Top Piscivores Research Articles

Generalist invasion in a complex lake food web

AbstractInvasive species constitute a threat not only to native populations but also to the structure and functioning of entire food webs. Despite being considered as a global problem, only a small number of studies have quantitatively predicted the food web‐level consequences of invasions. Here, we use an allometric trophic network model parameterized using empirical data on species body masses and feeding interactions to predict the effects of a possible invasion of Amur sleeper (Perccottus glenii), on a well‐studied lake ecosystem. We show that the modeled establishment of Amur sleeper decreased the biomasses of top predator fishes by about 10%–19%. These reductions were largely explained by increased larval competition for food and Amur sleeper predation on fish larvae. In contrast, biomasses of less valued fish of lower trophic positions increased by about 0.4%–9% owing to reduced predation pressure by top piscivores. The predicted impact of Amur sleeper establishment on the biomasses of native fish species vastly exceeded the impacts of current‐day fishing pressures.

Density‐dependent processes and population dynamics of native sculpin in a mountain river

AbstractUnderstanding the processes governing population dynamics is important for effective conservation and environmental management. Disentangling the relative role of density‐dependent versus density‐independent processes on population dynamics is often made difficult by the inability to control for abiotic or biotic factors, but long‐term datasets are invaluable in this pursuit. We used a 14‐year dataset from the Logan River, Utah, to assess long‐term trends in abundance and evidence of density‐dependent and density‐independent effects on population dynamics of Paiute sculpin (Cottus beldingii) across six sites. Additionally, we evaluated the feeding ecology of sculpin over 4 years. Sculpin densities generally increased from upstream to downstream, and the annual per capita rate of increase was negatively and significantly correlated with sculpin density at four of six sites. We observed a negative relationship between total gut content and sculpin density but did not observe a negative relationship between relative condition and density. Sculpin displayed a generalist feeding strategy, and interannual differences in diet composition appeared to be influenced by interannual differences in flow, particularly years with higher magnitude flow. The observed spatial patterns in sculpin abundance throughout the watershed matched those of invasive brown trout (Salmo trutta), the top piscivore in the Logan River, and likely represent affinities for the suite of ecological conditions associated with downstream sections of the Logan River. Our results suggest that sculpin populations are regulated largely by density‐dependent processes and match those from other studies on sculpin population dynamics including a range of species and habitats that differ vastly in abiotic conditions.

Kinetics and toxicity of an environmentally relevant mixture of halogenated organic compounds in zebrafish embryo

Persistent and semi-persistent halogenated compounds cause health problems for the animals occupying the upper level of the food web in the Baltic Sea. Atlantic salmon (Salmo salar), being a top piscivore in the Baltic Sea, has been observed to carry a large body burden of halogenated toxins. Here, a mixture of nine halogenated compounds belonging to different groups was created, based on the observed composition of halogenated toxins in salmon serum. The toxicokinetic properties of the compounds were studied in zebrafish (Danio rerio) embryos to achieve the same proportions between the internal doses of the compounds in the zebrafish as in the salmon. Toxicity was evaluated for the compounds dosed individually as well as in a mixture. Perfluorooctanesulfonic acid (PFOS) was the dominant compound in the salmon and was observed to be the driving force for effects on swimbladder inflation caused by the mixture with a 50% effect concentration of 4.8 µM nominal dose, or 1300 µMD based on the area under the internal concentration-time curve (AUC). The driving compound for other severe effects caused by the mixture, including lethality, spinal deformity, and edemas, was the hydroxylated polybrominated diphenyl ether 6-OH-BDE47, which was observed to have a 50% lethality concentration of 93 nM, corresponding to 94 µMD based on internal dose (AUC). The individual compounds were observed to act additively on most of the documented outcomes when dosed as a mixture.

