Hydrobiologia
DOI 10.1007/s10750-011-0925-8
LOCH LEVEN RESEARCH
Changes in the fish community of Loch Leven: untangling
anthropogenic pressures
I. J. Winfield • C. E. Adams • J. D. Armstrong • R. Gardiner
A. Kirika • J. Montgomery • B. M. Spears • D. C. Stewart •
J. E. Thorpe • W. Wilson
•
Ó Springer Science+Business Media B.V. 2011
Abstract Loch Leven, U.K., contains brown trout
(Salmo trutta), eel (Anguilla anguilla), minnow (Phoxinus phoxinus), perch (Perca fluviatilis), pike (Esox
lucius) and three-spined stickleback (Gasterosteus
aculeatus), with brook lamprey (Lampetra planeri)
and stone loach (Barbatula barbatula) also present in
its tributaries. Arctic charr (Salvelinus alpinus), Atlantic salmon (Salmo salar) and flounder (Platichthys
flesus) are now extinct. The brown trout population has
supported a world-renowned recreational fishery for
Guest editors: L. May & B. M. Spears / Loch Leven: 40 years
of scientific research
I. J. Winfield (&)
Centre for Ecology & Hydrology, Lancaster Environment
Centre, Library Avenue, Bailrigg, Lancaster LA1 4AP,
UK
e-mail: ijw@ceh.ac.uk
C. E. Adams
Scottish Centre for Ecology and the Natural Environment,
University of Glasgow, Glasgow G12 8QQ, UK
J. D. Armstrong R. Gardiner D. C. Stewart
J. E. Thorpe
Freshwater Laboratory, Marine Scotland, Faskally,
Pitlochry PH16 5LB, UK
over a century, although a decline in fishery performance led to extensive stocking between 1983 and
2006, including with non-native rainbow trout (Oncorhynchus mykiss). This review combines historical
information with contemporary gill-net and hydroacoustic surveys. In 2008, brown trout, perch and threespined sticklebacks were abundant, but pike and stone
loach were rare. The obstruction of migratory routes
was probably responsible for the loss of Atlantic
salmon and flounder, while a lowering of water level
likely caused the extinction of Arctic charr and
contributed to a reduction in pike abundance. Perch
abundance has fluctuated markedly, being influenced
by disease and eutrophication, although a reduction in
nutrients and associated recovery of macrophytes are
likely to have benefitted this species. Although the
brown trout population has undoubtedly shown a longterm decline, individuals are currently in excellent
condition.
Keywords Population Eutrophication Lake level
Fishery Brown trout (Salmo trutta) Rainbow trout
(Oncorhynchus mykiss) Perch (Perca fluviatilis)
Cormorant (Phalacrocorax carbo)
A. Kirika B. M. Spears
Centre for Ecology & Hydrology, Bush Estate, Penicuik,
Midlothian EH26 0QB, UK
Introduction
J. Montgomery W. Wilson
Kinross Estate Office, Kinross KY13 8AS, UK
Along with phytoplankton, which may periodically
appear to colour an entire lake vivid green, and birds,
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Hydrobiologia
which at certain times of year teem on its water
surface, terrestrial margins and in the skies above it, a
lake’s fish populations commonly enjoy the highest
public profiles of all of its flora and fauna. However, in
contrast to phytoplankton, which are easy to sample,
and birds, which are easy to count, the assessment and
monitoring of lake fish communities is technically
difficult and presents a number of continuing methodological challenges (Kubečka et al., 2009). When
the complexities and uncertainties of fisheries socioeconomics within a multi-use landscape (e.g. Arlinghaus et al., 2002) are added to this situation, objective
and confident understanding and management of lake
fish communities become extremely difficult to
achieve.
