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2013 SOUTHEASTERN NATURALIST 12(1):197–208
Effects of Predators on Fish and Crayfish Survival in
Matthew P. Dekar1,* and Daniel D. Magoulick2
Abstract - Predation from aquatic and terrestrial predators are important factors structuring
the size and depth distribution of aquatic prey. We conducted mesocosm and
tethering experiments on Little Mulberry Creek in northwest Arkansas during low flows
to examine the effects of predators on fish and crayfish survival in intermittent streams.
Using shallow artificial pools (10 cm deep) and predator exclusions, we tested the hypothesis
that large-bodied fish are at greater risk from terrestrial predators in shallow
habitats compared to small-bodied individuals. Twenty-four circular pools (12 open top,
12 closed top) were stocked with two size classes of Campostoma anomalum (Central
Stoneroller) and deployed systematically in a single stream pool. In addition, we used a
crayfish tethering experiment to test the hypothesis that the survival of small and large
crayfish is greater in shallow and deep habitats, respectively. We tethered two size classes
of Orconectes meeki meeki (Meek’s Crayfish) along shallow and deep transects in two
adjacent stream pools and measured survival for 15 days. During both experiments, we
monitored the presence or absence of predators by visual observation and from scat surveys.
We demonstrated a negative effect of terrestrial predators on Central Stoneroller
survival in the artificial pools, and larger individuals were more susceptible to predation.
In contrast, small crayfish experienced low survival at all depths and large crayfish were
preyed upon much less intensively during the tethering study, particularly in the pool
with larger substrate. More studies are needed to understand how stream drying and environmental
heterogeneity influence the complex interactions between predator and prey
populations in intermittent streams.
Predation threat in aquatic communities derives from both aquatic
and terrestrial predators. Due to the variable predation threat, it has been
hypothesized that large fish avoid terrestrial predators by occupying deep
habitats and small fish avoid aquatic predators by selecting shallow habitats
(Harvey and Stewart 1991, Power 1987). Similarly, Gelwick (2000) demonstrated
that crayfish occupied deep habitats to avoid terrestrial predators and
displayed nocturnal activity to avoid aquatic predators. Englund and Krupa
(2000) applied the depth- and size-distribution model to crayfish in a tethering
experiment. Results indicated that terrestrial predators, including mammals
and wading birds, had minimal impact on crayfish mortality in deep treatments
but consumed small and large crayfish in shallow treatments. Considering the
1Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701.
2USGS, Arkansas Cooperative Fish and Wildlife Research Unit, Department of Biological
Sciences, University of Arkansas, Fayetteville, AR 72701. *Corresponding author –
198 Southeastern Naturalist Vol. 12, No. 1
interaction between depth and predation risk, stream drying is likely an important
factor regulating predation in lotic ecosystems. However, few studies
examine the mechanisms that regulate community dynamics during stream
drying, and relatively little is known regarding the importance of predation
during drought (Matthews and Marsh-Matthews 2003). Further, it is important
to quantify the differential responses of fish and crayfish to predation during
stream drying to predict the size-structured population dynamics and re-colonization
potential of intermittent streams.
Stream drying can expose aquatic organisms to harsh abiotic and biotic
factors, including increased vulnerability to aquatic and terrestrial predators
(Magoulick and Kobza 2003). Magoulick (2004) added Micropterus dolomieu
Lacépède (Smallmouth Bass) to stream pools during summer drying to
examine micro-habitat selection by fish and crayfish in response to increased
predation threat. Bass additions displaced small minnows and small and large
crayfish into shallow habitats within pools. However, the resulting microhabitat
shift by large crayfish likely increased their vulnerability to terrestrial
predators that are more effective in shallow habitats (Power 1987, Power et
al. 1989). In contrast, stream drying and low flows may benefit prey populations
indirectly by negatively impacting aquatic predators. Increased crayfish
abundance following stream drying in intermittent streams in central Texas
was attributed to a lack of predatory fish that were absent due to intolerance
of low-flow conditions (Birnbaum et al. 2007). Therefore, the importance of
predation in intermittent streams will depend on predator and prey responses
to habitat availability and connectivity.
