arrows represent growth
processes (also identified by + sign); blue arrows represent
mortality processes (associated with - signs); thick dotted lines
and arrows represent complex systems/interactions not further
developed. E refers to environmental variables, F to fish
variables, L to limnological variables, and S to societal
variables. E1 = climate change; F1 = large Nile perch; F1' =
small Nile perch; F2 = Rastrineobola argentea; F3 =
haplochromines; L1 = nitrogen and phosphorus; L2 =
phytoplankton; L3 = zooplankton; L4 = detritus; L5 =
Caridina nilotica; L6 = shading; L6' = anoxia; S4 = local and
regional economy and society.
The shrimp C. nilotica is an important detritivore and became an
important prey for juvenile Nile perch as well as for the mostly
zooplanktivorous haplochromines that remained in the 1990s
(Downing et al. 2012a) (Fig. 1: C. nilotica --> small Nile perch
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
and haplochromines). Recovering haplochromines have mostly
reverted to feeding on zooplankton and other small invertebrates
but they include shrimp and larger prey in their diets when these
are abundant (Kishe-Machumu 2012, van Rijssel and Witte
2013). Light is of primary importance to haplochromines because
they rely on color vision for successful breeding and efficient
feeding (Seehausen et al. 1997a, Witte et al. 2013) (Fig.1: shading
--o haplochromines). Poor light can also alter haplochromine
species diversity by promoting hybridization (Seehausen et al.
1997a, Seehausen 2009).
economy to grow, fuels further investment in alternative sources
of income (such as agriculture and farming, or support businesses,
including housing, transport, or entertainment), and stimulates
human population growth. This growth feeds back into the
regional market. The local and regional markets invest in boat or
camp owners to ensure their supply of fish. Owners in turn convert
their capital into effort, thus further increasing total catch (Fig.
2: IntMar and LocMar --> owners; IntMar and LocMar -->
Soc&econ).
We connected anoxia negatively to all fish since reduced oxygen
is ultimately detrimental to them all, though usually through a
complex nonlinear process (Fig. 1: anoxia --o all fish). In a first
step, an anoxic deep water layer reduces the habitable part of the
water column for sensitive species, and increases habitat overlap
between species (Vonlanthen et al. 2012). Habitat overlap can
result in increased predation rates of Nile perch on
haplochromines, as well as hybridization rates between
haplochromine species. In a later step, the seasonal upwelling of
deep anoxic waters can lead to large fish kills (Ochumba 1990,
Gophen et al. 1995). Reduced oxygen concentrations also have a
chronic negative effect on species by influencing their metabolic
processes and growth rates (Kolding 1993, Rutjes et al. 2007).
Fig. 2. Socio-economic interactions. Green arrows represent
growth processes (also identified by + sign); blue arrows
represent mortality processes (associated with - signs); rust
arrows represent investment; thick dotted lines and arrows
represent complex systems/interactions not further developed.
E2 = international economy; E3 = policies; F1 = large Nile
perch; F1' = small Nile perch; F2 = Rastrineobola argentea;
F3 = haplochromines; F4 = Nile perch fishing effort; F5 = R.
argentea fishing effort; S1 = boat and camp owners; S2 =
international market; S3 = local and regional market; S4 =
local and regional economy; S4’ = local society.
Additionally, oxygen plays a complex role in biogeochemical
cycles: denitrification—a process that converts nitrate to
dinitrogen gas (N2)—takes place primarily in anoxic conditions,
and these conditions also promote phosphorus release (Fig. 1:
anoxia --> N&P). The effect of oxygen on biogeochemical cycles
is temperature sensitive (Veraart et al. 2011), and both weather
and temperature conditions influence the duration and extent of
water stratification in the lake (Sitoki et al. 2010). These processes
are thus likely to be greatly affected by climate change in complex
and nonlinear ways (Fig. 1: dashed line from E1 --> anoxia -->
N&P).
Besides the solid arrow indicating Nile perch predation on
haplochromines, a dashed arrow was included to indicate the
hypothesized negative effect of haplochromines on Nile perch
recruitment. It has long been speculated that through either
competition or predation, haplochromines have, or had, a
negative influence on the growth of small Nile perch, in a process
termed depensation (Fig. 1: haplochromines --o small Nile perch)
(Walters et al. 1997, Walters and Kitchell 2001, Goudswaard et
al. 2008, Downing et al. 2012b). However, a recent study suggests
that such an interaction probably did not play a significant role
at the scale of the whole lake (Downing et al. 2013a). Depensation
should be included only in cases and areas where there is a
demonstrated negative influence on the stock–recruitment
relationship.
Society and fisheries
Catches of small Nile perch, dagaa, and haplochromines feed a
regional market (in the lakeshore and neighboring countries) (Fig.
2: small Nile perch, dagaa, and haplochromines --> local and
regional market; herein abbreviated as LocMar), and catches of
large Nile perch go to a European export market (Fig. 2: Nile
perch --> international market; herein abbreviated as IntMar).
Markets convert catches into investment power, which allows the
By separating camp and boat owners from the rest of society, we
have the means to reflect social, political, and economic disparity.
Owners are entrepreneurs, drawing either on their own funds and
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
profits or on investments by industries, and they themselves can
invest in further effort and gain. Labor, however, is cheap, always
abundant enough, and plays a role in every industry: we therefore
neglected its impact on owners. Owners can switch between the
international Nile perch fishery and regional fisheries or be part
of both (Fig. 2: owners --> Nile perch and dagaa fishing effort),
depending on the status of the owners and the benefits each fishery
yields.
We removed small Nile perch from the fish groups. Instead, Nile
perch as a whole contribute to both the regional and international
markets (Fig. 3: Nile perch --> IntMar and LocMar). The
population size structure of Nile perch has changed since its first
introduction, though trends in these changes vary spatially
(Kolding et al. 2008) and are likely a response to both the
environment—through diet changes—and to size-selective
fishing (Downing et al. 2013c).
We added two exogenous processes to this diagram that influence
the cost-benefits that in turn regulate where owners focus their
investments: the international economy and policies (Fig. 2). The
international economy, driven by many more products, trends,
and fashions than the resources of Lake Victoria, can influence
the price Nile perch fetches, as well as quality requirements of the
Nile perch destined for export (Kambewa 2007, Ponte 2007,
Johnson 2010) (Box 1). This international influence is in large
part independent of conditions in the lake but nonetheless defines
many of the costs in the Nile perch industry (e.g., installing and
maintaining storage and hygiene facilities, or higher investment
in societies through eco-labeling requirements) and therefore
shapes the catch level necessary to offset costs and reap a benefit
(van der Knaap et al. 2002, van der Knaap and Ligtvoet 2010).
We merged society and the economy into one since they are
connected with positive interactions. Here, they directly
contribute to nutrient enrichment (Fig. 3: Soc&econ --> N&P).
Box 1: Influence of the international market
European markets set regulations on the quality of the fish they
imported, which created a need for filleting factories to invest in
technologies such as freezers and insulated trucks to obtain
quality certifications (Johnson 2010) (Fig. 3: international
economy(+) --> IntMar --> owners --o Nile perch fishing
effort…). European regulations caused import bans several times
in the late 1990s because of poor hygiene. During these periods,
Nile perch was exported to alternative markets, such as Japan,
though at lower prices, and fishing effort was temporarily reduced
(Fig. 3: international economy(-) --o IntMar --o Nile perch fishing
effort --> Nile perch…) (van der Knaap et al. 2002). Such
sanctions or a collapse in the international market can close up
the pathways that promote investment in fishing by owners. Such
effects could percolate down to show a positive effect on stocks
(e.g., Fig. 3: international economy(-) --> IntMar --> owners -->
Nile perch fishing effort and dagaa fishing effort --o Nile perch,
dagaa, and haplochromines). However, this depends on the
importance or adaptability of the local market relative to the
international one, as a collapse of the international economy
could simply shift dynamics toward the local and regional market,
if the local market has a substantial income-source independent
of fisheries.
The social-ecological system
We combined both diagrams and simplified the model to make it
manageable. We summarized the positive interactions that
dominate the ecological perspective into two main processes: the
positive effects of nutrient input that promote growth in fish
species (Fig. 3: N&P --> Nile perch, dagaa, and haplochromines),
and the negative effects that come from enrichment but are
represented by anoxia and shading (Fig. 3: N&P --> shading -->
Nile perch, dagaa, and haplochromines).
Fig. 3. Social-ecological system interactions. Green arrows
represent growth processes (also identified by + sign); blue
arrows represent mortality processes (associated with - signs);
rust arrows represent investment; thick dotted lines and arrows
represent complex systems/interactions not further developed.
E1 = climate change; E2 = international economy; E3 =
policies; F1 = Nile perch; F2 = Rastrineobola argentea; F3 =
haplochromines; F4 = Nile perch fishing effort; F5 = R.
argentea fishing effort; L1 = nitrogen and phosphorus; L6 =
anoxia and shading; S1 = boat and camp owners; S2 =
international market; S3 = local and regional market; S4 =
local and regional economy and society.
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Loop analysis
Our social-ecological system diagram has 25 loops. We excluded
the direct feedback between local market and society from our
analysis since it holds no emergent information. Among the
remaining 24 loops, we separated nutrient-driven processes,
including nine “enrichment” loops (Table 1), in which nutrients
lead to stock growth, increased fishing, and economic and
population growth, and thus more enrichment, and nine
“eutrophication” loops (Table 2), where stronger nutrient input
leads to anoxia and shading, influences stock growth as well as
economic and population expansion, and thus controls nutrient
input. Additionally, we have six “exploitation” loops (Table 3)
that describe dynamics from the different stocks through to the
markets and back.
Table 1. Enrichment loops and their signs. N;P = nitrogen and
phosphorus; Np = Nile perch; IntMar = international market;
Soc&econ = society and economy; LocMar = local and regional
market; Haplo = haplochromines; Dagaa = Rastrineobola
argentea; --> = positive interaction; --o = negative interaction.
Loop
1. N;P-->Np-->IntMar-->Soc&econ-->N;P
2. N;P-->Np-->LocMar-->Soc&econ-->N;P
3. N;P-->Haplo-->LocMar-->Soc&econ-->N;P
4. N;P-->Dagaa-->LocMar-->Soc&econ-->N;P
5. N;P-->Np--oHaplo-->LocMar-->Soc&econ-->N;P
6. N;P-->Np-->IntMar-->owners-->Dagaa effort--oHaplo-->
LocMar-->Soc&econ-->N;P
7. N;P-->Np-->IntMar-->owners-->Dagaa effort--oDagaa-->
LocMar-->Soc&econ-->N;P
8. N;P-->Haplo-->LocMar-->owners-->Np effort--oNp-->
IntMar-->Soc&econ-->N;P
9. N;P-->Dagaa-->LocMar-->owners-->Np effort--oNp-->
IntMar-->Soc&econ-->N;P
Sign
+
+
+
+
-
Positive enrichment loops go from nutrients promoting the
growth of stocks, through their positive effect on markets, then
society, and back to nutrient input (Table 1). Negative
eutrophication feedback loops follow the link between nutrients
and anoxia and shading. Anoxia and shading are the drivers on
stocks whose negative effects cascade to markets and society, and
thus lead to reduced nutrient input (Table 2). In the case of
negative exploitation loops, they link each stock, through to the
market it trades to, back to owners, increased effort, and thus a
reduction in stock (Table 3), which is ultimately self-regulating.