Rainbow smelt population responses to species invasions and change in environmental condition

Invasive species can cause major disruptions in native food webs, yet the impact of species introductions and whether they will become invasive appears to be context-dependent. Rainbow smelt and alewife co-exist as invasive species in the Laurentian Great Lakes and as native species on the Atlantic coast of North America, but in Lake Champlain rainbow smelt is the dominant native forage fish and alewife are invasive. Alewife became abundant by 2007, providing an opportunity to explore the dynamics of these two species in a system where only one is invasive. We used data from a 28-year forage fish survey to compare demographics of rainbow smelt populations in three basins of Lake Champlain with different volumes, nutrient levels, and predator abundances. Rainbow smelt catch-per-unit-effort (CPUE) remained constant in the large, deep Main Lake before and after alewife invaded, but decreased in the two smaller basins. Declines were primarily a result of increased age-0 and age-1 mortality. Predation by top piscivores, system productivity, and competition for resources alone could not explain the patterns in CPUE across the basins. The mechanisms that allow alewife and rainbow smelt to co-exist could be related to system volume and oxythermal habitat availability, and may explain why the two species do not negatively affect each other in the Great Lakes. Summer hypoxia in the smaller basins could force individuals into smaller habitat volumes with higher densities of competitors and cannibalistic adult rainbow smelt. Habitat availability may mediate the impact of invasive alewife on native rainbow smelt.

Legacy contaminant-stable isotope-age relationships in Lake Ontario year-class Alewife (Alosa pseudoharengus)

Alewife (Alosa pseudoharengus) are the preferred prey species of the top piscivore predators in the Lake Ontario food web and are an essential constituent in the bioaccumulation of persistent organic contaminants. Year-class samples collected in 2016 represent the alewife age ranges of 2015 (Age-01) sequentially dating back to 2008 (Age-08). The most abundant contaminant measured in Lake Ontario alewife (151.5 ng/g) were total polychlorinated biphenyls (PCBs), increasing at a rate of 11.8 ng/g per year on an age-averaged concentration basis. Total mercury demonstrated the largest percent increase (240%) accumulated over alewife ages of 1–8 years. Average total concentrations of the most abundant polychlorinated dibenzo-p-dioxin isomer (2378-tetrachlorinated dibenzo-p-dioxin (TCDD), 1.3 pg/g) and polychlorinated dibenzofuran isomer (2378-tetrachlorinated dibenzofuran (2378-TCDF), 6.6 pg/g) comprised most of the overall total dioxin (2.5 pg/g) and total furan concentrations (8.7 pg/g). The vast majority (69%) of alewife total toxic equivalence (TEQ) was comprised of the non-ortho coplanar PCBs. Both mammal and avian wildlife protection values based on total TEQ were uniformly exceeded for the dioxin-like compounds measured in Lake Ontario alewife. Ontogenetic dietary influences expressed a significant impact on Age-01 alewife age-contaminant relationships and age-stable isotope concentrations and trends for legacy contaminants. Total Hg and all dioxin-like contaminants did not demonstrate the Age-01 ontogenetic dietary effects found in legacy contaminants. A prominent polychlorinated naphthalene (PCN) concentration peak measured in year-class Age-04 alewife was followed by a corresponding lake trout peak 3–4 years later illustrating a unique example of trophic-level contaminant uptake and concomitant integration delay.

Genetic population structure of a top predatory fish (northern pike, Esox lucius) covaries with anthropogenic alteration of freshwater ecosystems

Abstract Esox lucius L. (northern pike) inhabits river, lake, and brackish water ecosystems of the temperate region in North America and Eurasia. In many areas of its native range, the species is confronted with human‐induced disturbances that can affect local and regional genetic diversity. Conservation of genetic diversity constitutes an important policy objective. Therefore, we investigated how the genetic population structure and patterns suggestive of hybridisation among subpopulations from different catchments vary with the type of ecosystem (river vs. lake) and the integrity of the ecosystem assessed using ecological status assessments of the European Water Framework Directive and indices of the wetland quality (in rivers) and trophic state (in lakes). In total, 1,384 pike samples from the North, Baltic, and Black Sea drainages in Germany were analysed. We differentiated between metapopulations from each hydrogeographic region using genotypes from 15 microsatellites and mitochondrial cyt b sequences. Individual populations showed signs of genetic admixture ranging from almost zero to complete replacement by foreign genotypes originating from different catchments. Hierarchical general linear modelling revealed a significantly positive association of the degree of genetic hybridisation with decreasing ecological status. An eroded population substructure indicative of hybridisation was also significantly elevated in rivers relative to lakes and in heavily modified water bodies. Results were consistent when using ecological status assessments by the Water Framework Directive and an alternative environmental quality index based on the health of wetlands in rivers and trophic state in lakes. We speculate that macrophyte loss through loss of wetlands or in response to eutrophication fostered genetic mixing among different subpopulations, which were brought into secondary contact either due to stocking or due to natural dispersal from foreign catchments. Our findings suggest that anthropogenic alteration of freshwater ecosystems may influence the genetic structures present in a freshwater top piscivore at both local and regional scales.