The history and contemporary state of the fish
community of Loch Leven in east-central Scotland,
U.K., is a good example of this complexity, even
though a total of only 12 resident and migratory fish
species has been recorded in this large, shallow and
eutrophic lake (see below). Amongst these species,
the local population of brown trout (Salmo trutta)
has received by far the greatest attention in terms of
research effort and fisheries exploitation, with commercial netting having been practised here from
1314 to 1873 and recreational fly fishing from 1844
onwards (Thorpe, 1974a; Munro, 1994). The latter,
at its height, supported 52 hire boats (W. W.,
unpublished data). Management activities in support
of angling activity have been documented by
Montgomery (1994), including an early example of
the use of hatchery and rearing ponds in 1882 when
60,000 fry and 4,000 2-year-old brown trout of
unrecorded origin were placed in inflowing streams,
a practice that was expanded from the 1920s to 1936
resulting in up to 300,000 fry being hatched each
year. Subsequently, the hatchery was closed during
the Second World War and only reopened in the
early 1980s using ‘home-bred’ stock of exclusively
local origin (Montgomery, 1994). The Loch Leven
trout fishery persists to the present with 13 boats
available for hire (W. W., unpublished data) and a
national and international reputation such that it was
included as a venue for the 29th FIPS-Mouche
World Fly Fishing Championships in June 2009. In
recent years, however, it has experienced turbulent
fortunes with various anthropogenic pressures (e.g.
Duncan, 1994), the socio-economic vagaries of the
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angling community (e.g. Montgomery, 1994) and
predation by cormorants (Phalacrocorax carbo)
(e.g. Stewart et al., 2005) all potentially involved.
Many of these influential factors have a long local
history (May & Spears, 2011) and so an understanding of the contemporary fish community
requires a correspondingly long-term perspective.
This article reviews historical published and
unpublished information in combination with contemporary gill net and hydroacoustic surveys to describe
long-term changes in the fish community of Loch
Leven. Observed alterations in the fish community are
then interpreted in the context of historical and current
anthropogenic pressures.
Materials and methods
Study site
Loch Leven is a large (surface area approximately
13.3 km2) but relatively shallow (mean depth 3.9 m,
maximum depth 25.5 m) lake located in east-central
Scotland, U.K. (56° 120 N, 3° 220 W; altitude 107 m).
This culturally eutrophicated water body drains a
catchment of 145 km2 and discharges into the Forth
Estuary via the River Leven. Further details of its
bathymetry (Kirby, 1971), water quality history
(Carvalho et al., 2011; May et al., 2011; Spears
et al., 2011), phytoplankton (Bailey-Watts, 1982;
Bailey-Watts et al., 1990), zooplankton (Gunn et al.,
2011), macrophytes (Dudley et al., 2011), macroinvertrbrates (Gunn et al., 2011) and birds (Carss et al.,
2011) are reported elsewhere.
Historical published and unpublished information
Published and unpublished information relevant to an
understanding of the fish community of Loch Leven
was collated by searches of the published and grey
literature, combined with unpublished data held by the
authors who collectively have been directly responsible for, or involved with, almost all fish research and
all fisheries operations on the loch. This resulted in the
identification of over 30 publications and reports
including, or supplemented with, data on fish populations from the 1960s, 1970s, 1980s, 1990s and 2000s
and fisheries data from 1900 to the 2000s.
Hydrobiologia
Contemporary hydroacoustic and gill net surveys
Before this study, the most recent sampling of the fish
populations of Loch Leven had been undertaken using
survey gill nets during March and December 2001
(Stewart et al., 2005). Consequently, a community
sampling programme using a combination of hydroacoustics and survey gill nets as used on a series of other
lochs by Winfield et al. (2009a) was begun in late 2007.
On 10 October 2007, day- and night-time hydroacoustic surveys were performed where water depth
exceeded approximately 5 m in the southern area of
the loch using a BioSonics DT6000 echo sounder
(BioSonics Inc., Seattle, U.S.A.) with a 200 kHz splitbeam vertical transducer following the approach
described by Winfield et al. (2009a). The transducer
was positioned approximately 0.5 m below the surface
of the water and data were recorded starting from a
range of 2 m. Following a standard hydroacoustic
analysis employed extensively elsewhere in the U.K.
by Winfield et al. (in press), the resulting data were
used to produce densities of ‘small’ (i.e. -52 to
-45 dB, length 40–99 mm), ‘medium’ (-44 to
-37 dB, length 100–249 mm) and ‘large’ (greater
than -37 dB, length greater than 250 mm) fish along
each of 18 transects.