Besides these expected loops, we obtained a few inverse loops,
where enrichment is self-regulating, while eutrophication and
exploitation are self-reinforcing (negative loops in Tables 1 and
3; positive loops in Table 2). These inverted-sign loops describe
longer—and presumably weaker through their indirectness—
chain reactions stemming from the presence of alternative sources
of income for society.
Exploitation loops are mostly self-regulating, implying that as a
stock dwindles, its exploitation costs increase and it invites less
fishing effort (Table 3: negative loops). However, here we see that
exploitation loops can connect two fisheries either directly (Table
3: loops 3 and 5) or indirectly through owners or the local market.
These stocks are thus not independent in the eyes of exploitation.
For example, pelagic, zooplankton-eating haplochromines can
make up a large fraction of the dagaa bycatch. This could
compensate for dwindling dagaa stocks since there is no barrier
to continuing exploitation in similar ways if, for a time at least,
the fish initially targeted is easily replaced (Box 2). In the same
way, local and international markets provide outlets for small and
large Nile perch, respectively. Therefore, if a size class becomes
less available, fishing camp owners would not necessarily see
diminishing returns immediately (Table 3: loops 1 and 2). The
exact dynamics would of course depend on the price that different
fish and fish-size classes fetch, and the fact that these prices are
subject to demand-driven fluctuations. In the short term, this
might be perceived as evidence of functional complementarity, a
benefit of diversity. However, it also signifies a dampening of the
self-regulating nature of the negative feedback.
Table 2. Eutrophication loops and their signs. N;P = nitrogen and
phosphorus; Anox&Shade = anoxia and shading; Np = Nile
perch; IntMar = international market; Soc&econ = society and
economy; LocMar = local and regional market; Haplo =
haplochromines; Dagaa = Rastrineobola argentea; --> = positive
interaction; --o = negative interaction.
Loop
1. N;P-->Anox&Shade--oNp-->IntMar-->Soc&econ-->N;P
2. N;P-->Anox&Shade--oNp-->LocMar-->Soc&econ-->N;P
3. N;P-->Anox&Shade--oHaplo-->LocMar-->Soc&econ-->N;P
4. N;P-->Anox&Shade--oDagaa-->LocMar-->Soc&econ-->N;P
5. N;P-->Anox&Shade--oNp--oHaplo-->LocMar-->Soc&econ-->
N;P
6. N;P-->Anox&Shade--oNp-->IntMar-->owners-->Dagaa
effort--oHaplo-->LocMar-->Soc&econ-->N;P
7. N;P-->Anox&Shade--oNp-->IntMar-->owners-->Dagaa
effort--oDagaa-->LocMar-->Soc&econ-->N;P
8. N;P-->Anox&Shade--oHaplo-->LocMar-->owners-->Np
effort--oNp-->IntMar-->Soc&econ-->N;P
9. N;P-->Anox&Shade--oDagaa-->LocMar-->owners-->Np
effort--oNp-->IntMar-->Soc&econ-->N;P
Sign
+
+
+
+
+
Table 3. Exploitation loops and their signs. Np = Nile perch;
IntMar = international market; Soc&econ = society and
economy; LocMar = local and regional market; Haplo =
haplochromines; Dagaa = Rastrineobola argentea; --> = positive
interaction; --o = negative interaction.
Loop
1
2
3
4
5
6
Np-->IntMar-->owners-->Np effort--oNp
Np-->LocMar-->owners-->Np effort--oNp
Haplo-->LocMar-->owners-->Dagaa effort--oHaplo
Dagaa-->LocMar-->owners-->Dagaa effort--oDagaa
Np--oHaplo-->LocMar-->owners-->Np effort--oNp
Np-->IntMar-->Soc&econ-->LocMar-->owners-->Np effort-oNp
Sign
+
-
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Box 2: Connectedness of the fisheries
Haplochromine populations in the offshore and pelagic waters
were on a steep decline from the late 1970s and collapsed to low
levels in the wake of the Nile perch boom (Witte et al. 1992).
Surprisingly, haplochromine biomass started recovering in the
early 1990s (Seehausen et al. 1997b) and quickly achieved
precollapse pelagic stock biomass (Witte et al. 2007b).
Haplochromines now represent up to 80% of the pelagic fish
biomass, and together with dagaa, they constitute more than 50%
of the lake’s total fish biomass (Tumwebaze 1997, Wanink 1999,
Kayanda et al. 2009). In Tanzania, since their resurgence,
haplochromines have shifted from being an important bycatch of
the dagaa fishery (sometimes constituting 50–90% of the catch)
(Witte et al. 2000) to becoming a target that is sold to regional
markets or is used as bait in the Nile perch fishery (Ngupula and
Mlaponi 2010). Indeed, since the mid-1990s, the dagaa fishery
has harvested an increasing amount of zooplanktivorous (genus
Yssichromis), benthivorous, and detritivorous haplochromines
(Haplochromis “paropius-like,” also referred to as Haplochromis
“broken bar” in Seehausen et al. [1997b]) (van Rijssel, personal
observation), indicating that benthic species migrate up at night
(Kishe-Machumu 2012), and that the dagaa nets—extending 10
m deep—can influence benthic populations. The relative
abundance of dagaa and haplochromines in catches is site- and
season-specific, suggesting that the dagaa–haplochromine
interaction hides multiple underlying drivers. While fishers and
markets in Tanzania have adapted and now specifically target
haplochromines, in Uganda and Kenya, haplochromines are still
only bycatch, though they are sold on local markets. The scale of
haplochromine bycatch is clearly indicative that dagaa fishing
methods are not selective enough, which is important because
haplochromines have slower growth rates than dagaa, are more
sensitive to eutrophication, and can therefore not sustain the same
levels of fishing mortality. Furthermore, since both dagaa and
haplochromines can be sold in multiple forms (bait, fodder,
human consumption), the actual taxonomic content of the dagaa
catch has little effect on fishers. Therefore, since the resurgence of
haplochromines, the dagaa and haplochromine fisheries and
stocks have become tightly connected, which reduces the strength
of negative feedbacks and the self-regulating ability of the system
(Fig. 3: dagaa fishing effort --o haplochromines versus dagaa
fishing effort --o dagaa; haplochromines and dagaa --> LocMar).
Indeed, if a reduced stock does not lead to a reduced catch (e.g.,
should dagaa replace haplochromines, or vice versa), the negative
feedback of a dwindling stock on fishers and markets is reduced.
This conclusion contrasts the view that selective fisheries—as
opposed to balanced harvesting (Garcia et al. 2012)—are
detrimental to the ecosystem as a whole. In this particular
instance, the lack of selectivity does not equate to a balanced
harvesting methodology since it is not adapted to the habitats,
life histories, and growth rates of dagaa and haplochromines.
A certain level of nutrient input can increase productivity for one
species (enrichment) but represent reduced fitness or increased
mortality (eutrophication) for another. Haplochromine species,
for example, are particularly sensitive to low light (Seehausen et
al. 1997a), whereas dagaa is probably relatively more sensitive to
deoxygenation than is Nile perch (Wanink et al. 2001). Without
explicit management, nutrient enrichment would most likely
always lead to further enrichment—i.e., it is not intrinsically selflimiting or regulating. Enrichment would, however, translate only
to increased stock growth until the negative consequences of
eutrophication start taking effect and while fishing effort does not
cause mortality rates that exceed growth rates (i.e., stock collapse).
Ultimately, sustainability of stocks is a function of the relative
strength of fishing-induced mortality versus a stock’s response in
growth through feeding. Since enrichment and eutrophication
alter growth rates, sustainable fishing mortality rates must be
variable, not constant. Also, although eutrophication affects the
whole lake, the distribution and effect of nutrients on the
biochemistry of the lake is spatially heterogeneous (Box 3, Table
4). Therefore, the processes that drive growth of stocks—and thus
dictate levels of sustainable harvesting—are site-specific and not
generalizable across the whole lake at any one time.
Box 3: Spatial heterogeneity of the lake
In inshore areas, terrestrial runoff and detritus are primary
sources of light limitation, whereas further offshore it is algal
biomass that induces light limitation, and thus restricts
photosynthesis to a narrower surface layer. Eutrophication affects
the whole lake, as half the total phosphorus input to the lake is
from atmospheric deposition (Tamatamah et al. 2005), though
there is a lot of spatial heterogeneity in its effects—not only in an
inshore-offshore gradient, but also between bays around the lake
(Silsbe et al. 2006, Loiselle et al. 2008, Cornelissen et al. 2013).
Total nitrogen concentrations have increased since the 1960s,
primarily through cyanobacteria fixation, and nitrogen generally
follows a decreasing concentration pattern from the coast to
offshore waters (Hecky et al. 2010). While nutrient loading has
created the light-limited conditions of the lake, seasonal and
interannual variations in primary productivity are now driven
primarily by climate (Silsbe et al. 2006, Cózar et al. 2012). This is
of particular relevance since it highlights an important
characteristic of qualitative models such as the one we use: they
do not account for time or effect lags (Justus 2005). The synergistic
and cumulative effects of warming and eutrophication on
phytoplankton and the food web will probably delay the effects
of eutrophication management (Schindler 2006).
Increases in nutrients no longer lead to increased phytoplankton
primary production in the northern Murchison Bay and
Napoleon Gulf of Lake Victoria, indicating that these areas are
phosphorus saturated (Silsbe et al. 2006) (Fig. 3: N&P -->
shading). However, recent experiments conducted on samples
from the Mwanza Gulf in southern Lake Victoria show that the
increase in primary productivity with further nutrient enrichment
can still take place (Cornelissen et al. 2013). Effects of further
nutrient enrichment are therefore likely to be area- and seasonspecific. Over the years, chlorophyll a measurements have shown
a great degree of variability (Table 4). Even though differences in
sampling frequency and measurement methods, as well as in
location, depth, and timing of sampling make it impossible to
draw accurate trends from these data, it appears that chlorophyll
concentrations in the northern parts of the lake are higher than
in the south (Table 4).
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Table 4. Variability in chlorophyll a (Chl) concentrations in the lake, as found in the literature. Seasons: DS = (dry season): June–
August; SR = (short rains): September–December, LR = (long rains): January–May. SD = standard deviation. N = number of measures.
LVRO = Lake Victoria Fisheries Organisation.