Mobile piscivores and the nature of top-down forcing in Upper Amazonian floodplain lakes

Shallow lakes can change states in response to manipulations of top predators. In most reported experiments, the top piscivore has been a fish. However, low-latitude lakes typically support non-piscine piscivores, including mammals, birds, and reptiles. The roles of these groups have been little investigated, and whether they are more important as piscivores than fish remains unknown. We report both a longitudinal (2001–2018) and a cross-sectional (2012) study of the bird, caiman, and giant otter populations of floodplain lakes in Peru’s Manu National Park. We compare the three groups, after removing the effects of taxonomic status and body mass, using allometric equations to estimate the field metabolic rate (FMR) for each group in each lake. Giant otters emerge as the dominant piscivore in lakes that support a resident family, with an energy requirement more than twice that of piscivorous birds and more than seven times that of caiman. However, giant otters were resident in only eight of 27 surveyed lakes. Indirect evidence suggests that these eight lakes are more productive than lakes not occupied by otters. The fact that lakes occupied by otters are dominated by phytoplankton suggests an underlying five-tiered trophic structure in keeping with the prominence of alternating levels in the structure of food webs.

Trophic dynamics of a reservoir fishery following an introduction of a top predator: Insights from stable carbon and nitrogen isotopes

AbstractTrophic dynamics and conceptual niche spaces of top piscivores were assessed using stable isotope analysis following the introduction of hybrid striped bass Morone saxatilis (Walbaum) × Morone chrysops (Rafinesque) into an established reservoir fishery devoid of gizzard shad, Dorosoma cepedianum (Lasueur). Hybrid striped bass were initially stocked into Three Mile Lake, Iowa as an attempt to biologically control pervasive yellow bass, Morone mississippiensis Jordan & Eigenmann, populations. From the analyses, it concluded that hybrid striped bass predominately targeted prey fish located in the littoral habitats of the reservoir and did not selectively consume yellow bass as intended. Bayesian isotope mixing models described the diets of hybrid striped bass to include a variety of prey fish, predominantly consisting of young bluegill Lepomis macrochirus and yellow bass. Food resource overlap among hybrid striped bass, largemouth bass, Micropterus salmoides Lacépède, and walleye, Sander vitreus (Mitchill), appeared substantial based on the isotopic niche overlap model. Results from this case study, in conjunction with the hindsight that yellow bass populations became prolific even with stockings of hybrid striped bass, suggest that their use to control yellow bass populations may not be a viable management strategy in southern Iowa.

Trophic ecology of piscivorous Arctic charr (Salvelinus alpinus (L.)) in subarctic lakes with contrasting food-web structures

The trophic ecology of piscivorous Arctic charr (Salvelinus alpinus (L.); charr) in the food webs of large subarctic lakes is not well understood. We assessed charr diets, parasites, growth, maturity, and stable isotope ratios in Fennoscandian subarctic lakes dominated by monomorphic or polymorphic whitefish (Coregonus lavaretus (L.)) populations. Charr density was low in all lakes, except in profundal habitats. Charr shifted to piscivory at small size (16–25 cm total length) and consumed a range of prey-fish sizes (2–25 cm). Cannibalism was observed in a few individuals from one monomorphic whitefish lake. Charr matured at 37–51 cm (5–8 years old), grew to 52–74 cm maximum observed length and 47–83 cm asymptotic length. Charr increased total area of convex hull and core stable isotopic diversity area of the fish community by 51–98% and 44–51% in monomorphic whitefish lakes, but only 8–11% and 7–10% in polymorphic whitefish lakes. The difference was due to increasing food-chain length in monomorphic whitefish lakes, whereas reliance on littoral carbon did not change. Charr were the top piscivores in monomorphic whitefish lakes, but played a less important role in polymorphic whitefish lakes, which contained a more diverse predator fauna.