Gill netting was undertaken using benthic and
pelagic versions of the Norden survey gill net as
described by Appelberg (2000). The benthic version is
bottom-set and is approximately 1.5 m deep and 30 m
long, with 12 panels of equal length having bar-mesh
sizes 5, 6.25, 8, 10, 12.5, 15.5, 19.5, 24, 29, 35, 43 and
55 mm, respectively. The pelagic version, which is set
floating from the water surface, is approximately
6.0 m deep and 27.5 m long, with 11 panels of equal
length having bar-mesh sizes 6.25, 8, 10, 12.5, 15.5,
19.5, 24, 29, 35, 43 and 55 mm, respectively. A total
of nine such nets was set overnight to sample three
inshore-bottom sites, three offshore-bottom sites and
three offshore-surface sites on 17 March, 20 August
and 6 October 2008. Upon lifting, all fish were
removed from the nets and killed before being
identified, counted and then frozen at -20°C for
future processing in the laboratory. The latter comprised measurements of individual length (fork length,
mm) and weight (g) and the removal of materials for
age and diet determination.
For comparison purposes, corresponding individual
data were assembled from brown trout and perch taken
from a total of seven other lochs sampled using the
same gill-netting methodology in 2007 and 2008 by
Winfield et al. (2008a, b, 2009b), and from Loch
Leven perch sampled in March 2001 (D. C. S.,
unpublished data from gill netting) and August 1968
(J. E. T., unpublished data from trawling).
Results
Historical fish community
A total of 11 native fish species has been recorded in
Loch Leven, i.e. Arctic charr (Salvelinus alpinus),
Atlantic salmon (Salmo salar), brook lamprey (Lampetra planeri), brown trout, eel (Anguilla anguilla),
flounder (Platichthys flesus), minnow (Phoxinus
phoxinus), perch (Perca fluviatilis), pike (Esox lucius),
stone loach (Barbatula barbatula) and three-spined
stickleback (Gasterosteus aculeatus). However, brook
lamprey and stone loach have always been largely
restricted to tributary streams and flounder was
probably only an occasional visitor (Day, 1887).
Three of the above species became locally extinct
during historical times. The Atlantic salmon and
flounder, which both used to migrate along the
outflowing River Leven to and from the Forth Estuary,
were lost during the eighteenth century (Day, 1887),
while Arctic charr have not been recorded since 1837
(Burns-Begg, 1874).
Only one fish species has subsequently been added
to the Loch Leven fish community, i.e. the non-native
rainbow trout (Oncorhynchus mykiss), which was
stocked for fishery purposes from 1993 to 2004.
Brown and rainbow trout fishery
The performance of the Loch Leven brown trout rod
fishery from 1900 to 2002 is given in Fig. 1a; after
2002, catch records collected by the fishery became
less reliable as an indicator of fishery performance as a
result of anglers releasing fish that they could have
retained. Between 1900 and 1970, the annual catch
frequently exceeded 40,000 brown trout with less than
20,000 fish landed in only a very few years. After
1970, however, catches declined markedly and rarely
exceeded 20,000 fish. Furthermore, this level of brown
trout catch was last surpassed in 1990. The post-1970
decline in absolute catch of brown trout was
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Hydrobiologia
100000
(a)
80000
Trout catch (numbers)
Fig. 1 Loch Leven fishery
a absolute catch for brown
trout (closed circles,
continuous line) and
rainbow trout (open circles,
broken line) from 1900 to
2002, b average weight for
brown trout (closed circles,
continuous line) and
rainbow trout (open circles,
broken line) from 1900 to
2002 and c catch-per-uniteffort (CPUE) for brown
trout (closed circles,
continuous line) and
rainbow trout (open circles,
broken line) from 1975 to
2002
60000
40000
20000
0
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
1970
1980
1990
2000
2010
Year
(b)
1000
Trout average weight (g)
800
600
400
200
0
1900
1910
1920
1930
1940
1950
1960
Year
Trout CPUE (number boat -1 hour -1)
(c)
0.6
0.4
0.2
0
1975
1980
1985
1990
Year
123
1995
2000
2005
Hydrobiologia
accompanied by a marked increase in individual
weight (Fig. 1b).
