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Effects of Predators on Fish and Crayfish Survival in Intermittent Streams
Matthew P. Dekar and Daniel D. Magoulick

Southeastern Naturalist, Volume 12, Issue 1 (2013): 197–208

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2013 SOUTHEASTERN NATURALIST 12(1):197–208 Effects of Predators on Fish and Crayfish Survival in Intermittent Streams 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. Introduction 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 – Matthew_Dekar@fws.gov. 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. Methods Study area 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). Fish enclosures 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. Crayfish tethering 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). Data analysis 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 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 October 2006. 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. Discussion 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. Acknowledgments 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 . Literature Cited Allouche, S., and P. Gaudin. 2001. 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