Shifting Distributions of Adult Atlantic Sturgeon Amidst
Post-Industrialization and Future Impacts in the Delaware
River: a Maximum Entropy Approach
Matthew W. Breece1*, Matthew J. Oliver1, Megan A. Cimino1, Dewayne A. Fox2
1 University of Delaware, Lewes, Delaware, United States of America, 2 Delaware State University, Dover, Delaware, United States of America
Abstract
Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) experienced severe declines due to habitat destruction and
overfishing beginning in the late 19th century. Subsequent to the boom and bust period of exploitation, there has been
minimal fishing pressure and improving habitats. However, lack of recovery led to the 2012 listing of Atlantic sturgeon
under the Endangered Species Act. Although habitats may be improving, the availability of high quality spawning
habitat, essential for the survival and development of eggs and larvae may still be a limiting factor in the recovery of
Atlantic sturgeon. To estimate adult Atlantic sturgeon spatial distributions during riverine occupancy in the Delaware
River, we utilized a maximum entropy (MaxEnt) approach along with passive biotelemetry during the likely spawning
season. We found that substrate composition and distance from the salt front significantly influenced the locations of
adult Atlantic sturgeon in the Delaware River. To broaden the scope of this study we projected our model onto four
scenarios depicting varying locations of the salt front in the Delaware River: the contemporary location of the salt
front during the likely spawning season, the location of the salt front during the historic fishery in the late 19th century,
an estimated shift in the salt front by the year 2100 due to climate change, and an extreme drought scenario, similar
to that which occurred in the 1960’s. The movement of the salt front upstream as a result of dredging and climate
change likely eliminated historic spawning habitats and currently threatens areas where Atlantic sturgeon spawning
may be taking place. Identifying where suitable spawning substrate and water chemistry intersect with the likely
occurrence of adult Atlantic sturgeon in the Delaware River highlights essential spawning habitats, enhancing
recovery prospects for this imperiled species.
Citation: Breece MW, Oliver MJ, Cimino MA, Fox DA (2013) Shifting Distributions of Adult Atlantic Sturgeon Amidst Post-Industrialization and Future
Impacts in the Delaware River: a Maximum Entropy Approach. PLoS ONE 8(11): e81321. doi:10.1371/journal.pone.0081321
Editor: Craig A Layman, North Carolina State University, United States of America
Received June 16, 2013; Accepted October 11, 2013; Published November 8, 2013
Copyright: © 2013 Breece et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Funding was provided by the following: NOAA NMFS Anadromous Fish Conservation Act (NOAA Award NA08NMF4050611; http://
www.nmfs.noaa.gov/sfa/state_federal/State-Federal-WEB/afca.htm), NOAA NMFS Species Recovery Grants to States (NOAA Award NA10NMF4720030;
http://www.nmfs.noaa.gov/pr/conservation/states/grant.htm), and DuPont Clear into the Future Fellowship Program (http://clearintothefuture.com/). The
funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors received funding through a commercial funder via a student fellowship (DuPont Clear into the Future Fellowship
Program), and this did not influence the research and/or manuscript in any way. Additionally, this does not alter the authors' adherence to all the PLOS
ONE policies on sharing data and materials.
* E-mail: mwbreece@udel.edu
Introduction
New York Bight Distinct Population Segment (DPS) of Atlantic
sturgeon, which includes individuals from the Delaware River,
was listed as endangered [5]. Although directed harvest of
Atlantic sturgeon ended in 1998 [6], the results of historic
overharvest coupled with habitat change and ongoing issues of
bycatch mortality have resulted in a > 99% decline from historic
abundance levels to < 300 spawning adults annually [7].
Like all sturgeons (family Acipenseridae), Atlantic sturgeon
require flowing freshwater and adherence of eggs to
appropriate substrate for successful spawning [8]. Eggs and
larvae of sturgeons are salt intolerant and require salinity levels
below 0.25 PSU to ensure survival [9]. Suitable spawning
substrates vary from coarse sands, hardpan clay, to bedrock
The Delaware River Estuary historically supported the
largest spawning population of Atlantic sturgeon (Acipenser
oxyrinchus oxyrinchus) until over-fishing and habitat
degradation caused sharp declines in their numbers [1]. The
Delaware River Atlantic sturgeon fishery was short lived; peak
landings of 2,900 mt occurred in 1888 and dropped more than
90% by the turn of the century [2]. In addition to over-fishing,
navigation projects in the Delaware River Estuary resulted in
the removal of over 1.5 x 108 m3 of sediments, causing major
changes in substrates, tidal flows, and salinity [3], [4], which
likely altered Atlantic sturgeon spawning habitats. Recently the
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Shifting Atlantic Sturgeon Distributions DE River
Study Area
[10], [11]. In contrast, unconsolidated fine grain materials will
adhere to the developing embryo resulting in abnormal
development or mortality [11].