Source
Location
Year
Season
Depth (m)
Chl (μg/l)
Acoustic survey LVFO
Murchinson Bay
Mwanza Gulf
Nyanza Gulf
Nyanza Gulf
Speke Gulf
Mwanza Gulf
Mwanza Gulf
Mwanza Gulf
Murchinson
Nyanza Gulf
Murchison Bay
Murchison Bay
Murchison Bay
Napoleon Gulf
Nyanza Gulf
Nyanza Gulf
Napoleon Gulf
Napoleon Gulf
Napoleon Gulf
Napoleon Gulf
Tanzania
Tanzania
Tanzania
Jinja
2005
2005
2005
2006
2005
1973–1974
1973–1974
1973–1974
2003
2005–2006
2003–2004
2003–2004
2003–2004
1992
1994–1995
1994–1996
1994–1998
1994–1999
1994–2000
1994–2001
2005–2007
2005–2007
2005–2007
2001–2002–
2004
2004
2004
2004
2005
2005
2005
2005
2005
2005
2005
2005
2005
2001–2002
2001–2002
2001–2003
2000
2000
2001
2001
2006
2006
2007
2007
2008
2008
2009
2009
1998
2009–2011
2009–2011
2009–2011
DS
LR
DS
LR
LR
DS
LR
SR
DS
All year
DS
LR
SR
SR
SR
LR
All year
LR
DS
SR
All year
All year
All year
SR
< 20
< 20
< 20
< 20
< 20
8
8
8
< 20
< 20
< 11
< 11
< 11
< 10
< 10.5
< 10.5
< 20
< 20
< 20
< 20
< 10
10–20
20–30
< 26
44.6
18.43
11.29
8.91
20.76
4.35
5.1
3.7
80.92
17.57
21.43
31.4
24.06
48
20.77
13.45
71
38.11
27.5
82.5
16
22.5
11
52.9
DS
DS
DS
DS
LR
SR
DS
LR
SR
DS
LR
SR
DS/SR
DS/SR
DS/SR
DS
LR
DS
LR
DS
LR
DS
LR
DS
LR
DS
LR
SR
DS
LR
SR
<5
<5
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 10
<8
< 14
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 20
< 15
< 25
< 25
< 25
9
18
10.5
14.67
18.17
13.67
20
18.17
17.56
9.83
13.67
17.56
42
69.5
26
12.8
13.2
10.9
10.8
16.2
14.8
14.4
15.7
14.5
16.8
14.8
12.7
34
13.24
14.96
13.45
Akiyama et al. (1977)
Cózar et al. (2007)
Gikuma-Njuru (2008)
Haande et al. (2011)
Lehman and Branstrator (1993)
Lung’Ayia et al. (2000)
Mugidde (2001)
Ngupula et al. (2011)
North et al. (2008)
Okello et al. (2009)
Shayo et al. (2011)
Silsbe et al. (2006)
Sitoki et al. (2010)
Yasindi and Taylor (2003)
Cornelissen et al. (2013)
Bunjako (Uganda)
Murchinson Bay
Napoleon Gulf
Kayenze (Speke Gulf)
Kayenze (Speke Gulf)
Kayenze (Speke Gulf)
Magu (Speke Gulf)
Magu (Speke Gulf)
Magu (Speke Gulf)
Mwanza Gulf
Mwanza Gulf
Mwanza Gulf
Fielding
Murchison Bay
Napoleon Gulf
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Whole lake
Napoleon Gulf
Mwanza Gulf
Mwanza Gulf
Mwanza Gulf
SD
N
8.19
2.97
9.18
1.60
2.01
1.10
13.73
3.60
6.63
10.41
6.54
1
1
2
4
9
6
6
8
13
7
14
20
16
7.48
2.77
100.4
20.33
18.37
79.94
6
8.5
5
10.60
11
11
47
9
6
10
10
11
9
12
1.41
7.07
0.71
7.58
3.76
6.46
10.12
12.19
7.21
2.79
4.72
6.17
23.06
24.33
6.292
11.3
4.4
7
6.7
8
10.9
8.4
8.5
11.2
8.6
6.1
12.5
2
2
2
6
6
9
6
6
9
6
6
9
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
4.71
8.05
4.60
68
73
72
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Fig. 4. Species and functional diversity loss in the soft-bottom
fish community of Lake Victoria. The bars represent the
number of haplochromine species per functional group
collected in the sublittoral waters (6–14 m) of the northern part
of the Mwanza Gulf, over soft bottom, in 1979–1982 and 2001–
2005. Between 1979–1982 and 2001–2005, 65–70% of species
and 50% of functional groups were irreversibly lost. One
unclassified species is omitted here. Data from Witte et al.
(2013). Not only does the number of functional group decrease,
but the degree of specialization within trophic groups also
decreases: e.g., former zooplanktivores have broader diets in
2001–2005 than in 1979–1982 (Kishe-Machumu 2012, van
Rijssel and Witte 2013).
Our combined social-ecological diagram allows us to see the
effects of change in a broader context, but it has a limited
predictive value: indeed, it cannot account for adaptation in either
human or fish populations to declines in stock or in environmental
conditions. However, with a limited time frame in mind, the
diagram can give an idea of the far-reaching effects of any single
action on the whole system. We indicated three exogenous
stressors: climate change, the international economy, and regional
policies. More might exist, and they could have broader effects
than illustrated here. These stressors are dubbed exogenous
because they are not solely or directly influenced by Lake
Victoria’s system. Exogenous stressors are important in that they
could shift the sign of loops in the feedback diagrams or change
the relative importance of any given interaction or loop and thus
alter system dynamics.
SYSTEM TRENDS AND CHANGES MATCHED TO THE
MODEL
Here we look into Lake Victoria’s past to contextualize the model
and identify how its elements and interactions illustrate trends
that have shaped the dynamics of the social-ecological system.
Through this juxtaposition of model and trends, we can identify
the model’s scope and limitations and indicate its best uses.
Furthermore, it allows us to explore how weights of different
interactions might have changed over time.
When using the diagrams, it is important to account for
underlying assumptions, and to look at different feedback loops
in concert. As an example, if nutrient enrichment leads to higher
Nile perch stocks (Table 1, row 7), this promotes investment from
the international market, which can drive owners to invest effort
in alternative stocks, such as dagaa, which would then reduce
dagaa stocks. This would thus negatively influence the local and
regional market and lead to a decrease in human population
growth, which would lead to a decrease in nutrient input. Taken
in isolation, such a scenario would be based on questionable
assumptions: it would assume that nutrient enrichment has a
positive effect only on Nile perch, not on dagaa. It would also
assume a single fishery target with no alternative stimuli for the
regional market, and that this dynamic is the major determinant
of human population growth. This type of loop would represent
a case where international economic input does not lead to social
and economic growth in the region, and thus does not lead to
increased nutrient input to the lake. The diagrams presented here
are tools that can enhance understanding and map multiple
system variables simultaneously, but even though they are simple
tools, their interpretation should not be oversimplified.
In the beginning…
Before the introduction of Nile perch, Lake Victoria harbored a
small though diverse, multispecies artisanal fishery. Fishing,
along with agriculture and livestock herding, constituted the
economic foundations of lakeshore societies. The main fishing
targets were the native tilapia Oreochromis esculentus and O.
variabilis (Balirwa et al. 2003, Pringle 2005b). Fish were initially
sold by number rather than weight, which invited catches of small
fish (Beverton 1959, Balirwa 1998). The introduction of gillnets
in the 1920s as a new tool quadrupled fishing efficiency, and the
fishery became commercial—a process further helped by urban
infrastructure development (including road and rail) around the
lake (Balirwa 1998) (Fig. 3, development of local/regional market:
dagaa and haplochromines --> LocMar --> owners and
Soc&econ…).
Since the 1960s, effects of eutrophication—driven by human
population growth around the lake and subsequent changes in
land use (Verschuren et al. 2002, Hecky et al. 2010)—became
apparent. The phytoplankton community underwent a transition
from a diatom-dominated community to one increasingly
dominated by nitrogen-fixing cyanobacteria, and primary
production increased. In turn, the zooplankton community
shifted to be dominated by small species (Mwebaza-Ndawula
1994, Gophen et al. 1995, Wanink et al. 2002). The dominance
shift in the phytoplankton community has been hypothesized to
be a cause in the zooplankton shift (Wanink et al. 2002) (Fig. 1:
change in relative importance of phytoplankton --> zooplankton
versus phytoplankton --> detritus and anoxia & shading).
Introduced species, changed ecosystem
As native stocks were declining from exploitation, eutrophication,
and habitat destruction, Nile tilapia (Oreochromis niloticus) and
other tilapiine species were introduced. Simultaneously (between
1954 and 1964), Nile perch were introduced to boost the
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
productivity of the fisheries and convert the native diversity of
haplochromines into a more desirable fishery product
(Kudhongania and Chitamwebwa 1995, Pringle 2005b). From
1965, Nile tilapia gradually replaced native tilapia, became a
major commercial catch (Balirwa 1998), and is currently an
important product for local and regional markets (Njiru et al.
2008). Tilapia does not figure explicitly in our diagrams because
it appears to interact little with other stocks (Downing 2012,
Downing et al. 2013b). It is, however, of course also influenced
by enrichment, eutrophication, and exploitation processes, and
contributes to the regional market, and can therefore be read in
the system in a similar way to haplochromines or dagaa, but with
tilapia-specific sensitivities to the different driving processes.
While the Nile perch introductions had little immediate effect, in
the 1980s many changes occurred quite rapidly. Nile perch
suddenly became dominant in the whole lake, and—in concert
with increased hypoxic and anoxic conditions—contributed to
the collapse of haplochromine stocks and mass disappearance or
extinction of native fish species (Witte et al. 1992, Hecky et al.
1994, 2010, Verschuren et al. 2002, Goudswaard et al. 2008). The
high abundance of haplochromines proved to be favorable for
Nile perch as growth estimates in both the Tanzanian and Kenyan
waters through the end of the 1980s—covering the time when Nile
perch were still feeding heavily on haplochromines—showed that
Nile perch grew at least twice as fast as in haplochromine-poor
lakes (e.g., Lake Chad, Lake Turkana, Lake Albert, Lake Nasser)
(Ligtvoet and Mkumbo 1990) (Fig. 3: Nile perch --o
haplochromines). Ecological opportunities afforded by the
vanishing haplochromines were exploited in short succession by
two native species, first the detritivorous shrimp C. nilotica
(Goudswaard et al. 2006) and then the zooplanktivorous cyprinid
dagaa (R. argentea) (Wanink et al. 2002).
Since the 1990s, effects of eutrophication and fishing have become
more apparent, and Lake Victoria’s ecosystem is still undergoing
many changes. The water hyacinth (Eichhornia crassipes), which
first appeared in the lake in 1989 (Chapman et al. 2008), had
produced large mats by 1995. These mats severely interfered with
fishing operations, with important social-economic consequences
(Opande et al. 2004), and altered the bio-physico-chemical
environment of the water under them (Masifwa et al. 2001,
Kateregga and Sterner 2008). This event supports the notion that
water quality influences stocks and catches, and importantly, that
when the lake’s environment is compromised in such a way, neither
international nor regional markets can act as backups, and society
has little (perhaps only agriculture) left to rest on (Fig. 3: N&P
and anoxia & shading --> Nile perch, dagaa, and haplochromines
--> IntMar and LocMar --> Soc&econ). The hyacinth invasion
was in large part managed between 1997 and 1999 through manual
removal and the introduction of a weevil (Neochetina
eichhorniae). The effects of these management procedures were
probably enhanced by an El Niño episode (1997–1998), during
which elevated water levels broke off large hyacinth stands, and
where low light conditions (clouds) and rain reduced hyacinth
growth (Njiru et al. 2002, Williams et al. 2005, 2007, Wilson et al.
2007). Even though the hyacinth bloom was probably triggered
by eutrophication, its decline is not a reaction to improved water
quality: there are multiple pathways to any single phenomenon.