This dramatic decline in fishery performance led to
the reintroduction, in 1983, of brown trout stocking
directly into the loch using 5,000 fish of local origin
from the re-opened hatchery and rearing-ponds (Montgomery, 1994). The number of brown trout stocked
each year steadily increased until 1988, when 166,000
fish were introduced. Stocking in excess of 100,000
brown trout continued in most years until 2004, when
the number was reduced to 5,000 and stocking was
discontinued in 2006. Unlike the earlier period when
the stocked fish consisted mainly of fry introduced to
tributary streams, from 1983 onwards the stocked fish
were released directly into the loch, generally as
yearlings of 120–180 mm in length (Montgomery,
1994). However, from 2004 to 2006, the emphasis was
on introducing older fish of approximately 280 mm in
length (W.W., unpublished data) which were already
large enough to be taken by anglers.
During the 1980s component of the stocking
programme, the performance of the brown trout fishery
in terms of catch-per-unit-effort (CPUE, calculated
from fishery returns from all anglers as the number of
brown trout caught per boat-hour) was relatively stable
but then decreased again in the 1990s (Fig. 1c). This
further deterioration in brown trout fishery performance,
coupled with poor environmental conditions (in terms of
low oxygen availability and low water clarity) and
changing angler preferences, led the fishery to stock
with rainbow trout from 1993 to 2004. During this
period at least 30,000 fish were stocked each year. This
stocking dramatically changed the nature of the Loch
Leven fishery such that the absolute catch (Fig. 1a) and
CPUE of rainbow trout (calculated as for brown trout,
Fig. 1c) almost equalled that of brown trout in the first
year of stocking of the latter species, and then greatly
exceeded it in all subsequent years with stocking and for
which catch data are available.
Perch population
Sampling in 2008 produced a total of 641 perch, of
which 45 individuals were taken in March when they
ranged between 83 and 251 mm in length and 6 and
292 g in weight. The length frequency distribution of
these fish contained only one individual above
180 mm in length (Fig. 2a), which amounted to 2%
of individuals and contrasted with corresponding data
from March 2001 (Fig. 2b) and August 1968 (Fig. 2c)
when 43% of 67 individuals and 48% of 180
individuals, respectively, exceeded this length. This
currently truncated length frequency distribution was
also evident when the 2008 Loch Leven length and
weight data were compared with corresponding data
from perch populations in two other lochs sampled
using identical methodology although, on the basis of
their relatively high weights at lengths, individual fish
were in good condition (Fig. 2d).
Brown trout population
Sampling in 2008 produced a total of 163 brown trout,
of which 25 individuals were taken in March when
they ranged between 135 and 577 mm in length and 27
and 2,317 g in weight. The length frequency distribution of the latter individuals contained relatively high
numbers of large fish (Fig. 3a), with a length-weight
relationship indicative of excellent individual condition and with markedly higher weights-at-lengths for
individuals greater than 400 mm when compared with
similar data from several other lochs (Fig. 3b).
Contemporary fish community
The population density of all fish recorded by hydroacoustics in the deeper (i.e. water depth 5 m and
greater) areas of Loch Leven during October 2007 was
72.7 fish ha-1 (geometric mean with lower and upper
95% confidence limits of 21.8 and 242.5 fish ha-1,
respectively). Note that these results were subject to a
near-surface blind zone of approximately 2.5 m within
which fish could not be detected. A breakdown by fish
length classes is given in Fig. 4a.