Terrestrial predators are a diverse group, and their impact on stream communities
will depend largely on foraging mode and prey type. Steinmetz et al. (2003)
argued that birds are important but largely overlooked top predators structuring
aquatic ecosystems. The authors manipulated avian predation threat in two Illinois
streams and demonstrated increased mean size of minnows, including
Luxilus chrysocephalus Rafinesque (Striped Shiner) and Campostoma anomalum
Rafinesque (Central Stoneroller), in reduced-predation treatments. Similarly, Allouche
and Gaudin (2001) demonstrated indirect effects of avian predation threat,
including reduced foraging and growth rates, on fish populations. In contrast to
some other predators, Lontra canadensis Schreber (River Otter) are able to dive
for extended periods of time (Ben-David et al. 2000), providing opportunities to
forage in both deep and shallow habitats. In addition, River Otters have a high
energetic demand and are known to consume large quantities of fish and crayfish
depending on the seasonal availability of prey (Dekar et al. 2010). Therefore, the
importance of predation in intermittent streams will depend on predator and prey
identities, including the seasonal availability of prey.
In this study, we used mesocosm and tethering experiments to examine the
effects of aquatic and terrestrial predators on fish and crayfish populations
in Little Mulberry Creek, an intermittent stream in northwest Arkansas. Central
Stonerollers and crayfish are widespread omnivores that often influence
2013 M.P. Dekar and D.D. Magoulick 199
benthic communities in North America (Evans-White et al. 2003). We hypothesized
that predation is a fundamental factor driving the depth-size distribution
of aquatic organisms in intermittent streams. We used prey enclosures to
quantify size-specific predation rates by terrestrial predators on Central
Stonerollers stocked in artificial stream pools positioned adjacent to the stream
bank. The experimental pools were closed to aquatic predators and either
open or closed to terrestrial predators. In addition, we examined depth- and
size-specific predation rates by aquatic and terrestrial predators on tethered
crayfish in stream pools. We predicted that large fish would be more susceptible
to terrestrial predators and demonstrate greater mortality compared to
small individuals in the experimental stream pools. We predicted that survival
rates for small crayfish would be greater in shallow tethering treatments and
large crayfish survival would be greater in deep treatments assuming that large
crayfish exceeded the gape-limitation of aquatic predators. However, if River
Otters were present, we predicted that large crayfish would also be susceptible
to predation in both deep and shallow treatments.
We conducted the predation experiments on Little Mulberry Creek in the
Boston Mountains ecoregion of northwest Arkansas. Mesocosm and tethering
experiments were conducted in a series of pools at a second-order site on Little
Mulberry Creek 14.4 km upstream from the confluence with the Mulberry
River (35º45'40"N, 93º35'40"W) in the Arkansas River Drainage Basin. The
study area, located in the Ozark National Forest, is 97% forested and dominated
by upland hardwood species, including Quercus alba L. (White Oak),
Quercus rubra L. (Northern Red Oak), Carya alba L. (Mockernut Hickory),
and Carya glabra Miller (Pignut Hickory), mixed with Pinus echinata Miller
(Shortleaf Pine). Little Mulberry Creek demonstrates riffle–pool morphology
dominated by bedrock and gravel substrates (Bennett et al. 1987). Seasonal
stream drying is common, and flow may become intermittent with dry riffles
and isolated or dry pools. A detailed description of the site characteristics,
including hydrologic variability and a map of the study area are presented in
Dekar et al. (2009).
We conducted a stream mesocosm experiment to quantify size-specific
survival rates on Central Stonerollers by terrestrial predators. Two mesocosm
trials were conducted during the study in 2005. The trials started on 31
August and 21 September and continued for six and eight days, respectively.
The trials were terminated before abundances reached zero. During each
trial, 24 plastic circular pools (1.4 m diameter x 30 cm deep) were filled to a
depth of 10 cm with stream water and placed adjacent to the stream bank at
200 Southeastern Naturalist Vol. 12, No. 1
the water’s edge (water depth ≤ 10 cm). Results from a pilot study suggested
that Central Stonerollers were unable to escape the pools at water depths
≤10 cm. The bottom of each pool was embedded in the stream substrate,
excluding aquatic predators. In addition, we placed two cobble-sized stones
from the stream into each pool for food (periphyton) and shelter. Pools were
stocked with six small (60–79 mm total length [TL]) or six large (80–99 mm
TL) Central Stonerollers collected in the study pool and adjacent pools and
riffles with a backpack electrofisher. Individuals of each size class were assigned
to pools at random. We assigned each pool systematically, alternating
between predator exclusion or open to predation, with the choice of the first
treatment selected at random. Terrestrial predators were excluded by covering
pools with metal hardware cloth (mesh size: 6 x 6 mm). During each trial,
the number of fish mortalities was compared among treatment combinations
to determine size-specific survival rates. Terrestrial predators were surveyed
qualitatively by visual observation and from scat samples collected during the
mesocosm trials. During the surveys, a single observer positioned downstream
from the study area recorded the presence and foraging status of terrestrial
predators within the study area for approximately an hour each day.