In the Delaware River, information on the location and timing
of Atlantic sturgeon spawning in relation to salinity is limited,
although it was believed to occur in a region ± 32 km
bracketing the freshwater/saltwater interface [12]. These
observations are based on fishery dependent data collected
from Atlantic sturgeon landed for caviar during the late 19th and
early 20th centuries [2] and are not consistent with recent
findings on spawning requirements in other rivers [9]. Although
these historic location observations may include both spawning
and non-spawning adults, they still provide insights into the
habitat usage of Atlantic sturgeon in the Delaware River.
Historic accounts suggest that Atlantic sturgeon primarily
utilized the area between Bombay Hook, DE (river kilometer
(rkm) 61) and Chester, PA (rkm 130), and occasionally moved
further upriver after the peak of the season in late-May [12].
Modifications made to the Delaware River over the last
century have altered many historic Atlantic sturgeon spawning
areas by changing the location of the salt front and composition
of substrates [3], [4]. In light of the recent decision to list the NY
Bight DPS as endangered, locating Atlantic sturgeon spawning
areas is a high priority research need for the conservation and
recovery of this imperiled species [5], especially given the
renewed channel-deepening efforts in the Delaware River by
the Untied States Army Corps of Engineers [13].
Unfortunately, the diminished population of spawning Atlantic
sturgeon in the Delaware River (< 300 adults) [7] creates an
impediment in identifying essential habitats. To overcome this
hurdle, we coupled multiple years of acoustic biotelemetry
detections with a presence-only modeling technique to
determine if the salt front location and sediment composition
are significant predictors of adult Atlantic sturgeon locations in
the Delaware River during the spawning season. We projected
our Atlantic sturgeon distribution model onto a series of historic
and future Delaware River flow scenarios to approximate the
likely location and distribution of suitable habitat available
under each scenario. The ability to forecast the locations of
Atlantic sturgeon and the distribution of suitable habitat given a
changing climate provides insights into what the future may
hold for a population showing sparks of recovery. Our analysis
is a significant step in developing a quantitative spatial
framework for managing this endangered species in an
urbanized river.
The study area focused on the tidal portion of the Delaware
River from the confluence with the Chesapeake and Delaware
Canal (C&D Canal; rkm 94) to the head of the tide at Trenton,
NJ (rkm 210). The Delaware River is the largest undammed
river in the Eastern United States and flows 530 km from New
York to the mouth of the Delaware Bay. It contains the worlds
largest freshwater, and one of the United States largest port
complexes hosting approximately 3,000 deep-draft vessels per
year [14]. The Delaware River Basin encompasses Delaware,
New Jersey, New York, and Pennsylvania and covers over
35,000 km2, providing drinking water to over 5% of the United
States population.
The main freshwater input into the estuary comes from the
Delaware River with a mean flow of 330 m3s-1 measured at
Trenton, NJ [4]. Since 1877, the removal of 3.7 x 108 m3 of
sediment through dredging increased the mean channel depth
of the tidal Delaware River (rkm 94-210) from 6.1 to 12.2 m,
increasing tidal amplitude at the head of the tide (rkm 210) by
nearly two-fold from 1.3 to 2.5 m [4].
Location Data
Atlantic sturgeon locations were estimated using passive
acoustic biotelemetry. During April and May of 2009-2012, a
total of 195 adult Atlantic sturgeon were implanted with longlived acoustic transmitters (VEMCO V-16, 6-H, ~ 6.4 year
battery life, 90 s mean transmission interval) 3-15 km off the
Delaware coast, using protocols developed previously for Gulf
sturgeon (A. o. desotoi) [15]. VEMCO VR-2W receivers were
utilized to detect telemetered individuals when in range ( > 80%
detection probability at distances of 1 km) [16]. Daily modal
locations of individual Atlantic sturgeon were defined as the
receiver location, which had the greatest number of hours with
detections for that individual in a given day. In the rare
occurrence of a daily mode occurring at multiple locations in a
given day, daily modal location was assigned randomly to one
of the locations with the highest number of hours. Atlantic
sturgeon are capable of transiting the study area on a daily
basis [16]; therefore, we treated daily modal locations as
independent observations.
Substrate composition
Substrate composition was provided by a hydrographic study
of the Delaware Estuary conducted from 2001-2002 by the
University of Delaware [17], which quantified substrate at
depths > 6 m in the Delaware River at a horizontal resolution of
1 m. This survey identified six sediment types: fine deposition
(mud and fluid mud), bedload (moderately well sorted sand and
gravel), fine reworking (mud), mixed reworking (mixed gravel,
sand, and mud), coarse reworking (poorly sorted sand and
gravel), and nondepositional (cobble and bedrock) (Figure 1)
[17]. Atlantic sturgeon occurrence in the Delaware River
corresponds with the deep-water habitat evaluated by the
hydrographic survey [16]. Therefore, passive acoustic receiver
locations were assigned a substrate type that covered the
majority of the area within 1 km of the receiver location, which
has previously been identified as the detection range of
receivers in this environment [16] (Figure 1). Substrate
Methods
Ethics Statement
Capture and handling of Atlantic sturgeon was authorized
under NMFS Permit No. 16507-01 and transmitter
implantations were performed under MS-222 (tricaine
methanesulfonate) to minimize stress and injury. Prior to any
sampling, all actions were approved by the Delaware State
University Institutional Animal Care and Use Committee. Daily
detection data are available through correspondence with D.