Exploring Crowded Trophic Niche Space in a Novel Reservoir Fish Assemblage: How Many is Too Many?

AbstractIn highly managed reservoir systems, species interactions within novel fish assemblages can be difficult to predict. In high‐elevation Scofield Reservoir in Utah the unintentional introduction of Utah Chub Gila atraria and subsequent population expansion prompted a shift from stocking exclusively Rainbow Trout Oncorhynchus mykiss to include tiger trout (female Brown Trout Salmo trutta × male Brook Trout Salvelinus fontinalis) and Bonneville Cutthroat Trout O. clarkii utah, which composed a novel suite of top predators and potential competitors. We examined the interspecific interactions among Scofield Reservoir piscivores using a multifaceted approach including gut analyses, stable isotopes, and gape limitation. Large Cutthroat Trout consumed 50–100% Utah Chub and tiger trout consumed 45–80%. In contrast, small and large Rainbow Trout consumed primarily invertebrate prey and exhibited significant overlap with small tiger trout, Cutthroat Trout, and Utah Chub. Large Cutthroat Trout and tiger trout occupy a top piscivore trophic niche and are more littoral, while Rainbow Trout occupy an omnivore niche space and are more pelagic. Both Cutthroat and tiger trout varied in niche space with respect to size‐class, demonstrating an ontogenetic shift to piscivory at approximately 350 mm TL. Cutthroat Trout and tiger trout are capable of consuming prey up to 50% of their own size, which is larger than predicted based on their theoretical gape limit. Because it appears food resources (Utah Chub) are not limited, and performance metrics are high, competition is unlikely between Cutthroat Trout and tiger trout. In contrast, apparent survival of Rainbow Trout has recently declined significantly, potentially due to shared food resources with Utah Chub or negative behavioral interactions with other members of the community. Collectively, this research aids in understanding biotic interactions within a top‐heavy and novel fish community and assists towards developing and implementing suitable management strategies to control nuisance species.

Evidence of the St. Clair-Detroit River System as a dispersal corridor and nursery habitat for transient larval burbot

Burbot Lota lota are distributed across the Laurentian Great Lakes where they occupy a top piscivore role. The St. Clair-Detroit River System is known to provide a migration corridor as well as spawning and nursery habitat for many indigenous fishes of economic and ecological significance. However, knowledge is scant of the early life history of burbot and the importance of this system in their dispersal, survival, and recruitment. In order to assess the role of the St. Clair-Detroit River System to burbot ecology, we collected larval burbot during ichthyoplankton surveys in this system from 2010 to 2013 as part of a habitat restoration monitoring program. More and larger burbot larvae were found in the St. Clair River than in the lower Detroit River, although this may be due to differences in sampling methods between the two rivers. Consistent with existing studies, larval burbot exhibited ontogenesis with a distinct transition from a pelagic zooplankton-based diet to a benthic macroinvertebrate-based diet. Our results demonstrate that the St. Clair-Detroit Rivers provide food resources, required habitat, and a migration conduit between the upper and lower Great Lakes, but the contribution of these fish to the lower lakes requires further examination.

Using time-varying asymptotic length and body condition of top piscivores to indicate ecosystem regime shift in the main basin of Lake Huron: a Bayesian hierarchical modeling approach

We evaluated the ecosystem regime shift in the main basin of Lake Huron that was indicated by the 2003 collapse of alewives (Alosa pseudoharengus) and dramatic declines in Chinook salmon (Oncorhynchus tshawytscha) abundance thereafter. We found that the period of 1995–2002 should be considered as the early phase of the final regime shift. We developed two Bayesian hierarchical models to describe time-varying growth based on the von Bertalanffy growth function and the length–mass relationship. We used asymptotic length as an index of growth potential and predicted body mass at a given length as an index of body condition. Modeling fits to length and body mass at age of lake trout (Salvelinus namaycush), Chinook salmon, and walleye (Sander vitreus) were excellent. Based on posterior distributions, we evaluated the shifts in among-year geometric means of the growth potential and body condition. For a given top piscivore, one of the two indices responded to the regime shift much earlier than the 2003 collapse of alewives, the other corresponded to the 2003 changes, and which index provided the early signal differed among the three top piscivores.