Gill-net sampling in 2008 produced a total of 1,139
individuals of five fish species, i.e. brown trout, perch,
pike, three-spined stickleback and stone loach
(Fig. 4b). However, catches were numerically dominated by brown trout (14% by numbers), perch (56%)
and three-spined sticklebacks (29%), with pike (less
than 1%) and stone loach (less than 1%) represented
by only single individuals.
Discussion
The fish community of Loch Leven is relatively simple
in terms of species number, with a cumulative total of
123
(a)
40
30
20
10
320
340
320
340
340
300
300
320
280
280
300
260
260
280
240
240
240
260
220
220
220
200
180
160
140
120
100
80
60
40
20
0
0
Length class (mm)
(b) 40
Percentage
Fig. 2 Length frequency
distributions of perch
sampled from Loch Leven
a 45 perch in March 2008,
b 67 perch in March 2001,
c 180 perch in August 1968
and d length-weight
relationship of 45 perch
sampled from Loch Leven in
March 2008. For
comparison, the latter also
shows corresponding data
from Loch Doon and Loch
Lomond
Percentage
Hydrobiologia
30
20
10
200
180
160
140
120
100
80
60
40
20
0
0
(c)
40
Percentage
Length class (mm)
30
20
10
200
180
160
140
120
100
80
60
40
20
0
0
Length class (mm)
(d)
300
Weight (g)
200
Leven
Doon
Lomond
100
0
0
100
200
300
Length (mm)
only 12 species including the rainbow trout which is
non-native to the U.K. and was introduced for fishery
purposes. However, it has displayed a number of
major changes over the long-term and the extensive
2008 sampling recorded only five species, i.e. brown
123
trout, perch, pike, three-spined stickleback and stone
loach. Furthermore, in numerical terms, the community is now dominated by just three species, i.e. brown
trout, perch and three-spined stickleback, with a total
abundance of 72.7 fish ha-1 as measured by the initial
Hydrobiologia
(a)
10
8
Percentage
6
4
2
600
550
500
450
400
350
300
250
200
150
50
100
0
0
Fig. 3 a Length frequency
distribution and b lengthweight relationship of 25
brown trout sampled from
Loch Leven in March 2008.
For comparison, the latter
also shows corresponding
data from Lochs Builg,
Doon, Eck, Insh, Lomond
and Skeen, and Loch of
Girlsta
Length class (mm)
(b) 2500
2000
Weight (g)
Leven
Builg
1500
Doon
Eck
Girlsta
Insh
1000
Lomond
Skeen
500
0
0
100
200
300
400
500
600
Length (mm)
extensive hydroacoustic survey of October 2007 being
near the middle of the range observed for 17 other
U.K. water bodies by Winfield et al. (in press).
The present absence of brook lamprey and rarity of
stone loach probably reflect the local distribution
patterns of these two riverine species, with their
populations primarily inhabiting the loch’s tributary
streams, and so are not of any significance in terms of
long-term changes. However, the current apparent
absence of Arctic charr, Atlantic salmon, eel (although
this species is poorly sampled by gill nets), flounder,
minnow and rainbow trout and the great scarcity of
pike are worthy of further discussion where data allow,
as are the observed changes in the perch and brown
trout populations. Unfortunately, most previous
assessments of the loch’s fish including Thorpe
(1974a, b), O’Grady et al. (1993) and Alexander
et al. (1999) have not attempted to sample the minnow
and three-spined stickleback populations. Consequently, while our 2008 sampling has shown that
these two species are, respectively, now apparently
absent and relatively abundant, nothing can be
concluded concerning their long-term population
dynamics.
The present Loch Leven fish community composition is also noteworthy in the context of absent species.