Fox.
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Figure 1. Acoustic receiver locations and 1 km detection radius overlain on the sediment composition map of the
Delaware River (adapted from Sommerfield and Madson 2003), location of the salt front shown given four different
scenarios, urban areas indicated through dark shading.
doi: 10.1371/journal.pone.0081321.g001
Presence-only Modeling
composition in the Delaware River has changed since the end
of the 19th century [4], unfortunately no known records of
historic sediments exist; therefore, substrate composition found
in the contemporary hydrographic survey [17] was used for the
projection of all model scenarios.
To determine if the distribution of sediment type utilized by
Atlantic sturgeon was significantly different than the distribution
of sediment occupied by the receivers in the Delaware River
we used a Chi-squared goodness-of-fit test [18]. Statistical
significance was determined at P < 0.05 for all analyses.
Recent advances in species distribution modeling have
utilized presence-only records to estimate the probability of a
species occurrence [19]. The MaxEnt program (MaxEnt v3.3.3)
has been used to mediate the need for presence-absence data
by relying solely on presence-only records along with
associated environmental and geographical attributes [20].
MaxEnt starts with the maximum entropy of a given area and
then constrains that area using provided environmental
characteristics associated with the species occurrence [20].
This creates a species distribution map of the probability of
occurrence given the input environmental features, which may
be considered a surrogate for habitat suitability modeling [20].
MaxEnt also generates a Receiver Operating Characteristics
(ROC) curve, which estimates goodness of fit by calculating the
Area Under the Curve (AUC). An AUC of 0.5 is considered the
result of a random distribution (non-predictive model) while an
AUC > 0.9 is considered to have outstanding discrimination
[21]. A cross-validation resampling procedure can be utilized to
determine out of sample estimates of performance and
uncertainty in the model [19].
We used the MaxEnt approach to model Atlantic sturgeon
distributions in the tidal Delaware River during the spawning
Location of the Salt Front
The daily location of the salt front in the Delaware River was
provided by the Delaware River Basin Commission (DRBC),
and was based on the rolling seven-day average location of the
250 mg/L chloride concentration. We computed the distance
between daily location of the salt front and daily modal Atlantic
sturgeon locations for every day a given individual was in the
Delaware River.
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season as provided by detections in the passive receiver array
from 2009-2012. Daily modal location estimates for individual
Atlantic sturgeon, with associated sediment type and distance
from the salt front, were used with a ten-fold cross-validation
procedure to train and test the MaxEnt model [22]. In the
MaxEnt program options, the regularization value was set at
one to avoid model over-fitting, convergence threshold was 10-5
and the maximum number of iterations was 500 [19,20].
Analysis of omission and ROC curves were examined to
determine the function of the omission rate in terms of the
cumulative threshold and sensitivity given the fractional
predicted area respectively [20]. The AUC was calculated for
the ROC to determine the models ability to discriminate
between its prediction and random occurrences.
Once the model was developed, we projected the estimated
probability of adult Atlantic sturgeon occurrence onto the
contemporary conditions of the tidal Delaware River.
Additionally, we determined the importance of covariates in the
model by computing the percent permutation importance to the
performance of the model by each covariate. Importance
values were calculated by random permutation of the
covariates independently and measuring the resulting decrease
in the AUC [20]. Covariate importance was also estimated with
jackknife tests where the model was run with the exclusion and
isolation of each covariate while recording the resulting
changes in performance [20].
To determine the effect of a migrating salt front on the
Delaware River, we projected the developed model onto three
additional scenarios of the location of the salt front. The first
scenario was the historical location of the salt front during the
peak of the sturgeon fishery in the late 19th century [12], as
estimated during the fishing season (rkm 92). Second, we used
an 11 km upstream shift (rkm 114) of the salt front in 2100
resulting from sea level rise as modeled by the Environmental
Protection Agency [23] (Figure 1). The final scenario used the
location of the salt front given the extreme drought conditions
that were observed in the early 1960’s and represent the
highest upstream records of the salt front (rkm 164) [23].
Ocean to the Delaware River between 2009-2012 (Table 1).
Atlantic sturgeon remained in the Delaware River for 7-70 d in
April-July, and traveled as far upstream as Roebling, NJ (rkm
201), occupying sediment types at a proportion that was
significantly different from the distribution of available sediment
types. Atlantic sturgeon selected for substrates consisting of
mixed and uniform-grained reworking material (χ2 = 63.9, df =
5, P < 0.0001) (Figure 2).
The average location of the salt front during adult Atlantic
sturgeon occupancy of the Delaware River during this study
ranged from rkm 92 (2011) to rkm 112 (2009 and 2012).
Atlantic sturgeon inhabited areas of the river ± 30 km from the
estimated salt front for 84% of the time with smaller peaks
occurring 60-100 km above the salt front for 16% of the time
(Figure 3).