Size- and depth-specific survival rates were measured on Orconectes meeki
meeki Faxon (Meek’s Crayfish) tethered in two adjacent stream pools, separated
by a 35-m long riffle, in 2006 for 15 days. Due to logistical constraints, the
tethering trials were initiated one week apart. We tethered crayfish in the downstream
and upstream pools on 12 September and 19 September, respectively.
We collected crayfish with scented round-funnel traps (bait not available for
consumption) positioned in the study pools and adjacent pools the night before
the beginning of each trial. We tethered small (18–29 mm carapace length [CL])
and large (32–45 mm CL) crayfish along shallow (depth range = 9–34 cm) and
deep (depth range = 45–84 cm) transects parallel to the stream bank. Fifty-two
crayfish were tethered downstream in the first trial, with 13 crayfish per treatment
combination. We tethered 56 crayfish upstream in the second trial, with 14 crayfish
per treatment. Individual crayfish were tethered with 40 cm of monofilament
fishing line (4.5 kg test, 0.3 mm diameter) attached to steel reinforcement bars
positioned in the substrate at 1.0-m intervals. The monofilament line was secured
to the dorsal side of the carapace with cyanoacrylate (Super Glue). Crayfish were
deployed systematically alternating between small and large individuals, with
the choice of the first size class selected at random. In addition, we alternated
between males and females for each treatment combination. Preliminary cage
experiments indicated that small and large crayfish were unable to escape tethers
after two weeks. Therefore, all missing individuals were interpreted as predation
events, excluding individuals that molted (exuviae present).
During the experiment, we surveyed each transect (elapsed days = 1−3,
8, 15) and recorded the number of missing or molted individuals. Aquatic
2013 M.P. Dekar and D.D. Magoulick 201
predators were surveyed by visual observation, and terrestrial predators were
identified by visual observation and from scat samples collected during the
tethering study. Qualitative surveys were conducted for approximately an hour
each day by a single observer. To compare habitat complexity between pools,
we recorded substrate categories at 10 equidistant points within 1.0 m2 of the
midpoint of each crayfish tethering position. Substrate categories followed a
modified Wentworth scale (Cummins 1962), but we added a bedrock category
and merged silt with sand (0.0039–2 mm). The remaining substrate categories
included gravel (>2–16 mm), pebble (>16–64 mm), cobble (>64–256 mm), and
boulder (>256 mm).
We used two-way ANOVA to test predator (open or closed) and size effects
(small or large) on percent survival of Central Stonerollers in experimental mesocosms
blocked by trial. However, the blocking effect was not significant, and
data from each trial was pooled. In addition, percent survival was arcsine square
root transformed to satisfy normality and variance assumptions. We used logistic
regression to model the presence or absence (survival) of tethered crayfish with
depth and size as factors blocked by pool. We calculated survival curves for each
treatment using the Kaplan-Meier estimator (Hosmer et al. 2008). The significance
level was α = 0.05 for both analyses.
Results from the two-way ANOVA indicated significant effects of predation
and size (P < 0.001; Table 1) on Central Stoneroller survival. Terrestrial predators
reduced Central Stoneroller abundance in open pools, and larger individuals
were more susceptible to predation compared to smaller individuals (Fig. 1). We
observed Megaceryle alcyon L. (Belted Kingfisher) actively foraging in the open
experimental pools daily, but no mammalian scat was collected. In addition, Ardea
herodias L. (Great Blue Heron) and Butorides virescens L. (Green Heron)
were observed in the study area, but predation events were not confirmed.
After 15 days of exposure, tethered crayfish survival was significantly related
to block (P = 0.003; Table 2) and size (P = 0.001; Table 2). Survival was greater
for large crayfish, and more crayfish survived in the upstream pool during trial 2
Table 1. Two-way ANOVA (n = 12) with predation (open or closed) and size (small: 60–79 mm TL,
large: 80–99 mm TL) factors for percent Central Stoneroller survival (arcsine square root transformed)
in artificial pools during two experimental trials (trial 1: 6 days, trial 2: 8 days) conducted
on Little Mulberry Creek in August and September 2005.