First, it appears that most or all of the 370,000 rainbow
trout stocked into the loch between 1993 and 2004
have now either died or been removed and that no
local reproduction has occurred. Although an extremely small number of juvenile and adult rainbow trout
have recently been caught by anglers, these are
thought to have entered the system from one or more
of several newly opened fisheries on the loch’s
tributaries (B. M. S. and W. W., personal observations). Second, a number of other major lakes in the
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Hydrobiologia
Abundance (fish ha -1)
(a) 140
120
100
80
60
40
20
0
Small
Medium
Large
Length class
(b)
60
Percentage
50
40
30
20
10
0
Brown trout
Perch
Pike
Three-spined
stickleback
Stone loach
Species
Fig. 4 a Abundance estimates (geometric means with 95%
confidence limits) by length class for small (length 40–99 mm),
medium (length 100–249 mm) and large (length equal to or
greater than 250 mm) fish recorded at Loch Leven on 10
October 2007, b Composition by numbers of the contemporary
fish community of Loch Leven based on sampling in March,
August and October 2008. Total sample size was 1,139 fish.
Note that single pike and stone loach were sampled, although
they are not visible on the figure at this scale
U.K. have recently experienced unconsented introductions of several fish species, most notably roach
(Rutilus rutilus) and ruffe (Gymnocephalus cernuus),
as a result of anglers fishing for pike using fish as live
baits (Winfield & Durie, 2004). Although there is thus
a potential for such unwanted introductions to Loch
Leven, with potential implications for the native fish
community, the extensive sampling of 2008 did not
record any new arrivals.
The three apparent local extinctions of fish species,
i.e. Arctic charr, Atlantic salmon and flounder, all
occurred before scientific studies of Loch Leven
began, so their causes can only be speculated upon.
However, Burns-Begg (1874) noted that the Arctic
charr was lost only a few years after the marked
lowering of the loch level in 1830, which is strong
circumstantial evidence that this was the major factor
123
behind its extinction. This potential pressure has
parallels with more recent and better-studied effects of
falls in lake levels on Arctic charr and other members
of the fish community of Haweswater, U.K. (Winfield
et al., 1998), and elsewhere (Maitland et al., 2007).
Furthermore, even if the Arctic charr had survived this
pressure, it would then have faced a significant
pressure from subsequent eutrophication which currently stresses other U.K. populations such as those in
Windermere (Winfield et al., 2008c). The local
extinctions of Atlantic salmon and flounder occurred
even earlier in the eighteenth century and may have
been due to the obstruction of migratory routes by
river pollution, although increasing physical difficulty
of passage due to sluice construction associated with
the subsequent lowering of the loch in 1830 and the
construction of weirs further downstream would
probably have formed additional factors.
The above local anthropogenic pressures together
with more widespread factors (Bonhommeau et al.,
2008) are also likely to have contributed to a decline in
the eel population of Loch Leven. The catches of
commercial eel fishings at the outflow from the loch
declined through the nineteenth century and these
operations finally came to an end in 1901 (Munro,
1994). Furthermore, this species was not recorded
during extensive sampling from 1968 to 1973 (J. E. T.,
personal observation). Even allowing for the inefficient sampling of eels by gill nets, it is remarkable that
the survey of O’Grady et al. (1993) and the present
sampling also failed to record any individuals of this
once ubiquitous species. However, recent very infrequent captures by anglers, the sighting of a dead eel in
the harbour at Kinross and very occasional captures of
eels in survey work in the Gairney Burn, a tributary of
Loch Leven, (B. M. S., W. W. and R. G., personal
observations) confirm the presence of at least a few
eels. It is possible that the eels now present may have
accessed the catchment via the River Devon, which
runs independently into the Firth of Forth, and is
linked with the Gairney Burn by the Pow Burn which
drains into both catchments.
The pike population of the loch has also declined
markedly since historic times, with the surveys of
O’Grady et al. (1993) and this study recording 0 and 1
individuals, respectively. Duncan (1994) considered
that a long-term pike removal programme operated by
the fishery was probably responsible for this decline,
although a reduction in macrophytes associated with
Hydrobiologia
eutrophication over the same period (Dudley et al.,
2011), which are used as spawning and nursery habitat
by pike (Craig, 1996), may also have been involved. In
addition, the earlier lowering of water levels may also
have had a negative effect on pike recruitment through
reducing spawning and nursery habitat.