Our MaxEnt model depicted the distribution of Atlantic
sturgeon by incorporating the passive acoustic detections of
telemetered individuals along with associated substrates and
distances to the salt front. The contemporary model estimated
Atlantic sturgeon distributions very well with an AUC of 0.900
with the relative location of the salt front providing the greatest
contribution (43.2%) to the model. The response curve for the
covariate of relative location of the salt front (all other
covariates held at average value) resembles the distribution of
occurrence histogram with noticeably strong peaks at 65 and
90 km above the salt front (Figure 4). Mixed-grained reworking
sediments (26.3%), and nondepositional substrates (18.9%)
where the next highest contributors, while the remaining four
substrate types combined contributed < 13% to the model
performance (Table 2). Permutation importance is in
agreement with the jackknife plots (Figure S1). Evaluation of
the response curves for the six sediments types reveal a higher
probability of occurrence in areas with nondepositional,
bedload, and coarse reworking substrates, while areas with
fine deposition substrate had a lower probability of occurrence
(Figure 4).
When the model was projected on to the three salt front
location scenarios, the model continued to preform at a high
level with AUCs of 0.893 for the historic location of the salt
front, 0.895 given climate change, and 0.898 under the
extreme drought scenario (Table 2). Additionally, the
permutation importance of the salt front location and substrate
distributions varied less than ±3% for all variables in the three
projected scenarios (Table 2). Response curves for the three
additional scenarios were similar to the response curves for the
contemporary model (Figure S2). MaxEnt ROC and omission
rate plots depicting the fraction of background habitat versus
the cumulative threshold of suitable habitat for the
contemporary analysis and three scenarios are available in
Figure S3.
The contemporary probability distribution map (Figure 5a)
suggests that Atlantic sturgeon occupy the region from New
Castle, DE (rkm 99) to Tinicum Island, PA (rkm 137), with high
concentration areas near Claymont, DE (rkm 125) and Chester,
PA (rkm 130). These high concentration areas contain coarsegrained and nondepositional bedrock habitat suitable for
spawning (Figure 1). Additionally, there were signals of
Results
Forty-six receivers were deployed and maintained in the tidal
Delaware River from 2009-2012. The mean distance between
receiver locations was 2.5 km enabling near continuous
detection of telemetered individuals with individuals being
detected at least 20 hours of every day while in the study area.
Thirty-six of the 46 receiver locations fell within the confines of
the recent hydrographic survey of the sediments (Figure 1).
The majority of receiver locations were associated with mixedgrained reworking substrate (44%) followed by fine-grained
reworking
(17%),
coarse-grained
bedload
(17%),
nondepositional (17%), coarse-grained reworking (8%) and
fine-grained deposition (3%). The ten receivers that fell outside
the confines of the hydrographic survey were utilized for
location information (i.e. distance to salt front) only and not
substrate type.
Of the 195 adult Atlantic sturgeon telemetered, 12 individuals
made a total of 20 likely spawning migrations from the Atlantic
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Table 1. Individual adult Atlantic sturgeon that entered the Delaware River from 2009-2012 during the likely period of
spawning with sex, length, weight, timing of occupancy, and maximum river kilometer.
ID
Capture date
Sex
Fork length (cm)
Weight (kg)
Date of arrival
Date of departure
Days of occupancy
Max rkm*
1225
4/27/2006
Male
158
n/a
5/11/2009
n/a
n/a
141
2433
4/5/2009
Male
178
57
5/14/2009
5/30/2009
16
125
2442
4/8/2009
Male
166
50
5/5/2009
6/15/2009
41
129
2433
4/5/2009
Male
178
57
4/27/2010
5/19/2010
22
130
2442
4/8/2009
Male
166
50
5/7/2010
6/4/2010
28
129
4954
4/9/2010
Male
154
41
4/15/2010
5/2/2010
17
115
4965
4/20/2010
Male
171
53
5/8/2010
6/6/2010
29
130
2433
4/5/2009
Male
178
57
4/29/2011
5/29/2011
30
135
2442
4/8/2009
Male
166
50
5/1/2011
5/28/2011
27
122
4954
4/9/2010
Male
154
41
4/14/2011
4/17/2011
3
92
4956
4/12/2010
Male
184
58
5/8/2011
6/8/2011
31
201
4958
4/12/2010
Female
196
79
5/7/2011
5/23/2011
16
135
4965
4/20/2010
Male
171
53
5/13/2011
5/30/2011
17
190
4980
4/22/2010
Female
206
84
5/21/2011
7/30/2011
70
148
2433
4/5/2009
Male
178
57
4/22/2012
5/23/2012
32
129
2442
4/8/2009
Male
166
50
4/17/2012
6/6/2012
50
135
2465
4/3/2012
Male
180
63
5/6/2012
5/25/2012
20
120
2466
4/3/2012
Male
167
42
5/13/2012
6/22/2012
39
176
2488
4/9/2012
Female
208
98
5/9/2012
5/29/2012
10
130
2493
4/12/2012
Male
163
43
5/6/2012
6/1/2012
26
135
*. rkm – River Kilometer
doi: 10.1371/journal.pone.0081321.t001
occupancy as far upstream as Burlington, NJ (rkm 187; Figure
5a).