Dominance of cyanobacteria in the phytoplankton community has
led to an increase in concentrations of toxins produced by
cyanobacteria (e.g., microcystin). Higher trophic levels—including
humans—are increasingly exposed to such toxins, both directly
through water and through their accumulation in fish tissues (Poste
et al. 2011). Low fish quality would probably reduce fish prices on
(or even access to) the international market, but decreased
wellbeing or increased illness in lakeshore societies would probably
have more complex and further reaching consequences. At this
point, however, the extent to which cyanotoxins affect human
health around Lake Victoria is unknown, though it is important to
keep toxicity in mind as a process in eutrophication.
Societal adaptations
The advent of the Nile perch boom in the 1980s changed societies
dramatically (Fig. 3: IntMar --> owners and Soc&econ versus
LocMar --> owners and Soc&econ). People migrated to the
lakeside to enter the fisheries, which accelerated human population
growth and altered the kinship relations that had existed previously.
Human populations around the lake continue to grow at a higher
rate than the African average (Odada et al. 2009).
Filleting factories were constructed around the lake, and the fish
were exported to Europe (Pringle 2005a, Johnson 2010). The major
filleting factories were built to process up to 25 tons of Nile perch
each, per day, but would break-even on costs when operating at
about 30% capacity (Johnson 2010). Nile perch catches reached a
peak in 1990, but effort has continued to increase since.
Changes in dagaa, haplochromines, and the regional market
Although dagaa is native to Lake Victoria, a directed fishery for it
first developed only in the 1960s. The stocks of dagaa rapidly
increased after the Nile perch boom and haplochromine collapse.
Dagaa became the lake’s second most important fishery by the
1990s (Wanink 1999), when landing sites started organizing
themselves into camps, also presenting a strong social structure,
from trader to owner, through camp supervisors and laborers, with
women drying the fish. Dagaa is currently a dominant commercial
fishery of the lake in terms of biomass (Tumwebaze et al. 2007),
and it trades on domestic and regional markets (Gibbon 1997).
Catch that is successfully dried and cleaned of sand is destined for
human consumption as it serves as a cheap source of protein for
the poor and middle class (Medard 2012), and the rest sells as
animal feed: chicken fodder, for example. Traders play an important
role, checking the quality of the product as well as establishing its
price (Gibbon 1997). Despite no apparent decrease in its stock size,
dagaa has undergone behavioral, morphological, and life history
changes in some regions of the lake, which may reflect local adaptive
response to predation by Nile perch, fishing pressure, and/or
eutrophication and associated hypoxia (Wanink 1999, Wanink and
Witte 2000, Manyala and Ojuok 2007, Sharpe et al. 2012). Fishing
effort has been on a steady increase (Mkumbo et al. 2007), at least
until 2006, from where data suggest a stabilization in effort (Kolding
et al. 2014) (Fig. 3: N&P --> dagaa versus dagga fishing effort --o
dagaa). Dagaa remains a product destined mostly for a local and
regional market, and is, therefore, less well monitored, regulated,
and studied than Nile perch (Medard 2012).
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
SOCIAL-ECOLOGICAL CHANGE SEEN IN MODEL
STRUCTURE
The single diagram we developed to represent Lake Victoria’s
social-ecological system can be used to analyze and represent a
whole variety of dynamics. This is necessary because system
dynamics have seen both short- and long-term dynamics. Seasons
influence light, temperature, weather, and wind conditions, which
influence the growth of organisms as well as access to different
areas of the lake. Dynamics also have a heterogeneous spatial
distribution, considering, for example, the different nutrient
environments in the north and south of the lake (Box 3, Table 4),
or the atmosphere of conflict in contested Ugandan—Kenyan
fish camps (Box 4), or differences in fishing practices (Beuving
2014). All of these factors imply that a quantitative model of Lake
Victoria’s social-ecological dynamics ought to be set in a clearly
defined spatio-temporal setting. Meanwhile, this purely
qualitative model allows us to analyze three types of structural
changes to the system and their influence on dynamics and role
in the effectiveness of policies and regulations: (1) a change in the
relative weight of different interactions, (2) the change in the sign
of interactions or loops, and (3) the addition or removal of
interactions. Here, we analyze—through examples of past
changes and hypothetical management policies—the effects of
these three types of changes on the dynamics on Lake Victoria’s
social-ecological system.
Box 4: Societal reorganization and inequity
To ensure constant supply to the factories after the trawling ban
became effective in the 1990s (Mbuga et al. 1998), a new socialeconomical interaction emerged: processors invested in building
infrastructure to better store Nile perch at landing sites and
sponsored fishermen with nets and gear via agents, who would
then ensure fixed prices and supply of fish (Mbuga et al. 1998,
Geheb et al. 2008, Johnson 2010). Lakeshore societies thus
developed a stronger economic hierarchy from processor through
agent to fisher, with many fishers then entering a situation of
credit towards agents or processors (Geheb et al. 2008). Fishing
effort continues to increase and fisher communities to restructure.
In the Nile perch fishery in some areas, many agents (middlemen)
have become boat owners and landing site or camp managers who
employ camp supervisors and laborers to operate full fleets and
with whom they arrange credits and loans to ensure a constant
fish supply. The structure is likely quite diverse, however, and there
are reports of wealthy boat owners striking deals with factories
and thus taking a trader’s role.
Economic input to society is nonlinear, with larger swaths of
income distributed to big owners, and much less reaching fishers
and laborers. Besides stocks and the international economy and
regulations, local social-economic settings and interactions can
have complex effects on both social stability and benefits reapable
by owners through fishing. Individual cost-to-benefit ratio of
fishing and risk-taking behavior is variable. Competition for
landing sites among owners, for example, can influence costs and
prices by leading to a local monopoly of the fisheries by few
owners. Competition among owners can lead to conflict and
violence: there are reports of piracy in the southern part of the
lake, which leads to increased costs in security and uneven
distribution of fish resources. Also, there are chronic territorial
conflicts over fish camp islands between Kenya and Uganda
(Otieno 2013). Knowledge of the distribution of power,
opportunities, and value of risk in society is key to the selection
and implementation of appropriate policy measures. Here, we
lack representation of the potential response diversity of society,
which is a great impediment to measuring and predicting society’s
reactions and adaptations to ongoing changes in the system.
Changes to the relative weight of interactions
Management policies can profoundly influence the connectedness
of a system, for example, by creating tighter networks of
communication. Increased economic disparity caused an increase
in illegal fishing practices in the 1990s. In an effort to improve
compliance and better implement fishing regulations, Tanzania
established a comanagement approach to the fisheries in 1997, in
the form of Beach Management Units (BMUs) (Kateka 2010).
These local groups were established to increase stakeholder
participation in surveillance and management of the fisheries, and
similar structures were later implemented in Uganda and Kenya
(Mkumbo 2002, Kateka 2010). Their success has been mixed,
however: BMUs have effectively suppressed poisoning and
dynamiting as fishing techniques, but they lack an adequate legal
status, and by establishing their own bylaws, BMUs can
sometimes abuse their position and authority (Geheb et al. 2007,
Njiru et al. 2007). Importantly, they have also increased fishing
efficiency through improving communication networks (Eggert
and Lokina 2009) (Fig. 3: strengthening of interactions Nile perch
and dagaa fishing effort has a negative effect on Nile perch, dagaa,
and haplochromines), helping fishers get to good fishing sites
faster and reducing the negative feedback of declining stocks on
fishers.
Changes in the sign of interactions or feedbacks
Fertilizer subsidies, which might promote agriculture and
alternative sources of income, would nonetheless reinforce
positive feedback loops through nutrient enrichment, and thus
be overall destabilizing. A fishing limiting policy, such as a
licencing fee, could add a minus to fishing effort and change the
signs of the loops in that way, though if it is not applied to all
fisheries, such a policy could have a backlash effect and lead to
higher exploitation of alternative fisheries or to increased land
use for agriculture. In short, the effect of any single policy or
change depends on its ripple effects through the broader system.
Changing the number of pathways in the system
In a bid to prevent and eliminate illegal, unregulated, and
unreported fishing, the Lake Victoria Fisheries Organisation has
pushed to promote eco-labeling of Nile perch. For this, they
established codes of practice with help from the Marine
Stewardship Council (Kolding et al. 2014). In 2009, the German
cooperative Naturland launched its eco-label for the small capture
Nile perch fishery, certifying the sustainability of Nile perch
products exported to Germany. The label is based on ecosystem,
fishery, social, and hygiene criteria, and covers eight landing sites
and 1000 fishermen in Tanzania (Bukoba), though there is
currently no known study that evaluates the effects of eco-labeling
on the stocks, society, or economy. However, seen through our
diagram, Naturland’s policy—i.e., supporting a small-scale
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
fishery on the Nile perch—might be translated by adding a role
to some owners, whereby their connection between the regional
market and the Nile perch fishery is subject to regulations (Fig.
3: LocMar --> owners --> Nile perch fishing effort) and where
owners connect the regional and international market (Fig. 3,
IntMar --> owners --> LocMar). One of the loops that would
then emerge is a negative feedback loop connecting the regional
market to fisheries, Nile perch stocks, the international market,
local societies, and the regional market (Fig. 3: LocMar -->
owners --> Nile perch fishing effort --o Nile perch --> IntMar -->
Soc&econ (or owners) --> LocMar). This constraint on the
middleman (owner) would differ from the loops we have so far
accounted for in that the owner’s role merges with that of the local
and regional economy and society (Fig. 3, Soc&econ). Such a
loop might have beneficial social-economic repercussions since it
could reduce economic disparity (note that we separated owners
from society to be able to reflect differences in economic benefits
and incentives). However, in itself, this policy still only adds a
pathway to harvesting Nile perch: a small-scale fishery might be
sustainable if taken on its own, but as long as it operates in parallel
with other harvesting policies, the stocks might yet be vulnerable
to unsustainable harvesting. Therefore, for a policy like that of
Naturland to actually ensure sustainable Nile perch harvesting,
it needs to outcompete existing harvesting strategies (effectively
connecting itself to other fisheries with a minus sign in our
diagram). While not all owners are party to the policy, sustainable
harvesting might happen if the label succeeds in influencing
consumption patterns in the international market (and) or if
different incentives to harvest sustainably are presented directly
to fishermen and fishing site owners.
Quantitative versus structural changes to the system
Overall, we find that policies and changes that alter the
connectedness of the system influence the self-regulatory capacity
of the system. Connection strength changes are probably the most
common and insidious changes in the system, as they can be an
uncalculated side effect of various other policies (as for example,
the case of BMUs). Also, given the heterogeneity in conditions
and processes over the whole lake, differences in connectedness
across the system influence the generalizability of policy
interventions: interaction strengths are not the same across the
whole lake or even permanent in time (Boxes 2 and 3). Changes
in the sign and number of interactions, however, as for example
the Naturland eco-labeling policy, can alter system dynamics
without necessarily altering the integrity of regulating feedback
mechanisms, though the effects of such structural changes are
dependent on the configuration and dynamics of the system as a
whole.
DISCUSSION
Here, we put together pieces of Lake Victoria’s social-ecological
system in a qualitative, conceptual model and achieve greater
understanding of the system’s functioning from a good prior
knowledge of the processes behind dynamics, as well as from the
key elements and interactions that are specific to Lake Victoria.