Drought-driven lake level decline: effects on coarse woody habitat and fishes

Research testing for the effects of climate change on lentic fishes has focused on changing thermal and dissolved oxygen regimes, but has often overlooked potential influences of altered lake levels on littoral habitat availability and species interactions. Natural littoral structures such as coarse woody habitat (CWH) can be critical to fishes for prey production, refuge, and spawning. Drought-driven lake level declines may strand these structures above the waterline and thereby remove them from littoral zones. A prolonged drought in northern Wisconsin, USA, allowed us to test for effects of lake level decline on CWH and the response of a fish community. During our study (2001–2009), the lake level of Little Rock Lake South declined over 1.1 m and >75% of the previously submerged CWH was lost from the littoral zone. The loss of CWH coincided with the forage fish species (yellow perch, Perca flavescens) falling below detection and reduced growth of the top piscivore (largemouth bass, Micropterus salmoides). Our study highlights the importance of lake level fluctuations as a mechanism by which climate change may affect aquatic ecosystems and species interactions.

Forebay use and entrainment rates of resident adult fish in a large hydropower reservoir

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 19:253-263 (2013) - DOI: https://doi.org/10.3354/ab00536 Forebay use and entrainment rates of resident adult fish in a large hydropower reservoir Eduardo G. Martins1,*, Lee F. G. Gutowsky1, Philip M. Harrison2, David A. Patterson3, Michael Power2, David Z. Zhu4, Alf Leake5, Steven J. Cooke1 1Fish Ecology and Conservation Physiology Laboratory, Ottawa-Carleton Institute for Biology, Carleton University, 1125 Colonel By Dr., Ottawa, Ontario K1S 5B6, Canada 2Department of Biology, University of Waterloo, 200 University Ave., Waterloo, Ontario N2L 3G1, Canada 3Fisheries and Oceans Canada, Cooperative Resource Management Institute, School of Resource and Environmental Management, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada 4Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta T6G 2W2, Canada 5Environmental and Social Issues, BC Hydro, 6911 Southpoint Drive, Burnaby, British Columbia V3N 4X8, Canada *Email: egmartins@gmail.com ABSTRACT: Fish entrainment occurs when individuals are displaced from reservoirs to downstream waters by way of water diversion through turbines or other water release structures. While much effort has been put into quantifying and reducing entrainment of downstream migrating salmonids, considerably less research has focused on fish that are residents in reservoirs and in many cases are the top piscivores in these systems. We used 2 yr of acoustic telemetry data on adult bull trout Salvelinus confluentus and burbot Lota lota tagged in Kinbasket Reservoir (British Columbia, Canada) to investigate entrainment vulnerability by measuring forebay use and entrainment rates. Our results revealed that (1) adult bull trout and burbot made little use of the forebay; (2) bull trout used the forebay more and had higher rates of entrainment than burbot; and (3) both forebay use and entrainment varied among seasons. Bull trout used the forebay more intensively and were mostly entrained in the fall and winter. Limited forebay use and entrainment data for burbot suggest that they are more vulnerable to entrainment in the fall. These findings should be generally applicable for adult bull trout and burbot in reservoirs that maximize operations in the fall and winter, and could help managers and industry to make informed decisions on how to mitigate and compensate for entrainment. KEY WORDS: Entrainment · Hydropower · Bull trout · Burbot · Forebay · Turbines · Acoustic telemetry · Detection efficiency Full text in pdf format Supplementary material PreviousNextCite this article as: Martins EG, Gutowsky LFG, Harrison PM, Patterson DA and others (2013) Forebay use and entrainment rates of resident adult fish in a large hydropower reservoir. Aquat Biol 19:253-263. https://doi.org/10.3354/ab00536 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 19, No. 3. Online publication date: October 24, 2013 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2013 Inter-Research.