The reduction in macrophyte abundance mentioned
above is also likely to have had a negative effect on the
perch population of Loch Leven because this species
uses their physical complexity for spawning and
nursery habitat and it also influences competitive
interactions involving adult members of the population (Craig, 2000). In addition, Duncan (1994)
considered that a perch disease that he reported as
first observed in Loch Leven in 1983 may also have
contributed to a marked decline, although the first
signs of this disease were actually observed over a
decade earlier in 1971 (J. E. T., personal observation).
Long-standing and marked fluctuations in the perch
population are evident from abundance estimates of
968,000 individuals and 253,000 individuals in 1970
and 1971, respectively (Thorpe, 1974a), by the
scarcity of perch in the 1993 sampling of O’Grady
et al. (1993), and by their limited size distribution in
the present sampling of 2008. Even allowing for the
substantial natural variation in abundance of perch
observed in relatively undisturbed habitats such as that
of the north basin of Windermere (Paxton et al., 2004),
the decline of the perch population in the eutrophicated Loch Leven has been particularly marked. However, some recovery of the perch population might
now be expected as the loch’s level of eutrophication
reduces (Carvalho et al., 2011) and its macrophytes
increase in abundance (Dudley et al., 2011; May &
Carvalho, 2010). As adult perch consume over 6% of
their body weight per day during the summer (Thorpe,
1977) and young perch are probably a major planktivore within the Loch Leven system, such population
changes are likely to have important implications for
the loch’s food web through impacts on their prey
populations.
The brown trout population has undoubtedly
received by far the most research attention of all of
Loch Leven’s fish species, driven in large part by its
fishery importance and thus high public profile.
Although documented and undocumented changes
over time in fishing practices and equipment mean that
trends in fishery performance must be interpreted with
caution, the decline of the Loch Leven trout fishery has
been so marked that it undoubtedly reflects an actual
decline in the local brown trout population. This
problem was widely acknowledged by the early 1990s
and led to much previous research being presented or
reviewed during a dedicated conference on the trout
fishery reported by Hutchinson & Walker (1994).
Consequently, this earlier research will not be revisited here in any detail.
Brown trout spawn in flowing water and the streams
that feed the loch are therefore a vital component of
the habitat for this species. Despite heavy impacts of
agriculture and some localised gravel abstraction on
parts of the catchment, the streams entering Loch
Leven are highly productive of brown trout. Surveys
conducted in the early 1990s (Armstrong et al., 1994;
Armstrong & Gardiner, 1995) assessed the strength
and aspects of the demographics of trout in the
streams. Strong relationships were evident with key
components of the habitat such that the degree of bank
overhangs (which correlated strongly with bankside
vegetation) together with coarse in-stream substratum
explained 76% of the variation in trout densities in
winter. Trout densities in autumn were strongly
influenced by the degree of overhead cover together
with densities of trout in May/June. Trout older than
1? in autumn were scarce in the sampled sites. These
results were consistent with a possible limitation of
spawning on production, through one or a combination
of low numbers of eggs, or insufficient distribution of
spawning substratum. There was no evidence of trout
tending to remain in the streams as older fish rather
than migrating to the loch, as observed also in an
earlier study (Thorpe, 1974a). Despite this potential
limitation of recruitment on production, densities of
trout were high in Loch Leven streams compared with
other populations and at least as high as those
determined by Thorpe (1974a). The presence of the
remnants of yolk sacs and a wide range of lengths in
samples collected in May/June were consistent with
the occurrence of a wide range of spawning times and
may facilitate high levels of production through
heterogeneous advantage (Griffiths & Armstrong,
2001) due to young fish of different sizes using
different local niches and therefore competing little
with one another. Further examination of the streams
would be enhanced by trapping of trout migrating to
and from the loch to measure production of smolts and
migrant fry directly, to assess the numbers of spawners, and to derive indices of stock size.