The interconnection diagram highlights the multiple pathways to
any single phenomenon in the system. As an example, the threat
to fisheries stems not only from fishing effort but also from other
environmental factors: eutrophication, hypoxia, and anoxia
reduce fish growth and survival rates, thereby increasing the
stocks’ vulnerability to fishing pressure. Complete overharvesting
of Lake Victoria’s fishes, be it Nile perch, dagaa, or tilapia, would
not necessarily be an easy task: the number of fishers and crafts
on the lake has increased dramatically since the early 1990s, but
technologies have not followed suit everywhere. Paddling is still
the dominant method of propulsion in a large part of the lake,
and where engines are available, fuel costs, and in the case of the
Nile perch fishery, costs related to the hygienic storage of fish
during longer periods, as well as dangerous boating conditions,
all impose limits on the distance from shore that is exploited.
Dagaa is also difficult to harvest efficiently, despite being a
productive species: fishing of dagaa occurs mostly on moonless,
windless nights, using lamps that attract insects and zooplankton
and then the zooplanktivores, including dagaa. However, even if
all stocks are not currently overfished, the threat to Lake Victoria’s
fisheries is real. Indeed, native tilapia (ngege [Oreochromis
esculentus]) was successfully reduced to low numbers using quite
simple methods before the introduction of Nile perch and Nile
tilapia. Even though the dagaa fishery might not be the most
important direct threat to dagaa stocks, it could be an important
influence on haplochromines since the two fisheries are connected
through fishing methods and markets. The adaptability of fishers
and markets to changes in stocks might reduce the self-regulating
character of exploitation (i.e., dampen the negative feedback of
exploitation loops) and make a stock collapse even less predictable
or even avoidable. In this way, while overfishing is not necessarily
currently the biggest threat to fisheries on Lake Victoria per se, it
becomes an important threat in the presence of environmental
change and eutrophication, and in the context of the full,
interconnected system.
Interestingly, here we find that even though high connectivity of
fisheries at the level of owners and markets may be detrimental
to stocks (Box 2), it can also make the social-economic system
more robust to the loss of the international market. A loss of the
international market would cause a dramatic loss of revenue, but
there is no shortage of demand for fish products, in general, and
fishing markets have diversified. Dagaa, haplochromines, and
small Nile perch are sold locally and regionally for human
consumption and as animal feed. Tilapia and Nile perch fetch
higher prices on the same markets. Furthermore, a shutdown of
all international trade is considered highly unlikely. In the same
way as when the European market closed in 1997, Israel and Asia
and other African countries could perhaps serve as alternative
markets (van der Knaap et al. 2002). Nonetheless, it is important
to remember that lakeshore societies have grown and developed
fast, and a lack of long-term investments or permanent structures
contributes to the high and increasing income disparity in
lakeshore populations, which in turn can influence societies and
contribute to their increased volatility (Wilkinson and Pickett
2009).
The rapid Nile perch invasion created a strong dependence on
fisheries (as exemplified during the water hyacinth blooms), which
probably makes societies quite vulnerable to a loss of stocks,
especially since there are few other sources of steady income.
Indeed, as the Nile perch fishery grew, migrant settlements that
started off as temporary became villages—with bars, makeshift
cinema halls, and brothels—though they often lack essential
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
permanent structures such as medical units or schools (Geheb et
al. 2008). Investment opportunities lie mostly in high profit
margin activities rather than in durable or saving schemes, the
economic life is deeply monetized since there is little selfsustenance, and most goods must be bought (Beuving 2010).
People in these boom villages easily fall into a poverty trap.
General health status is low, and there is a high HIV-AIDS
infection and prevalence rate (van der Knaap and Ligtvoet 2010).
Therefore, alternative sources of income are undoubtedly highly
dependent on the fisheries—the services sold in villages and camps
are targeted at fishers. These will not provide a safety net should
fisheries collapse. Agriculture, chicken farming, and livestock
herding are among the few sources of income that are independent
of fisheries. Following this analysis, we suggest new metrics with
which to bridge social and ecological changes in future studies:
investments in infrastructure and the sources of these investments
might provide a usable proxy for the dependence of societies on
their resources. Such data might also inform the sustainability or
durability of development around Lake Victoria. While
investment in urban and transport infrastructure in the early 20th
century and in fish processing infrastructures in the 1980s
heralded priority areas of future development around the lake,
there was no comparable spending on agricultural infrastructure.
This model does not yet encompass the full complexity of Lake
Victoria’s societal components (e.g., Box 4). We lacked the
references and detail needed to generalize the different processes
that drive societal dynamics, and how these induce individuals to
take risks, drive investment, and frame decision-making
processes. In short, our model strongly underrepresents the
response diversity of society (all those processes hidden in the
feedback between society and the local market). In the same way,
our model lacks resolution in terms of biodiversity (Box 5). We
can represent functional diversity to a small degree—which
underlies in part the haplochromine-dagaa interrelation—but the
full biodiversity and richness of the ecosystem, which sets the
stage for the adaptability of the system to changes and
disturbances, is not represented. These two elements—
biodiversity and social structure—are directly relevant to any
medium-term predictions on system dynamics, to policy-making,
and to the assessment of the social-ecological system’s current
state. Also, it is important to remind the reader that the ecosystem
we describe has a strong aquatic bias. In truth, any visitor to the
lake will first encounter a large diversity of birds, insects, and
vegetation, all of which influence and are influenced by changes
in the aquatic, terrestrial, climatic, and human systems. This,
however, is at least a first step in bridging the socialeconomic-(aquatic)-ecosystem, in hopes that encouraging further
work on bridges to avian and terrestrial ecology might be possible
in the future.
Box 5: Diversity changes
Even though the resurging offshore haplochromine community
has largely recovered in biomass and retained a 40-strong species
diversity (Seehausen et al. 1997b), of which 15–20 species are
pelagic, this is only a small fraction of the 1970s haplochromine
diversity. Indeed, 65–70% of both benthic and pelagic species
dwelling over soft bottoms in the sublittoral waters (6–14 m deep)
of the Mwanza Gulf seem to have been irreversibly lost (Kishe-
Machumu 2012, Witte et al. 2013) (Fig. 4). In the pre-Nile perch
system, an estimated minimum of 500 different species of
haplochromines occupied almost every function in the food web,
from detritivorous species through to piscivores. Fishing pressure
on Nile perch (Fig. 3: Nile perch fishing effort --o Nile perch --o
haplochromines), as well as morphological adaptations to
changed environmental conditions and major habitat shifts are
possibly associated with the reappearance of haplochromines
(Kitchell et al. 1997, Seehausen et al. 1997b, Schindler et al. 1998,
Witte et al. 2000, 2008, Balirwa et al. 2003, Mkumbo and Mlaponi
2007, van Rijssel and Witte 2013). Dominant haplochromines of
the Mwanza Gulf in 2006–2008 and 2010–2011 were found to be
predominantly the former zooplanktivores and detritivores/
phytoplanktivores (Kishe-Machumu 2012). However, diets of
haplochromine species recovering since the 1990s changed: they
contained more macro-invertebrates than in the 1970s and early
1980s (Kishe-Machumu et al. 2008). Although in recent years
haplochromines seem to have reverted to zooplanktivory, they are
less specialized than they were in the 1970s and 1980s (KisheMachumu 2012, van Rijssel and Witte 2013).
In our feedback diagrams, we linked all fields of research at their
boundaries, thus shifting the focus from each field’s central themes
to the interconnections between fields. The diagrams show how
a change in any part of Lake Victoria’s system can trigger chain
reactions across the system. Many of these chain reactions
contain or are part of feedback loops, indicating that changes in
the system can initiate and drive new dynamic regimes. By
analyzing this interconnected system, we find multiple pathways
to what may at first appear to be singular, linear phenomena. The
presence of multiple pathways can be dangerous since it can lead
to a misinterpretation of mechanisms driving change, but it can
also be advantageous: by understanding different pathways that
regulate a single process, we obtain multiple tools with which to
manage change.
We investigate the mechanisms of change through the socialecological model. Changes to the structure of the system, which
can be brought about through evolution and adaptation in the
social-ecological system (for example, a broadening of a species’
diet creating a new interaction, a species extinction leading to a
loss of an interaction, or technical or cultural changes in the social
system) or through the implementation of policies, can
dramatically alter systems dynamics and responses to change.
Though our analysis is not quantitative, we can nonetheless see
that differences in relative strengths of interactions, which can be
understood as a change in the system state, can greatly influence
the vulnerability of system elements to further change. Such
quantitative changes and differences can be quite pervasive in
both time and space. The dominant processes shaping the Lake
Victoria’s dynamics have changed over time and are
heterogeneous in space. This heterogeneity implies that multiple
system states exist, which means on the one hand that studies on
any part of the system must be understood in their specific space–
time context, and on the other hand that the scope of management
actions must be scale adapted.
The system’s heterogeneity can also explain some of the scientific
disagreement that prevails regarding the state of the fisheries.
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Efforts ought to be made to determine the spatio-temporal scales
at which different processes dominate. Lake Victoria’s complex,
changing, and interconnected system calls for an adaptive
management approach capable of embracing this heterogeneity.
Adaptive management requires in large part that stakeholders
have a common understanding and knowledge of their system
(Ostrom 2009). By combining knowledge from different fields and
perspectives, we tried to develop such common knowledge. This
knowledge should not be interpreted as absolute. Rather, it will
wax and develop as users and scientists learn from further changes
in Lake Victoria’s system and from the system’s responses to
management. Choices of management policies need not and will
not wait for more or better knowledge of the system’s dynamics:
they are part of the learning processes.
This little history of Lake Victoria highlights how nearly all
regulations or policies that have been put in place in Lake Victoria
have only ever targeted fishing. Almost nothing has been done to
halt or reverse eutrophication. In part, this is because the main
sources of eutrophication—i.e., land burning—have only recently
been defined. Though eutrophication already displayed effects in
the 1960s, it has only recently been acknowledged as a potential
and in fact important driver of change in Lake Victoria’s system.
Indeed, this is to our knowledge the first interdisciplinary study
to put eutrophication and fisheries in Lake Victoria on an equal
footing, and perhaps in this way signals a turning point in the
understanding of drivers of change in Lake Victoria.
Eutrophication is ongoing, has damaging consequences for Lake
Victoria’s system, cannot compensate for or be remediated by
fishery policies, and must be addressed for the general health of
the system, its users, and fisheries. Additionally, any fisheryoriented policy is likely to be ineffective in the absence of a parallel
eutrophication and broader lake basin management policy.
Associated policy options must be processed and understood by
societies to be made operational, yet we still lack a lot of
knowledge about social dynamics and interactions. Furthermore,
biodiversity is likely to be key in helping to process excess
nutrients, restore better water quality, and value and enjoy the
host of nonfishery-related ecosystem services that the lake also
provides. The full role of biological diversity and its potential to
recover in Lake Victoria are largely unknown. Our study points
to society and diversity as two key subjects Lake Victoria scientists
need to unravel.
LITERATURE CITED
Abila, R. O., and E. G. Jansen. 1997. From local to global markets.