Food web structure in a near-pristine mangrove area of the Australian Wet Tropics

Carbon and nitrogen isotopic composition was used to identify the main sources of carbon and describe the main trophic pathways in Deluge Inlet, a near-pristine mangrove estuary in tropical north Queensland, Australia. Producers' δ 13C varied from −28.9‰ for mangroves to −18.6‰ for seagrass. Animals were also well separated in δ 13C (−25.4‰ to −16.3‰ for invertebrates and −25.2‰ to −17.2‰ for fish), suggesting considerable differences in ultimate sources of carbon, from a substantial reliance on mangrove carbon to an almost exclusive reliance on seagrass. In general, invertebrates had lower δ 15N than fish, indicating lower trophic levels. Among fish, δ 15N values reflected well the assumed trophic levels, as species from lower trophic levels had lower δ 15N than species from higher trophic levels. Trophic levels and trophic length were estimated based on δ 15N of invertebrate primary consumers (6.1‰), with results suggesting a food web with four trophic levels. There was also evidence of a high level of diet overlap between fish species, as indicated by similarities in δ 13C for fish species of higher trophic levels. Stable isotope data was also useful to construct a general model for this food web, where five main trophic pathways were identified: one based on both mangrove and microphytobenthos, one on plankton, two on both microphytobenthos and seagrass, and one based mainly on seagrass. This model again suggested the presence of four trophic levels, in agreement with the value calculated based on the difference in δ 15N between invertebrate primary consumers and top piscivores.

Linking terrestrial and aquatic ecosystems: The role of woody habitat in lake food webs

Trees that fall into lakes from riparian forest become habitat for aquatic organisms, and are potentially an important link between terrestrial and aquatic ecosystems. Coarse woody habitat (CWH) promotes the production of benthic invertebrate prey and offers refuge for prey fishes, which are in turn consumed by piscivorous fishes. We used a simulation model to explore responses of an aquatic food web to changes in CWH caused by lakeshore residential development and windstorm, as well as to harvest of adult piscivores. Residential development had a negative effect on fishes, and could lead to extirpation of benthivorous prey fish species. In contrast, pulsed addition of CWH following a windstorm had little effect on the aquatic food web. Our results suggest that CWH is more important as shelter for prey fishes than as a substrate for benthic invertebrate production. However, piscivore harvest can supersede the role of CWH as prey shelter, leading to piscivore collapse and prey persistence even when CWH levels are low enough to promote piscivore dominance. Thus, the effects of lakeshore residential development on fishes can be masked by angler harvest of top piscivores.

Ontogenetic Trophic Interactions and Benthopelagic Coupling in Lake Washington: Evidence from Stable Isotopes and Diet Analysis

AbstractStable isotopes of nitrogen and carbon and stomach content analysis were used to determine the trophic position and relative importance of benthic and pelagic pathways for different life stages and species of the major fishes and invertebrate prey in Lake Washington. Significant coupling of the benthic and pelagic pathways was evident in this complex food web across seasons and fish ontogenies. Among apex predators, cutthroat trout Oncorhynchus clarkii and northern pikeminnow Ptychocheilus oregonensis shifted ontogenetically from benthic omnivory to pelagic piscivory, whereas yellow perch Perca flavescens shifted from pelagic zooplanktivory to benthic piscivory. Apex predators continued to rely on benthic prey seasonally, particularly in winter and spring. Benthic pathways were less important to the current diets of apex predators than they were during the recovery from eutrophication in the 1970s. Surprisingly, the δ15N values for copepods during winter and for zooplanktivorous longfin smelt Spirinchus thaleichthys and threespine sticklebacks Gasterosteus aculeatus were similar to those for top piscivores, whereas the significantly lower values for zooplanktivorous juvenile sockeye salmon O. nerka were more similar to expectations. Nitrogen and carbon isotope ratios of pelagic planktivores and invertebrates also varied seasonally. Mixing model results showed that stable isotopes and stomach contents were comparable for determining ontogenetic trends, but stable isotopes established these trends with many fewer samples and less variability and accurately portrayed ontogenetic trends when few stomach samples were available. However, stomach content analysis was critical in delineating seasonal trends in diets and for identifying specific prey species.

Fish humps in two Colorado River fishes: a morphological response to cyprinid predation?