Source SS DF MS F value P
Predation 10.79 1 10.79 94.64 less than 0.001
Size 1.33 1 1.33 11.66 0.001
Predation × size 0.04 1 0.04 0.35 0.556
Error 5.01 44 0.11
202 Southeastern Naturalist Vol. 12, No. 1
(Fig. 2). In contrast, we did not detect an effect of depth on crayfish survival (P =
0.226; Table 2). Substrate in the downstream pool (trial 1) was 63% gravel, 33%
pebble, 3% cobble, and 1% boulder. In contrast, substrate in the upstream pool
(trial 2) was 60% pebble, 24 % gravel, 12% cobble, 2% sand, 1% boulder, and 1%
bedrock. In terms of predators, we observed frequent foraging by Smallmouth
Figure 1. Percent Central Stoneroller survival (± standard error) in artificial stream pools
on Little Mulberry Creek in August and September 2005 (n = 12). Twelve pools each
were open or closed to terrestrial predators and stocked with six small (60–79 mm TL) or
six large (80–99 mm TL) individuals.
Table 2. Logistic regression coefficients (β), standard errors (SE), and odds ratios (OR) of tethered
crayfish survival (presence or absence) after 15 days of exposure with depth (shallow: 9–34 cm,
deep: 45–84 cm) and size (small: 18–29 mm CL, large: 32–45 mm CL) as factors blocked by pool
(n = 108). Crayfish were tethered in adjacent pools on Little Mulberry Creek in September and
Source β SE P OR SE
Constant 1.10 0.52 0.035
Block -1.74 0.58 0.003 0.18 0.10
Depth 0.76 0.63 0.226 2.13 1.34
Size -3.84 1.13 0.001 0.02 0.02
Depth × size 0.46 1.36 0.735 1.58 2.15
2013 M.P. Dekar and D.D. Magoulick 203
Figure 2. Tethered crayfish survival probability in adjacent pools (trials) on Little Mulberry
Creek in September and October 2006 (n = 108). Each trial lasted 15 days and
included small (18–29 mm CL) and large (32–45 mm CL) crayfish tethered along shallow
(9–34 cm) and deep (45–84 cm) transects.
204 Southeastern Naturalist Vol. 12, No. 1
Bass, Belted Kingfishers, and Great Blue Herons. In addition, fresh River Otter
scat that contained crayfish remains was collected at both study pools on September
14th and again on September 28th at the downstream pool. Although predation
was not confirmed, Lepomis cyanellus Rafinesque (Green Sunfish), Micropterus
punctulatus Rafinesque (Spotted Bass), and Procyon lotor L. (Raccoon) were
also present during the study.
Predation is an important factor structuring aquatic and terrestrial communities
(Sih et al. 1985). However, predation may be less important in harsh
environments relative to abiotic factors. Creed (2006) hypothesized that abiotic
factors would be more influential compared to biotic factors in temporary streams
with harsh disturbance regimes. Similarly, Grossman et al. (1998) demonstrated
that flow variability had the greatest impact on fish assemblage structure compared
to resource limitation and biotic interactions during a long-term study on
a stream in North Carolina. In contrast, stream drying may increase predator and
prey densities and exacerbate biotic interactions, including competition and predation
(Magoulick and Kobza 2003). In this study, we quantified fish and crayfish
survival rates in an intermittent stream. Although we demonstrated the importance
of predators in drying stream pools, results varied depending on predator
and prey identities.
In aquatic ecosystems, predation threat derives from aquatic and terrestrial
sources. In response to the dual threat, it is hypothesized that large fish occupy
deep habitats to escape terrestrial predators and small fish avoid aquatic predators
by occupying shallow habitats (Harvey and Stewart 1991, Power 1987). Power
(1984) demonstrated the size and depth distribution of armored catfish (Loricariidae)
inhabiting a stream in Panama. Small catfish avoided aquatic predators
in shallow habitats, and large catfish avoided bird and mammalian predators by
occupying deep habitats during the dry season. Power et al. (1985) showed strong
interactions among bass (Micropterus salmoides Lacépède [Largemouth Bass] and
Spotted Bass), Central Stonerollers, and algae in a small stream in Oklahoma. Bass
removals from a stream pool resulted in increased grazing by Central Stonerollers
and reduced algal standing crop. We predicted that Central Stoneroller survival in
our shallow study pools would be negatively related to body size. Our results supported
this hypothesis, with large individuals demonstrating decreased survival
compared to small individuals.