Our estimates, given the historic location of salt front (rkm
92), suggest adult Atlantic sturgeon distributions were
concentrated over a wide area (~35 km) extending from
downstream of Delaware City (rkm 96) to Chester, PA (rkm
130) with a low probability of occurrence in the reaches from
Chester, PA (rkm 130) to Burlington, NJ (rkm 187; Figure 5b).
The likely distribution of adult Atlantic sturgeon under the
scenario of increased seawater intrusion due to climate change
and sea level rise is constrained and shifted upstream. Under
this scenario, Atlantic sturgeon would principally occur from
Wilmington, DE (rkm 122) to Tinicum Island, PA (rkm 137) with
the highest concentrations near Claymont, DE (rkm 125) and
Chester, PA (rkm 130; Figure 5c). Under extreme historic
drought conditions, the distribution of adult Atlantic sturgeon
was constricted to several localized spots occurring near
Claymont, DE (rkm 125), Chester, PA (rkm 130), and in the
waters adjacent to Philadelphia (rkm 153; Figure 5d).
acoustic telemetry, sediment information, and location of the
salt front to determine if these factors play a significant role in
the occurrence of adult Atlantic sturgeon during the spawning
season in the Delaware River. Additionally, we were able to
project the impact of sediment type and salt front location onto
historic, contemporary, and future scenarios in the region to
determine how these environmental predictors might mediate
the distribution of adult Atlantic sturgeon in the Delaware River.
Limited knowledge of riverine requirements by adult Atlantic
sturgeon [7] coupled with their current conservation status [5]
underscores the need for an improved understanding of the
drivers that dictate habitats utilized during spawning migrations.
While in the riverine environment, Atlantic sturgeon require
both staging and spawning habitats, occupying the staging
habitats for the majority of the time, making several short,
directed spawning runs of 10-50 km to and from spawning
habitats during the spawning season [24]. The use of MaxEnt
coupled with passive telemetry has enabled us to identify
important stretches of the Delaware River for adult Atlantic
sturgeon, providing an alternative to intense, in-river sampling
in a heavily trafficked urban river.
Adult Atlantic sturgeon in the Delaware River showed an
affinity for areas near the salt front. The majority of time was
spent ~ 30 km downstream to 30 km upstream of the salt front,
typically between New Castle, DE (rkm 99) and the Schuylkill
River, PA (rkm 148). With adjustments taken for the migration
of the salt front over the last century, our findings mirror
accounts from the 1880’s: “From this point southward 20 miles,
and northward as many more, it is probable that a large part of
the spawning [of Atlantic sturgeon] now occurs” [12]. This
Discussion
Historic overfishing followed by habitat degradation of
riverine environments essential to Atlantic sturgeon spawning
and rearing has depleted their populations and stalled
recovery. The lack of population increase given improved
conditions in the Delaware River and the absence of directed
fishing pressure were listed as the main drivers for the recent
listing of the Atlantic sturgeon NY Bight distinct population
segment under the Endangered Species Act [5]. We used
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Figure 2. Number of days adult Atlantic sturgeon were observed occupying each substrate type with expected number
of days by each sediment type: FD- Fine Deposition, UR- Uniform Reworking, MR- Mixed Reworking, CR- Coarse
Reworking, BL- Bedload, ND- Nondepositional.
doi: 10.1371/journal.pone.0081321.g002
By relating the locations of various substrates to the distance
from the salt front, we were able to model distributions of
Atlantic sturgeon and highlight key areas of congregation
based on these features. Our model estimates a distribution
bracketing the salt front and indicates two areas of high
concentration occurring upstream of the salt front (Figure 5a).
These two areas near Claymont, DE (rkm 125) and Chester,
PA (rkm 130) are largely composed of bedrock habitat that is
ideal for spawning [11]. The high probability of Atlantic
sturgeon occupancy in these two relatively small areas and the
presence of suitable spawning habitat provide strong evidence
that spawning is taking place in these reaches. Without the
collection of eggs or larvae we cannot confirm spawning
activity in these areas; however, young-of-the-year Atlantic
sturgeon have been collected in close proximity suggesting
nearby spawning [28].
By projecting the model onto historic conditions, we
estimated the probability of occurrence when Atlantic sturgeon
harvests were at their peak in the late 19th century (Figure 5b)
[12]. These historic conditions, with the salt front depressed
further downstream, allowed adult Atlantic sturgeon to utilize
large expanses of suitable habitat during the spawning season.
The probability of occurrence map for historic conditions
account references the location of the salt front and believed
spawning to take place ± 32 km from this point [12]. Although,
we now know that Atlantic sturgeon are incapable of
successfully reproducing in areas downstream of the salt front
[9] this historic account alludes to important habitats in close
proximity to the salt front.