The fish exporting and fishmeal industries of Lake Victoria—
structure, strategies and socio-economic impacts in Kenya. IUCN,
Nairobi, Kenya.
Akiyama, T., A. A. Kajumulo, and S. Olsen. 1977. Seasonal
variations of plankton and physico-chemical condition in
Mwanza Gulf, Lake Victoria. Bulletin of Freshwater Fisheries
Research Laboratory 27:49–61.
Awange, J. L., and O. Ong’ang'a. 2006. Lake Victoria, ecology,
resources, environment. Springer, Victoria.
Balirwa, J. S. 1998. Lake Victoria wetlands and the ecology of the
Nile tilapia, Oreochromis niloticus Linne. Dissertation,
Wageningen University, Balkema, Rotterdam, The Netherlands.
Balirwa, J. S., C. A. Chapman, L. J. Chapman, I. G. Cowx, K.
Geheb, L. Kaufman, R. H. Lowe-McConnell, O. Seehausen, J.
H. Wanink, R. L. Welcomme, and F. Witte. 2003. Biodiversity
and fishery sustainability in the Lake Victoria basin: an
unexpected marriage? BioScience 53:703–715. http://dx.doi.
org/10.1641/0006-3568(2003)053[0703:BAFSIT]2.0.CO;2
Beuving, J. J. 2010. Playing pool along the shores of Lake Victoria.
Fishermen, careers and capital accumulation in the Ugandan Nile
perch business. Journal of the International African Institute
(Africa) 80:1–27.
Beuving, J. 2013. Chequered fortunes in global exports: the
sociogenesis of African entrepreneurship in the Nile perch
business at Lake Victoria, Uganda. European Journal of
Development Research 25:501–517. http://dx.doi.org/10.1057/
ejdr.2013.28
Beuving, J. 2014. Spatial diversity in small-scale fishing: a sociocultural interpretation of the Nile perch sector on Lake Victoria,
Uganda. Tijdschrift voor economische en sociale geografie. http://
dx.doi.org/10.1111/tesg.12081
Beverton, R. J. H. 1959. A report on the state of Lake Victoria
fisheries. Mimeo, Fisheries Laboratory, Lowestoft.
Chapman, L. J., C. A. Chapman, L. Kaufman, F. Witte, and J.
Balirwa. 2008. Biodiversity conservation in African inland waters:
lessons of the Lake Victoria region. Verhandlungen des
Internationalen Verein Limnologie 30:16–34.
Responses to this article can be read online at:
http://www.ecologyandsociety.org/issues/responses.
php/6965
Cornelissen, I. J. M., G. M. Silsbe, J. A. J. Verreth, E. van Donk,
and L. A. J. Nagelkerke. 2013. Dynamics and limitations of
phytoplankton biomass along a gradient in Mwanza Gulf,
southern Lake Victoria (Tanzania). Freshwater Biology 59:127–
141. http://dx.doi.org/10.1111/fwb.12253
Acknowledgments:
Cózar, A., N. Bergamino, S. Mazzuoli, N. Azza, L. Bracchini, A.
M. Dattilo, and S. A. Loiselle. 2007. Relationships between
wetland ecotones and inshore water quality in the Ugandan coast
of Lake Victoria. Wetlands Ecology and Management 15:499–507.
http://dx.doi.org/10.1007/s11273-007-9046-6
We would like to thank Koos Vijverberg for his very constructive
comments on the manuscript. Many thanks to Tijs Goldschmidt and
the Artis-bibliotheek for hosting interesting debates on Lake
Victoria. This work is part of the integrated project “Exploitation
or eutrophication as threats for fisheries? Disentangling social and
ecological drivers of ecosystem changes in Lake Victoria
(SEDEC),” supported by the Netherlands Organisation for
Scientific Research (NWO/WOTRO) grant number W01.65.304.00.
Cózar, A., M. Bruno, N. Bergamino, B. Úbeda, L. Bracchini, A.
M. Dattilo, and S. A. Loiselle. 2012. Basin-scale control on the
phytoplankton biomass in Lake Victoria, Africa. PloS ONE 7:
e29962. http://dx.doi.org/10.1371/journal.pone.0029962
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Dobiesz, N. E., R. E. Hecky, T. B. Johnson, J. Sarvala, J. M.
Dettmers, M. Lehtiniemi, L. G. Rudstam, C. P. Madenjian, and
F. Witte. 2010. Metrics of ecosystem status for large aquatic
systems—a global comparison. Journal of Great Lakes Research
36:123–138. http://dx.doi.org/10.1016/j.jglr.2009.11.003
Downing, A. S. 2012. Seeing the water for the fish: building on
perspectives of Lake Victoria. Dissertation, Wageningen
University, The Netherlands.
Downing, A. S., N. Galic, K. P. C. Goudswaard, E. H. van Nes,
M. Scheffer, F. Witte, and W. M. Mooij. 2013a. Was Lates late?
A null model for the Nile perch boom in Lake Victoria. PloS ONE
8:e76847. http://dx.doi.org/10.1371/journal.pone.0076847
Downing, A. S., E. H. van Nes, J. H. Janse, F. Witte, I. J. M.
Cornelissen, M. Scheffer, and W. M. Mooij. 2013b. Assembling
the pieces of Lake Victoria’s many food webs: Reply to Kolding.
Ecological Applications 23:671–675. http://dx.doi.org/10.1890/12-1418.1
Downing, A. S., E. H. van Nes, J. H. Janse, F. Witte, I. J. M.
Cornelissen, M. Scheffer, and W. M. Mooij. 2012a. Collapse and
reorganization of a food web of Mwanza Gulf, Lake Victoria.
Ecological Applications 22:229–239. http://dx.doi.org/10.1890/11-0941.1
Downing, A. S., E. H. van Nes, W. M. Mooij, and M. Scheffer.
2012b. The resilience and resistance of an ecosystem to a collapse
of diversity. PloS ONE 7:e46135. http://dx.doi.org/10.1371/
journal.pone.0046135
Downing, A. S., E. H. van Nes, K. E. van de Wolfshaar, M.
Scheffer, and W. M. Mooij. 2013c. Effects of resources and
mortality on the growth and reproduction of Nile perch in Lake
Victoria. Freshwater Biology 58:828–840. http://dx.doi.
org/10.1111/fwb.12089
Eggert, H., and R. B. Lokina. 2009. Regulatory compliance in
Lake Victoria fisheries. Environment and Development Economics
15:197.
Garcia, S. M., J. Kolding, J. Rice, M.-J. Rochet, S. Zhou, T.
Arimoto, J. E. Beyer, L. Borges, A. Bundy, D. Dunn, E. A. Fulton,
M. Hall, M. Heino, R. Law, M. Makino, A. D. Rijnsdorp, F.
Simard, and A. D. M. Smith. 2012. Reconsidering the
consequences of selective fisheries. Science 335:1045–1047. http://
dx.doi.org/10.1126/science.1214594
Geheb, K., S. Kalloch, M. Medard, A.-T. Nyapendi, C. Lwenya,
and M. Kyangwa. 2008. Nile perch and the hungry of Lake
Victoria: gender, status and food in an East African fishery. Food
Policy 33:85–98. http://dx.doi.org/10.1016/j.foodpol.2007.06.001
Geheb, K., M. Medard, M. Kyangwa, and C. Lwenya. 2007. The
future of change: roles, dynamics and functions for fishing
communities in the management of Lake Victoria’s fisheries.
Aquatic Ecosystem Health & Management 10:467–480. http://dx.
doi.org/10.1080/14634980701704098
Gibbon, P. 1997. “The poor relation. A political economy of the
marketing chain for dagaa in Tanzania.” Pages 1–67 in CDR
Working Paper 97.2, Center for Development Research,
Copenhagen, Denmark.
Gikuma-Njuru, P. 2008. Physical and biogeochemical gradients
and exchange processes in Nyanza Gulf and main Lake Victoria
(East Africa). Dissertation, University of Waterloo, Waterloo,
Ontario, Canada.
Gophen, M., P. B. O. Ochumba, and L. S. Kaufman. 1995. Some
aspects of perturbation in the structure and biodiversity of the
ecosystem of Lake Victoria (East Africa). Aquatic Living
Resources 8:27–41. http://dx.doi.org/10.1051/alr:1995003
Goudswaard, K. P. C., F. Witte, and E. F. B. Katunzi. 2008. The
invasion of an introduced predator, Nile perch (Lates niloticus,
L.) in Lake Victoria (East Africa): chronology and causes.
Environmental Biology of Fishes 81:127–139. http://dx.doi.
org/10.1007/s10641-006-9180-7
Goudswaard, K. P. C., F. Witte, and J. H. Wanink. 2006. The
shrimp Caridina nilotica in Lake Victoria (East Africa), before
and after the Nile perch increase. Hydrobiologia 563:31–44. http://
dx.doi.org/10.1007/s10750-005-1385-9
Haande, S., T. Rohrlack, R. P. Semyalo, P. Brettum, B. Edvardsen,
A. Lyche-Solheim, K. Sørensen, and P. Larsson. 2011.
Phytoplankton dynamics and cyanobacterial dominance in
Murchison Bay of Lake Victoria (Uganda) in relation to
environmental conditions. Limnologica 41:20–29. http://dx.doi.
org/10.1016/j.limno.2010.04.001
Hecky, R. E., H. A. Bootsma, and E. O. Odada. 2006. African
lake management initiatives: the global connection. Lakes &
Reservoirs: Research & Management 11:203–213. http://dx.doi.
org/10.1111/j.1440-1770.2006.00307.x
Hecky, R. E., F. W. B. Bugenyi, P. Ochumba, J. F. Talling, R.
Mugidde, M. Gophen, and L. Kaufman. 1994. Deoxygenation of
the deep water of Lake Victoria, East Africa. Limnology and
Oceanography 39:1476–1481. http://dx.doi.org/10.4319/lo.1994.39.6.1476
Hecky, R. E., R. Mugidde, P. S. Ramlal, M. R. Talbot, and G. W.
Kling. 2010. Multiple stressors cause rapid ecosystem change in
Lake Victoria. Freshwater Biology 55:19–42. http://dx.doi.
org/10.1111/j.1365-2427.2009.02374.x
Johnson, J. L. 2010. From Mfangano to Madrid: the global
commodity chain for Kenyan Nile perch. Aquatic Ecosystem
Health & Management 13:20–27. http://dx.doi.org/10.1080/1463
4980903584694
Justus, J. 2005. Qualitative scientific modeling and loop analysis.
Philosophy of Science 72:1272–1286. http://dx.doi.org/10.1086/508099
Kambewa, E. V. 2007. Balancing the people, profit and planet
dimensions in international marketing channels. A study on
coordinating mechanisms in the Nile perch channel from Lake
Victoria. Wageningen University, The Netherlands.
Kateka, A. G. 2010. Co-management challenges in the Lake
Victoria fisheries. Stockholm University, Stockholm, Sweden.
Kateregga, E., and T. Sterner. 2008. Lake Victoria fish stocks and
the effects of water hyacinths on the catchability of fish. Pages 1–
28 in Environment for Development Discussion Paper Series.
Katunzi, E. F. B., W. L. T. Van Densen, J. H. Wanink, and F. Witte.
2006. Spatial and seasonal patterns in the feeding habits of
juvenile Lates niloticus (L.), in the Mwanza Gulf of Lake Victoria.