123
Hydrobiologia
Estimates of the brown trout population within
Loch Leven have been undertaken during three
different periods, including an examination of the
effect of an aeromonad epidemic on the spawning
population (Thorpe & Roberts, 1972). However,
caution is urged with respect to the interpretation of
long-term dynamics because outside of the population
estimates by Thorpe (1974a), which themselves
ranged from 127,000 fish in 1968 to 52,000 fish in
1971, later assessments by O’Grady et al. (1993) and
Alexander et al. (1999) were undertaken using different techniques, for different components of the
population and for different times of the year.
Consequently, their assembly into a single time series
cannot be performed robustly.
These latter data have subsequently been used by
both Wright (2003) and Stewart et al. (2005) to assess
the controversial subject of the impact of cormorants
overwintering at Loch Leven. In common with other
inland locations across the U.K., the number of
cormorants wintering at Loch Leven increased substantially in the late 1980s (Wright, 2003). By
comparing CPUE for gill-net and angling catches
before and after the arrival of wintering cormorants,
Wright (2003) concluded that cormorants had no
effect on brown trout abundance or fishery performance. Using diet analysis coupled with a consumption model, Stewart et al. (2005) estimated that
cormorants consumed 80,803 brown and 5,213 rainbow trout over a 7 month period and concluded that
there was a high potential for competition between the
birds and the fishery for available fish. Stewart et al.
(2005) also found a link between brown trout stocking
level in spring and cormorant abundance the following
winter, suggesting that increased stocking could have
attracted more cormorants to the site and that the
subsequent increase in predation would cancel out any
benefits of stocking on the fishery yield.
The numbers of brown and rainbow trout estimated to have been removed by cormorants (Stewart
et al., 2005) suggested high levels of impact relative
to any of the estimates of the trout populations that
have been attempted (Wright, 2003). However, even
in Loch Leven, which has been more intensively
studied than most other large water bodies in the
U.K., there is considerable uncertainty regarding the
size of the trout populations such that precise
estimates of the impacts of piscivorous birds on
them are not possible.
123
A clear feature of the brown trout population is that
the length frequency distribution and length-weight
relationship observed in 2008 indicates that those
individuals present are in excellent individual condition. This observation is consistent with the marked
increase in the average weight of brown trout in the
fishery, which accompanied the decline in their
apparent abundance in the 1970s. This relationship is
consistent with density-dependent increase in size
rather than reduction in density due to mortality
associated with poor average condition.
Conclusion
The fish community of Loch Leven is relatively simple
with a cumulative total of 12 species, but it has
undergone a number of extinctions and major changes
over historical and recent times and is currently
dominated by just brown trout, perch and three-spined
stickleback. A lowering of water level probably
caused the local extinction of Arctic charr and
contributed to the deterioration of the pike population,
while the obstruction of migratory routes was likely
responsible for the loss of Atlantic salmon and
flounder. The perch population has shown marked
fluctuations in abundance typical of this species,
although its local dynamics have also probably been
influenced by a perch-specific disease outbreak and
increasing eutrophication. The present lowering of
nutrient levels in the loch and associated recovery of
macrophytes are likely to benefit this species, which,
given its position as a major planktivore within the
system, is likely to have important implications for the
loch’s food web. The size of the brown trout population was initially estimated by application of mark and
recapture methodology on an unusually large scale.
However, differences in methodologies applied subsequently make it difficult to compare the size of the
population over time and this has hampered robust
analysis of the factors that may be involved in the
demise of the trout fishery. Nonetheless, the decline
has been so marked that it undoubtedly reflects an
actual decline in the local brown trout population.
Acknowledgments We would like to thank Janice Fletcher,
Ben James and other colleagues at the Centre for Ecology &
Hydrology for help in the field and laboratory during the recent
study of the fish community of Loch Leven. Colin Bean of
Scottish Natural Heritage kindly allowed the use of individual
Hydrobiologia
brown trout and perch data collected under contract at a number
of other locations. We also gratefully acknowledge appropriate
sampling permissions from the Kinross Estates, Scottish Natural
Heritage and The Scottish Government. The preparation of this
review was primarily funded by the Natural Environment
Research Council. Loch Leven is part of the UK Environmental
Change Network (http://www.ecn.ac.uk/).
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