Extant fishes endemic to the upper Colorado River of the American southwest include only cyprinids and catostomids. A curious attribute in species of both groups is the presence of a large nuchal hump. Largest cyprinid humps occur in humpback chub, Gila cypha, and largest catostomid humps occur in razorback sucker, Xyrauchen texanus. Several authors have suggested the humps confer a hydrodynamic advantage to life in fast flow, but this premise has not been confirmed with experimental work. To test the role of humps in Colorado River fishes, we subjected whole-body casts of preserved specimens with humps and with humps removed to controlled flows in an experimental tank. These tests confirmed that humps increased drag coefficients for X. texanus and G. cypha with no additional lift component. High energetic costs of locomotion and position-holding with a large hump, and the additional metabolic expense of forming large humps, suggest that the humps are not relict structures. Instead, we argue that these large humps represent convergent evolution prompted by predation from a cyprinid piscivore. Colorado pikeminnow, Ptychocheilus lucius, top piscivore in the Colorado River system, is the only native fish capable of consuming large X. texanus and G. cypha, and it also is sympatric with them. However, lack of jaw teeth and a relatively small jaw gape limit the maximum prey size that P. lucius can consume. Based on gape size, about 55% of X. texanus and 71% of G. cypha could be consumed by even the largest P. lucius. However, vulnerability would increase to 73 and 83% respectively if these species did not have humps. Coevolution tends to favor predator defense mechanisms in prey most vulnerable to such a voracious predator. Development of a large nuchal hump provides a deep body that is difficult or impossible for P. lucius to ingest.

Migration supports uneven consumer control in a sewage‐enriched river food web

Summary We studied the response of the St Lawrence River food web to enrichment by primary‐treated sewage from the city of Montreal, Canada. Despite the richness and complexity of this food web, the openness of the system and the limited production possible in this temperate river's short growing season, we found that biomass densities of consumers responded to enrichment according to the qualitative predictions of a simple bioenergetic model. All consumer trophic levels responded to sewage enrichment, but not all members of each trophic level: top piscivores seemed to control small, benthivorous fish but were unable to control larger fish, so the intermediate consumer level became dominated by Catastomidae (suckers). Consequently, the epiphytic prey of small fish were free to respond to sewage enrichment, but the benthic prey of suckers were not. The important elements of complexity seemed to be the factors that determine the realized pattern of feeding linkages: heterogeneity in prey vulnerability (predator–prey size ratios) and predator efficiency (vegetation cover for ambush predators, inaccessibility of epiphytic prey to bottom‐feeding suckers). Because of the openness of the system, the community was able to respond to enrichment by a combination of demographic (production) and behavioural (migration) mechanisms. We argue that the food web model described an energetically favourable state, and that migration provided a ‘shortcut’ for the system to approach this state within a short growing season.

Structure of tropical river food webs revealed by stable isotope ratios

Fish assemblages in tropical river food webs are characterized by high taxonomic diversity, diverse foraging modes, omnivory, and an abundance of detritivores. Feeding links are complex and modified by hydrologic seasonality and system productivity. These properties make it difficult to generalize about feeding relationships and to identify dominant linkages of energy flow. We analyzed the stable carbon and nitrogen isotope ratios of 276 fishes and other food web components living in four Venezuelan rivers that differed in basal food resources to determine 1) whether fish trophic guilds integrated food resources in a predictable fashion, thereby providing similar trophic resolution as individual species, 2) whether food chain length differed with system productivity, and 3) how omnivory and detritivory influenced trophic structure within these food webs. Fishes were grouped into four trophic guilds (herbivores, detritivores/algivores, omnivores, piscivores) based on literature reports and external morphological characteristics. Results of discriminant function analyses showed that isotope data were effective at reclassifying individual fish into their pre‐identified trophic category. Nutrient‐poor, black‐water rivers showed greater compartmentalization in isotope values than more productive rivers, leading to greater reclassification success. In three out of four food webs, omnivores were more often misclassified than other trophic groups, reflecting the diverse food sources they assimilated. When fish δ15N values were used to estimate species position in the trophic hierarchy, top piscivores in nutrient‐poor rivers had higher trophic positions than those in more productive rivers. This was in contrast to our expectation that productive systems would promote longer food chains. Although isotope ratios could not resolve species‐level feeding pathways, they did reveal how top consumers integrate isotopic variability occurring lower in the food web. Top piscivores, regardless of species, had carbon and nitrogen profiles less variable than other trophic groups.