Hydrobiologia 568:121–133. http://dx.doi.org/10.1007/s10750-006-0033-3
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Katunzi, E. F. B., J. Zoutendijk, T. Goldschmidt, J. H. Wanink,
and F. Witte. 2003. Lost zooplanktivorous cichlid from Lake
Victoria reappears with a new trade. Ecology of Freshwater Fish
12:237–240. http://dx.doi.org/10.1046/j.1600-0633.2003.00023.x
Kayanda, R., A. M. Taabu, R. Tumwebaze, L. Muhoozi, T.
Jembe, E. Mlaponi, and P. Nzungi. 2009. Status of the major
commercial fish stocks and proposed species-specific
management plans for Lake Victoria. African Journal of Tropical
Hydrobiology and Fisheries 21:15–21.
Kishe-Machumu, M. A. 2012. Inter-guild differences and possible
causes of the recovery of cichlid species in Lake Victoria, Tanzania.
Dissertation, Leiden University, Leiden, The Netherlands.
Kishe-Machumu, M., F. Witte, and J. H. Wanink. 2008. Dietary
shift in benthivorous cichlids after the ecological changes in Lake
Victoria. Animal Biology 58:401–417. http://dx.doi.
org/10.1163/157075608X383700
Kishe-Machumu, M. A., F. Witte, J. H. Wanink, and E. F. B.
Katunzi. 2012. The diet of Nile perch, Lates niloticus (L.) after
resurgence of haplochromine cichlids in the Mwanza Gulf of
Lake Victoria. Hydrobiologia 682:111–119. http://dx.doi.
org/10.1007/s10750-011-0822-1
Kitchell, J. F., D. E. Schindler, R. Ogutu-Ohwayo, and P. N.
Reinthal. 1997. The Nile perch in Lake Victoria: interactions
between predation and fisheries. Ecological Applications 7:653–
664. http://dx.doi.org/10.1890/1051-0761(1997)007[0653:TNPILV]
2.0.CO;2
Kolding, J. 1993. Population dynamics and life-history styles of
Nile tilapia, Oreochromis niloticus, in Ferguson’s Gulf, Lake
Turkana, Kenya. Environmental Biology of Fishes 37:25–46.
http://dx.doi.org/10.1007/BF00000710
Kolding, J., M. Medard, O. Mkumbo, and P. A. M. van Zwieten.
2014. Status, trends and management of the Lake Victoria
fisheries. In R. L. Welcomme, J. Valbo-Jørgensen, and A. S. Halls,
editors. Inland fisheries evolution and management—case studies
from four continents. FAO Fisheries and Aquaculture Technical
Paper 579. In press.
Kolding, J., P. van Zwieten, O. Mkumbo, G. Silsbe, and R. Hecky.
2008. Are the Lake Victoria fisheries threatened by exploitation
or eutrophication? Towards an ecosystem-based approach to
management. Pages 309–354 in G. Bianchi and H. R. Skjoldal,
editors. The ecosystem approach to fisheries. http://dx.doi.
org/10.1079/9781845934149.0309
Kudhongania, A. W., and D. B. R. Chitamwebwa. 1995. Impact
of environmental change, species introductions and ecological
interactions on the fish stocks of Lake Victoria. Pages 19–32 in
T. J. Pitcher and P. J. B. Hart, editors. The impact of species changes
in African Lakes. Chapman & Hall, London, UK. http://dx.doi.
org/10.1007/978-94-011-0563-7_2
Lehman, J. T., and D. K. Branstrator. 1993. Effects of nutrients
and grazing on the phytoplankton of Lake Victoria.
Verhandlungen der internationale Vereinigung für Limnologie
25:850–855.
Levins, R. 1974. The qualitative analysis of partially specified
systems. Annals of the New York Academy of Sciences 231:123–
138. http://dx.doi.org/10.1111/j.1749-6632.1974.tb20562.x
Ligtvoet, W., and O. C. Mkumbo. 1990. Synopsis of ecological
and fishery research on Nile perch (Lates niloticus) in Lake
Victoria, conducted by HEST/TAFIRI in Report of the fifth
session of the sub-committee for the development and
management of the fisheries of Lake Victoria. Pages 35–74 FAO
Fisheries Report 430. Rome, Italy.
Loiselle, S. A., N. Azza, A. Cózar, L. Bracchini, A. Tognazzi, A.
Dattilo, and C. Rossi. 2008. Variability in factors causing light
attenuation in Lake Victoria. Freshwater Biology 53:535–545.
http://dx.doi.org/10.1111/j.1365-2427.2007.01918.x
Lung’Ayia, H. B. O., A. M’Harzi, M. Tackx, J. Gichuki, and J. J.
Symoens. 2000. Phytoplankton community structure and
environment in the Kenyan waters of Lake Victoria. Freshwater
Biology 43:529–543. http://dx.doi.org/10.1046/j.1365-2427.2000.
t01-1-00525.x
Manyala, J. O., and J. E. Ojuok. 2007. Survival of the Lake
Victoria Rastrineobola argentea in a rapidly changing
environment: biotic and abiotic interactions. Aquatic Ecosystem
Health & Management 10:407–415. http://dx.doi.
org/10.1080/14634980701704155
Masifwa, W. F., T. Twongo, and P. Denny. 2001. The impact of
water hyacinth, Eichhornia crassipes (Mart) Solms on the
abundance and diversity of aquatic macroinvertebrates along the
shores of northern Lake Victoria, Uganda. Hydrobiologia
452:79–88. http://dx.doi.org/10.1023/A:1011923926911
Mbuga, J. S., A. Getabu, A. Asila, M. Medard, and R. Abila.
1998. Trawling in Lake Victoria: its history, status and effects.
Socio-economics of the Lake Victoria Fisheries Project Report No.
3. IUCN, Nairobi, Kenya.
Medard, M. 2012. Relations between people, relations about
things: gendered investment and the case of the Lake Victoria
fishery, Tanzania. Signs 37:555–566. http://dx.doi.org/10.1086/662704
Mkumbo, O. C. 2002. Assessment and management of Nile perch
(Lates niloticus L.) stocks in the Tanzanian waters of Lake
Victoria. Dissertation, University of Hull, Hull, UK.
Mkumbo, O. C., and E. Mlaponi. 2007. Impact of the baited hook
fishery on the recovering endemic fish species in Lake Victoria.
Aquatic Ecosystem Health & Management 10:458–466. http://dx.
doi.org/10.1080/14634980701704197
Mkumbo, O. C., P. Nsinda, C. N. Ezekiel, I. G. Cowx, and M.
Aeron. 2007. Towards sustainable exploitation of Nile perch
consequential to regulated fisheries in Lake Victoria. Aquatic
Ecosystem Health & Management 10:449–457. http://dx.doi.
org/10.1080/14634980701708057
Mugidde, R. 2001. Nutrient status and planktonic nitrogen fixation
in Lake Victoria. Dissertation, University of Waterloo, Waterloo,
Ontario, Canada.
Muhoozi, L. I. 2002. Exploitation and management of the artisanal
fisheries in the Ugandan waters of Lake Victoria. Dissertation,
University of Hull, Hull, UK.
Mwebaza-Ndawula, L. 1994. Changes in relative abundance of
zooplankton in northern Lake Victoria, East Africa.
Hydrobiologia 272:259–264. http://dx.doi.org/10.1007/BF00006526
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Ngupula, G. W., A. S. E. Mbonde, and C. N. Ezekiel. 2011. Spatial
and temporal patterns of phytoplankton abundance and
composition in three ecological zones in the Tanzanian waters of
Lake Victoria. African Journal of Aquatic Science 36:197–206.
http://dx.doi.org/10.2989/16085914.2011.589118
Ngupula, G. W., and E. Mlaponi. 2010. Changes in abundance
of Nile shrimp, Caridina nilotica (Roux) following the decline of
Nile perch and recovery of native haplochromine fishes, Lake
Victoria, Tanzanian waters. Aquatic Ecosystem Health &
Management 13:196–202. http://dx.doi.org/10.1080/14634988.2
010.483188
Njiru, M., P. Nzungi, A. Getabu, E. Wakwabi, A. Othina, T.
Jembe, and S. Wekesa. 2007. Are fisheries management, measures
in Lake Victoria successful? The case of Nile perch and Nile tilapia
fishery. African Journal of Ecology 45:315–323. http://dx.doi.
org/10.1111/j.1365-2028.2006.00712.x
Njiru, M., J. Ojuok, a. Getabu, T. Jembe, M. Owili, and C. Ngugi.
2008. Increasing dominance of Nile tilapia, Oreochromis niloticus
(L) in Lake Victoria, Kenya: consequences for the Nile perch
Lates niloticus (L) fishery. Aquatic Ecosystem Health &
Management 11:42–49. http://dx.doi.org/10.1080/14634980701878090
Otieno, E. 2013. Kenya and Uganda clash over Migingo. Daily
Nation, Monday, July 1. 2013. http://www.nation.co.ke/news/
Kenya+and+Uganda++clash+over+Migingo/-/1056/1901406/-/cljpxw/-/
index.html
Njiru, M., A. N. Othina, A. Getabu, D. Tweddle, and I. G. Cowx.
2002. Is the invasion of water hyacinth, Eichhornia crassipes Solms
( Mart .), a blessing to Lake Victoria fisheries? Page 396 in I. G.
Cowx, editor. Management and Ecology of Lake and Reservoir
Fisheries. Blackwell Science, Oxford, UK.
North, R. L., S. J. Guildford, R. E. H. Smith, M. R. Twiss, and
H. J. Kling. 2008. Nitrogen, phosphorus, and iron colimitation of
phytoplankton communities in the nearshore and offshore
regions of the African Great Lakes. Verhandlungen der
internationale Vereinigung für Limnologie 30:259–264.
Ochumba, P. B. O. 1990. Massive fish kills within the Nyanza Gulf
of Lake Victoria, Kenya. Hydrobiologia 208:93–99. http://dx.doi.
org/10.1007/BF00008448
Odada, E. O., W. O. Ochola, and D. O. Olago. 2009. Drivers of
ecosystem change and their impacts on human well-being in Lake
Victoria basin. African Journal of Ecology 47:46–54. http://dx.
doi.org/10.1111/j.1365-2028.2008.01049.x
Okello, W., C. Portmann, M. Erhard, K. Gademann, and R.
Kurmayer. 2009. Occurrence of microcystin-producing
cyanobacteria in Ugandan freshwater habitats. Environmental
Toxicology 25:367–380. http://dx.doi.org/10.1002/tox.20522
Opande, G. O., J. C. Onyango, and S. O. Wagai. 2004. Lake
Victoria: the water hyacinth (Eichhornia crassipes [Mart.] Solms):
its socio-economic effects, control measures and resurgence in the
Winam gulf. Limnologica 34:105–109. http://dx.doi.org/10.1016/
S0075-9511(04)80028-8
Ostrom, E. 2009. A general framework for analyzing
sustainability of social-ecological systems. Science 325:419–422.
http://dx.doi.org/10.1126/science.1172133
Ponte, S. 2007. Bans, tests, and alchemy: food safety regulation
and the Uganda fish export industry. Agriculture and Human
Values 24:179–193. http://dx.doi.org/10.1007/s10460-006-9046-9
Poste, A. E., R. E. Hecky, and S. J. Guildford. 2011. Evaluating
microcystin exposure risk through fish consumption.