Crayfish may demonstrate similar depth-size patterns in response to aquatic
and terrestrial predators. Similar to grazing fish, the depth distribution of
crayfish may have important cascading effects on communities and habitat
heterogeneity. Creed (1994) demonstrated that large crayfish dramatically
reduced filamentous algae (Cladophora glomerata L.) from deep habitats in
a Michigan stream but not in shallow habitats. Consequently, the exclusion of
filamentous algae facilitated the growth and abundance of epilithic microalgae
2013 M.P. Dekar and D.D. Magoulick 205
and sessile grazing insects. The observed pattern was attributed to the avoidance
of terrestrial predators in shallow habitats by large crayfish. Similarly,
Englund and Krupa (2000) surveyed headwater stream pools in Kentucky and
demonstrated that small crayfish shifted their distribution to shallow habitats
in pools with predatory fish. In contrast, large crayfish remained in deep habitats
regardless of the presence of aquatic predators.
In the present study, we tested the hypothesis that small and large crayfish
would demonstrate increased survival in shallow and deep habitats, respectively.
In contrast to our prediction, we did not detect a depth effect, and small
crayfish survival was significantly reduced compared to large crayfish at all
depths. We used relatively large individuals for the large size class (32–45
mm CL), and predation by aquatic predators was expected to be minimal. In
addition, the range of depths in our shallow treatment (9–34 cm) may have
exceeded, in some cases, the effective foraging depth of wading birds. For
example, Power et al. (1989) demonstrated substantial predation by birds on
loricariid catfish in open-topped pens at shallow depths (10 and 20 cm) but
not in deeper pens (30 and 50 cm). Similarly, Lantz et al. (2011) manipulated
depth and emergent vegetation density in experimental macrocosms in Florida.
Results indicated a strong preference of foraging wading birds for shallow
habitat (10 cm) compared to deep habitat (25 cm). Although observed capture
efficiency was similar and fish prey densities were the same among treatments,
the preference for sparse vegetation was hypothesized to be related to anticipated
higher prey densities and higher capture efficiency relative to open-water
habitat and densely vegetated habitat, respectively. In terms of refuge from
predators in drying streams, larger substrate in the upstream pool (trial 2) may
account for increased crayfish survival observed in the upstream pool compared
to the downstream pool (trial 1). Therefore, refuge from predators is dynamic in
drying streams, a product of both depth and suitable cover.
Indirect effects of multiple predators may also impact aquatic communities
(Carey and Wahl 2010, Crowder et al. 1997, Hoeinghaus and Pelicice
2010). Huxel (2007) demonstrated that food-web stability generally increased
when multiple predators demonstrated synergistic interactions, whereas stability
decreased with antagonistic interactions, in a simulation study with a
five-species food-web model. In terms of aquatic and terrestrial predators,
Crowder et al. (1997) demonstrated experimentally that juvenile Leiostomus
xanthurus Lacépède (Spot) survival was greater in the presence of both Paralichthys
lethostigma Jordan and Gilbert (Southern Flounder) and wading birds
compared to the additive effects of each predator alone. Steinmetz et al. (2008)
demonstrated facilitation between herons and Smallmouth Bass, resulting in
risk-enhancement for Striped Shiners and Central Stonerollers. Therefore,
facilitation among aquatic and terrestrial predators resulting from predator
avoidance may increase predation rates on fish and crayfish. In this study,
we showed that terrestrial predators significantly reduced fish abundance in
shallow experimental pools, and effects were stronger on large individuals.
206 Southeastern Naturalist Vol. 12, No. 1
Although aquatic and terrestrial predation was confounded in our tethering
study, we demonstrated relatively low survival of small crayfish in shallow and
deep habitats compared to large crayfish. Considering the dramatic depth-size
distribution of fish and crayfish, emergent effects of multiple predators are likely
important factors structuring communities in intermittent streams. However,
our study suggests that large crayfish may be more resistant to predators in drying
streams relative to fish, with important consequences to the re-colonization,
size structure, and trophic dynamics of intermittent streams.
We thank G. Huxel, J. Ludlam, and K. Morgan for assistance in the field. We are grateful
for the comments provided by J. Ludlam, B. Wagner, and two anonymous reviewers
that improved this manuscript. Additional thanks to P. Kowalewycz and the United States
Forest Service for river access and camping facilities. Partial funding for this study was
provided by the Arkansas Cooperative Fish and Wildlife Research Unit in conjunction
with the University of Arkansas, the Arkansas Game and Fish Commission, the United
States Geological Survey, and the Wildlife Management Institute. The use of trade, product,
industry, or firm names or products or software or models, whether commercially
available or not, is for informative purposes only and does not constitute an endorsement
by the US Government or the US Geological Survey .
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