Our modeling efforts suggest adult Atlantic sturgeon
locations within the Delaware River are influenced by substrate
composition and the location of the salt front. While the majority
of Atlantic sturgeon acoustic detections were within 30 km of
the salt front, the response curves show additional high
probability of occurrence around 65 km above the salt front and
then again at areas > 90 km above the salt front. While
occurrences well upstream of the salt front were observed in
the telemetry data, the high contribution to the model above 90
km is likely an artifact of lack of substrate data at the upper end
of the study area. Atlantic sturgeon appear to select for areas
with coarse reworking and nondepositional substrates, which
corresponds with previous findings for juvenile Atlantic
sturgeon in the Delaware River [16], adults and late stage
juveniles in the Hudson River [25], and is a common theme
throughout the range of Atlantic sturgeon [26],[27].
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Figure 3. Density of observations by distance from salt front for telemetered adult Atlantic sturgeon in the Delaware
River during the likely period of spawning from 2009-2012.
doi: 10.1371/journal.pone.0081321.g003
showed a reduction in Atlantic sturgeon in the lower extent of
our study area compared with the contemporary model. This
reduction is likely a combination of lack of available substrate
data for that stretch of the Delaware River as well as a shift in
the range of adult Atlantic sturgeon downstream of the area
covered by this study and not a true lack of occurrence during
that time period. In contrast, increased salt-water intrusion from
the marine environment that is likely to occur under predicted
scenarios of climate change and sea level rise [23], markedly
constrain areas of suitable habitat for Atlantic sturgeon in the
Delaware River (Figure 5c). The increased seawater inundation
of the Delaware River due to sea level rise will soon threaten
the two small reaches this study has highlighted as potential
spawning areas.
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Under projected climate change by 2100, an 11 km upstream
shift in the salt front is predicted as a direct result of sea level
rise [23]. This compression of the freshwater portion of the
Delaware River is further exacerbated by the ongoing
Delaware River Main Channel Deepening Project [13], which
upon completion will increase channel depths from 12.2 to 13.7
m. This depth increase is expected to shift the salt front an
additional 4 km upstream [4] for a total shift of 15 km.
Additionally, the expansion of the Panama Canal, allowing
vessels with a draft of 15 m compared to the current maximum
draft of 12 m, will increase the demand for deeper draft East
Coast ports and may increase pressure to deepen the
Delaware River beyond 13.7 m [29]. Further deepening of the
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Shifting Atlantic Sturgeon Distributions DE River
Figure 4. Contemporary MaxEnt mean response curves of the covariates distance to the salt front (km), mixed reworking
(mixed gravel, sand, and mud), nondepositional (cobble and bedrock), uniform reworking (mud), bedload (moderately well
sorted sand and gravel), coarse grained reworking (poorly sorted sand and gravel), and fine deposition (mud and fluid
mud). (0 = absence of covariate, 1 = presence of covariate). Mean is in red with ± one standard deviation in blue (two shades for
the categorical substrate covariates).
doi: 10.1371/journal.pone.0081321.g004
Delaware River will likely result in increased inundation and
further restriction of suitable habitat for Atlantic sturgeon.
Compounding the effect of increased salinity from seawater
inundation is the upstream movement of the turbidity maximum
PLOS ONE | www.plosone.org
zone, which is directly linked to the location of the salt front in
the Delaware River [30]. The upstream shift of the turbidity
maximum zone will likely increase sedimentation rates [30] in
these highlighted reaches and transform the existing hard-
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Shifting Atlantic Sturgeon Distributions DE River
concentrates Atlantic sturgeon in areas of the river with the
highest volume of commercial traffic. This overlap of an
endangered species that is vulnerable to vessel strikes [28],
[31] [32], with deep draft vessels could result in a situation that
hinders recovery of Atlantic sturgeon in the Delaware River.
The characterization of adult Atlantic sturgeon riverine
habitat has proven notoriously difficult, mainly because of their
large size and occupancy of the main channels of large river
systems, which often directly overlap with commercial shipping
activity. With the aid of biotelemetry and newly developed
species distribution models, which utilize presence-only
records, we were able to estimate the occupancy of adult
Atlantic sturgeon during spawning migrations, a critical stage in
their life history. Furthermore, the convergence of model
outputs with historic information on Atlantic sturgeon
distributions not only provides support for our model, but also
suggests that this approach is a valuable hind-casting tool.
Even though the exact geographic locations from the historic
fishery are vague, the description of habitats was very similar
to habitats that were estimated through our modeling efforts.
While the Delaware River has undergone large alterations
since the peak of the Atlantic sturgeon fishery in the late 19th
century [4], the habitat requirements for Atlantic sturgeon have
remained constant, resulting in a reduction in available staging
and spawning habitat. Additionally, we were able to employ our
model to forecast the distributions of Atlantic sturgeon given
alterations to the location of the salt front in the Delaware
River. These forecasts provide insights into the challenges we
will face as we struggle to conserve and recover this imperiled
species while balancing the economic needs of the mid-Atlantic
region.