Environmental Science & Technology 45:5806–5811. http://dx.doi.
org/10.1021/es200285c
Pringle, R. M. 2005a. The Nile perch in Lake Victoria: local
responses and adaptations. Africa: Journal of the International
African Institutes 75:510–538. http://dx.doi.org/10.3366/afr.2005.75.4.510
Pringle, R. M. 2005b. The origins of the Nile perch in Lake
Victoria. BioScience 55:780–787. http://dx.doi.org/10.1641/0006-3568
(2005)055[0780:TOOTNP]2.0.CO;2
Rutjes, H. A., M. C. Nieveen, R. E. Weber, F. Witte, and G. E. E.
J. M. Van den Thillart. 2007. Multiple strategies of Lake Victoria
cichlids to cope with lifelong hypoxia include hemoglobin
switching. American Journal of Physiology – Regulatory,
Integrative and Comparative Physiology 293:R1376-1383. http://
dx.doi.org/10.1152/ajpregu.00536.2006
Schindler, D. W. 2006. Recent advances in the understanding and
management of eutrophication. Limnology and Oceanography
51:356–363. http://dx.doi.org/10.4319/lo.2006.51.1_part_2.0356
Schindler, D. E., J. F. Kitchell, and R. Ogutu-Ohwayo. 1998.
Ecological consequences of alternative gill net fisheries for Nile
perch in Lake Victoria. Conservation Biology 12:56–64.
Seehausen, O. 2009. Speciation affects ecosystems. Nature
458:1122–1123. http://dx.doi.org/10.1038/4581122a
Seehausen, O., J. J. M. van Alphen, and F. Witte. 1997a. Cichlid
fish diversity threatened by eutrophication that curbs sexual
selection. Science 277:1808–1811. http://dx.doi.org/10.1126/
science.277.5333.1808
Seehausen, O., F. Witte, E. F. Katunzi, J. Smits, and N. Bouton.
1997b. Patterns of the remnant cichlid fauna in southern Lake
Victoria. Conservation Biology 11:890–904. http://dx.doi.
org/10.1046/j.1523-1739.1997.95346.x
Sharpe, D. M. T., S. B. Wandera, and L. J. Chapman. 2012. Life
history change in response to fishing and an introduced predator
in the East African cyprinid Rastrineobola argentea. Evolutionary
Applications 5:677–693. http://dx.doi.org/10.1111/j.1752-4571.2012.00245.
x
Shayo, S. D., C. Lugomela, and J. F. Machiwa. 2011. Influence of
land use patterns on some limnological characteristics in the
south-eastern part of Lake Victoria, Tanzania. Aquatic
Ecosystem Health & Management 14:246–251. http://dx.doi.
org/10.1080/14634988.2011.599607
Silsbe, G. M., R. E. Hecky, S. J. Guildford, and R. Mugidde. 2006.
Variability of chlorophyll a and photosynthetic parameters in a
nutrient-saturated tropical great lake. Limnology and
Oceanography 51:2052–2063. http://dx.doi.org/10.4319/lo.2006.51.5.2052
Sitoki, L., J. Gichuki, C. Ezekiel, F. Wanda, O. C. Mkumbo, and
B. E. Marshall. 2010. The environment of Lake Victoria (East
Africa): current status and historical changes. International
Review of Hydrobiology 95:209–223. http://dx.doi.org/10.1002/
iroh.201011226
Ecology and Society 19(4): 31
http://www.ecologyandsociety.org/vol19/iss4/art31/
Tamatamah, R. A., R. E. Hecky, and H. C. Duthie. 2005. The
atmospheric deposition of phosphorus in Lake Victoria (East
Africa). Biogeochemistry 73:325–344. http://dx.doi.org/10.1007/
s10533-004-0196-9
Tumwebaze, R. 1997. Application of hydroacoustics in fish stock
assessment of Lake Victoria. Thesis, University of Bergen, Bergen,
Norway. http://dx.doi.org/10.1080/14634980701709527
Tumwebaze, R., I. Cowx, S. Ridgway, A. Getabu, and D. N.
MacLennan. 2007. Spatial and temporal changes in the
distribution of Rastrineobola argentea in Lake Victoria. Aquatic
Ecosystem Health & Management 10:398–406.
van der Knaap, M., and W. Ligtvoet. 2010. Is Western
consumption of Nile perch from Lake Victoria sustainable?
Aquatic Ecosystem Health & Management 13:429–436. http://dx.
doi.org/10.1080/14634988.2010.526088
van der Knaap, M., M. J. Ntiba, and I. G. Cowx. 2002. Key
elements of fisheries management on Lake Victoria. Aquatic
Ecosystem Health & Management 5:245–254. http://dx.doi.
org/10.1080/14634980290031947
Van Rijssel, J. C., and F. Witte. 2013. Adaptive responses in
resurgent Lake Victoria cichlids over the past 30 years.
Evolutionary Ecology 27:253–267. http://dx.doi.org/10.1007/
s10682-012-9596-9
Rastrineobola argentea (Cyprinidae). Aquatic Living Resources
15:37–43. http://dx.doi.org/10.1016/S0990-7440(01)01145-7
Wanink, J. H., and F. Witte. 2000. Rapid morphological changes
following niche shift in the zooplanktivorous cyprinid
Rastrineobola argentea from Lake Victoria. Netherlands Journal
of Zoology 50:365–372.
Wilkinson, R. G., and K. Pickett. 2009. The spirit level: why more
equal societies almost always do better. Allen Lane, London, UK.
Williams, A. E., H. C. Duthie, and R. E. Hecky. 2005. Water
hyacinth in Lake Victoria: Why did it vanish so quickly and will
it return? Aquatic Botany 81:300–314. http://dx.doi.org/10.1016/
j.aquabot.2005.01.003
Williams, A. E., R. E. Hecky, and H. C. Duthie. 2007. Water
hyacinth decline across Lake Victoria—Was it caused by climatic
perturbation or biological control? A reply. Aquatic Botany 87:94–
96. http://dx.doi.org/10.1016/j.aquabot.2007.03.009
Wilson, J. R. U., O. Ajuonu, T. D. Center, M. P. Hill, M. H. Julien,
F. F. Katagira, P. Neuenschwander, S. W. Njoka, J. Ogwang, R.
H. Reeder, and T. Van. 2007. The decline of water hyacinth on
Lake Victoria was due to biological control by Neochetina spp.
Aquatic Botany 87:90–93. http://dx.doi.org/10.1016/j.aquabot.2006.06.006
Veraart, A. J., J. J. M. de Klein, and M. Scheffer. 2011. Warming
can boost denitrification disproportionately due to altered oxygen
dynamics. PloS ONE 6:e18508. http://dx.doi.org/10.1371/
journal.pone.0018508
Witte, F., T. Goldschmidt, J. Wanink, M. van Oijen, K.
Goudswaard, E. Witte-Maas, and N. Bouton. 1992. The
destruction of an endemic species flock: quantitative data on the
decline of the haplochromine cichlids of Lake Victoria.
Environmental Biology of Fishes 34:1–28. http://dx.doi.
org/10.1007/BF00004782
Verschuren, D., T. C. Johnson, H. J. Kling, D. N. Edgington, P.
R. Leavitt, E. T. Brown, M. R. Talbot, and R. E. Hecky. 2002.
History and timing of human impact on Lake Victoria, East
Africa. Proceedings of the Royal Society of London B Biological
Sciences 269:289–294. http://dx.doi.org/10.1098/rspb.2001.1850
Witte, F., B. S. Msuku, J. H. Wanink, O. Seehausen, E. F. B.
Katunzi, P. C. Goudswaard, and T. Goldschmidt. 2000. Recovery
of cichlid species in Lake Victoria: an examination of factors
leading to differential extinction. Reviews in Fish Biology and
Fisheries 10:233–241. http://dx.doi.org/10.1023/A:1016677515930
Vonlanthen, P., D. Bittner, A. G. Hudson, K. A. Young, R. Müller,
B. Lundsgaard-Hansen, D. Roy, S. Di Piazza, C. R. Largiader,
and O. Seehausen. 2012. Eutrophication causes speciation
reversal in whitefish adaptive radiations. Nature 482:357–362.
http://dx.doi.org/10.1038/nature10824
Witte, F., O. Seehausen, J. H. Wanink, M. A. Kishe-Machumu,
M. Rensing, and T. Goldschmidt. 2013. Cichlid species diversity
in naturally and anthropogenically turbid habitats of Lake
Victoria, East Africa. Aquatic Sciences 75:169–183. http://dx.doi.
org/10.1007/s00027-012-0265-4
Walters, C., V. Christensen, and D. Pauly. 1997. Structuring
dynamic models of exploited ecosystems from trophic massbalance assessments. Reviews in Fish Biology and Fisheries 7:139–
172. http://dx.doi.org/10.1023/A:1018479526149
Witte, F., J. H. Wanink, and M. Kishe-Machumu. 2007a. Species
distinction and the biodiversity crisis in Lake Victoria.
Transactions of the American Fisheries Society 136:1146–1159.
http://dx.doi.org/10.1577/T05-179.1
Walters, C., and J. F. Kitchell. 2001. Cultivation/depensation
effects on juvenile survival and recruitment: implications for the
theory of fishing. Canadian Journal of Fisheries and Aquatic
Sciences 58:39–50. http://dx.doi.org/10.1139/f00-160
Witte, F., J. H. Wanink, M. Kishe-Machumu, O. C. Mkumbo, P.
C. Goudswaard, and O. Seehausen. 2007b. Differential decline
and recovery of haplochromine trophic groups in the Mwanza
Gulf of Lake Victoria. Aquatic Ecosystem Health & Management
10:416–433. http://dx.doi.org/10.1080/14634980701709410
Wanink, J. H. 1999. Prospects for the fishery on the small pelagic
Rastrineobola argentea in Lake Victoria. Hydrobiologia 407:183–
189. http://dx.doi.org/10.1023/A:1003708624899
Wanink, J. H., J. J. Kashindye, K. P. C. Goudswaard, and F. Witte.
2001. Dwelling at the oxycline: Does increased stratification
provide a predation refugium for the Lake Victoria sardine
Rastrineobola argentea? Freshwater Biology 46:75–85.
Wanink, J. H., E. F. B. Katunzi, K. P. C. Goudswaard, F. Witte,
and W. L. T. van Densen. 2002. The shift to smaller zooplankton
in Lake Victoria cannot be attributed to the ‘sardine’
Witte, F., M. Welten, M. Heemskerk, I. Van Der Stap, L. Ham,
H. Rutjes, and J. Wanink. 2008. Major morphological changes in
a Lake Victoria cichlid fish within two decades. Biological Journal
of the Linnean Society 94:41–52. http://dx.doi.org/10.1111/
j.1095-8312.2008.00971.x
Yasindi, A. W., and W. D. Taylor. 2003. Abundance, biomass and
estimated production of planktonic ciliates in Lakes Victoria and
Malawi. Aquatic Ecosystem Health & Management 6:289–297.
http://dx.doi.org/10.1080/14634980301496