Table 2. Model and covariate evaluation for the
contemporary analysis and three projected scenarios of salt
front location: historical location (rkm 92), location given
climate change (rkm 114), and location given drought
conditions (rkm 164).
Location of Salt Front (RKM)
Climate
Average AUC
Distance from Salt
Front Importance
Mixed Reworking
Importance
Nondepositional
Importance
Uniform Reworking
Importance
Bedload Importance
Fine Deposition
Importance
Coarse Reworking
Importance
Contemporary
Historic
Change
(103)
(92)
(114)
Drought
(164)
0.89
0.89
0.90
0.90
43.2
44.7
40.4
42.2
26.3
25.4
28.4
25.6
18.9
19.5
19.9
20.3
3.8
3.5
4.0
3.9
2.1
2.0
2.1
2.2
1.5
1.5
1.2
1.7
4.2
3.4
4.1
4.1
doi: 10.1371/journal.pone.0081321.t002
bottom substrates into areas of fine deposition substrate in
which our model has shown the probability of occurrence of
adult Atlantic sturgeon is low.
In addition to severely reducing available staging and
spawning habitats, upstream movement of the salt front also
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Figure 5. Projected areas of adult Atlantic sturgeon occurrence for 4 different salt front location scenarios: acontemporary location (rkm 103), b-historical location (rkm 92), c- location given climate change (rkm 114), d- location
given drought conditions (rkm 164).
doi: 10.1371/journal.pone.0081321.g005
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Shifting Atlantic Sturgeon Distributions DE River
Supporting Information
location of the salt front as well as the three different
scenarios of the location of the salt front.
(TIFF)
Figure S1.
Jackknife plots of the test gain of the
covariates for the contemporary analysis as well as the
three additional scenarios of the location of the salt front.
(TIFF)
Acknowledgements
We thank Kevin Wark, Lori Brown, Michael Lohr, Kim DamonRandall, Matthew Fisher and the Delaware Department of
Natural Resources and Environmental Control for their
assistance during this research. We thank two anonymous
reviewers for their critical reviews of previous versions of this
manuscript.
Figure S2. Mean response curves for the MaxEnt analysis
of the three scenarios for the covariates distance to the
salt front (km), mixed reworking (mixed gravel, sand, and
mud), nondepositional (cobble and bedrock), uniform
reworking (mud), bedload (moderately well sorted sand
and gravel), coarse grained reworking (poorly sorted sand
and gravel), and fine deposition (mud and fluid mud). (0 =
absence of covariate, 1 = presence of covariate). Mean is in
red with ± one standard deviation in blue (two shades for the
categorical substrate covariates.
(TIFF)
Author Contributions
Conceived and designed the experiments: MWB DAF.
Performed the experiments: MWB DAF. Analyzed the data:
MWB MAC. Contributed reagents/materials/analysis tools: DAF
MJO. Wrote the manuscript: MWB.
Figure S3. MaxEnt receiver operating curves and plots of
the omission rate for test model runs of the contemporary
References
1. Secor DH, Waldman JR (1999) Historical abundance of Delaware Bay
Atlantic sturgeon and potential rate of recovery. Am Fish Soc Symp 23:
203-216.
2. Cobb JN (1900) The sturgeon fishery of Delaware River and Bay.
Report of Commissioner of Fish and Fisheries 25: 369-380.
3. DiLorenzo JL, Huang P, Thatcher ML, Najarian TO (1993) Effects of
historic dredging activities and water diversions on the tidal regime and
salinity distribution of the Delaware Estuary. Final Report Submitted to
Delaware River Basin Commission pp.. p. 124.
4. Walsh DR (2004) Anthropogenic Influences on the Morphology of the
Tidal Delaware River and Estuary: 1877-1987. Master's Thesis p. 85.
5. United States Office of the Federal. Registry (2012) Endangered and
threatened wildlife and plants; threatened and endangered status for
distinct population segments of Atlantic sturgeon in the northeast
region. Federal Register 77: 5880-5912
6. States Atlantic (1998) Marine Fisheries Commission. Amendment 1 to
the interstate fishery management plan for Atlantic sturgeon.
Washington DC: Atlantic States Marine Fisheries Commission, pp. 25
7. Sturgeon Atlantic Status Review Team (2007) Status Review of Atlantic
sturgeon (Acipenser oxyrinchus oxyrinchus). Report to National Marine
Fisheries Service. Northeast Regional Office. p. 175.
8. Bemis WE, Kynard B (1997) Sturgeon rivers: an introduction to
acipenseriform biogeography and life history. Environ Biol Fishes 48:
167-183. doi:10.1023/A:1007312524792.
9. Van Eenennaam JP, Doroshov SI, Moberg GP, Watson JG, Moore DS
et al. (1996) Reproductive conditions of the Atlantic sturgeon
(Acipenser oxyrinchus) in the Hudson River. Estuaries 19: 769-777.
doi:10.2307/1352296.
10. Borodin N (1925) Biological observations on the Atlantic sturgeon
(Acipenser sturio). Trans Am Fish Soc 55: 184-190. doi:
10.1577/1548-8659(1925)55[184:BOOTAS]2.0.CO;2.
11. Billard R, Lecointre G (2001) Biology and conservation of sturgeon and
paddlefish. Rev Fish Biol Fish 10: 355-392.
12. Ryder JA (1890) The sturgeons and sturgeon industries of the eastern
coast of the United States: with an account of experiments bearing
upon sturgeon culture. Bulletin of US Fish Commission 8: 231-329
13. United States Army Corp of Engineers (2009) Delaware River Main
Channel Deepening Project:General conformity analysis and mitigation
report. p. 131.
14. Economic League of; Greater Philadelphia (2008) Maritime commerce
in greater Philadelphia: assessing industry trends and growth
opportunities of Delaware River ports. pp. 78
15. Fox DA, Hightower JE, Parauka FM (2000) Gulf sturgeon spawning
migration and habitat in the Choctawhatchee River system, Alabama–
Florida.
Trans
Am
Fish
Soc
129:
811-826.
doi:
10.1577/1548-8659(2000)129.
PLOS ONE | www.plosone.org
16. Simpson PC (2008) Movements and habitat use of Delaware River
Atlantic Sturgeon. Nat Resources Mastersthesis Pp. 141.
17. Sommerfield CK, Madsen JA (2003) Sedimentological and Geophysical
Survey of the Upper Delaware Estuary: Final Report to the Delaware
River Basin Commission. University of Delaware Sea Grant College
Program Newark, Delaware. pp. 126 p
18. Larntz K (1978) Small-sample comparisons of exact levels for chisquared goodness-of-fit statistics. J Am Stat Assoc 73: 253-263. doi:
10.1080/01621459.1978.10481567.
19. Elith J, Phillips SJ, Hastie T, Dudík M, Chee YE et al. (2011) A
statistical explanation of MaxEnt for ecologists. Divers Distrib 17:
43-57. doi:10.1111/j.1472-4642.2010.00725.x.
20. Phillips SJ, Anderson RP, Schapire RE (2006) Maximum entropy
modeling of species geographic distributions. Ecol Modell 190:
231-259. doi:10.1016/j.ecolmodel.2005.03.026.
21. Landis JR, Koch GG (1977) The measurement of observer agreement
for categorical data. Biometrics 33: 159-174. doi:10.2307/2529310.
PubMed: 843571.
22. Redon M, Luque S (2010) Presence-only modelling for indicator
species distribution: Biodiversity monitoring in the French Alps. In 6th
Spatial Analysis and Geomatics International Conference 1. pp. 42-55.
23. Hull CHJ, Titus JG (1986) Greenhouse effect, sea level rise, and
salinity in the Delaware Estuary. Agency: United States Environmental
Protection. p. 36.
24. Hatin D, Fortin R, Caron F (2002) Movements and aggregation areas of
adult Atlantic sturgeon (Acipenser oxyrinchus) in the St Lawrence River
estuary, Quebec, Canada. J Appl Ichthyol 18: 586-594. doi:10.1046/j.
1439-0426.2002.00395.x.
25. Bain M, Haley N, Peterson D, Waldman JR (2000) Harvest and habitats
of Atlantic sturgeon Acipenser oxyrinchus Mitchill, 1815 in the Hudson
River estuary: lessons for sturgeon conservation 16. Instituto Espanol
de Oceanografia Boletin. pp. 43-53.
26. Smith TIJ (1985) The fishery, biology, and management of Atlantic
sturgeon, Acipenser oxyrhynchus, in North America. Environ Biol
Fishes 14: 61-72. doi:10.1007/BF00001577.
27. Gilbert CR (1989) Atlantic and shortnose sturgeons. United States
Department of Interior Biological Report 82 p. 28.
28. Fisher MT (2011) State of Delaware annual compliance report for
Atlantic sturgeon. Report to the Atlantic States Marine Fisheries
Commission, Atlantic Sturgeon Plan Review Team pp. 19.
29. Rodrigue J-P (2010) Factors impacting North American freight
distribution in view of the Panama Canal expansion. Calgary: Van
Horne Institute. p. 64.
30. Cook TL, Sommerfield CK, Wong K-C (2007) Observations of tidal and
springtime sediment transport in the upper Delaware Estuary.
11
November 2013 | Volume 8 | Issue 11 | e81321
Shifting Atlantic Sturgeon Distributions DE River
Estuarine, Coast Shelf Sci 72: 235-246. doi:10.1016/j.ecss.
2006.10.014.
31. Simpson PC, Fox DA (2009) A contemporary understanding of the
Delaware River Atlantic sturgeon: survival in a highly impacted aquatic
ecosystem 69. American Fishery Society. pp. 1-4.
PLOS ONE | www.plosone.org
32. Balazik MT, Reine KJ, Spells AJ, Fredrickson CA, Fine ML et al. (2012)
The potential for vessel interactions with adult Atlantic sturgeon in the
James River, Virginia. North American J Fish Manag 32: 1062-1069.
doi:10.1080/02755947.2012.716016.
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