Comparative functional responses of native and high‐impacting invasive fishes: impact predictions for native prey populations

Comparative functional responses (FRs) can predict impacts of invasive species, including piscivorous fishes, via quantifying their depletion of native food resources as a function of prey density. The utility of FRs for predicting impacts on prey populations by invasive fishes of different trophic guilds was tested here by comparing the FRs of the invaders Cyprinus carpio and Carassius auratus, with three native, trophically analogous fishes, Barbus barbus, Squalius cephalus and Tinca tinca. Chironomid larvae and Gammarus pulex were used as prey items. Predictions, developed from studies on the foraging of C. carpio and C. auratus in the literature, were that the invaders would have significantly higher consumption rates for chironomids than the native fishes, but not for G. pulex. Mean consumption rates for chironomids were significantly lower for both invaders than B. barbus and S. cephalus, but were similar to T. tinca. Barbus barbus had a significantly lower consumption rate of G. pulex than both invaders, but there were no significant differences between S. cephalus, T. tinca and the invaders. All FRs were type II, with FR curves for the invaders preying upon chironomids never being significantly higher than the native fishes, contrary to predictions. For G. pulex, some significant differences were apparent between the invaders and native fishes, but again were contrary to predictions. These results indicated that when predation impacts of invasive fishes could also be a function of their population density and body sizes, these parameters should be incorporated into FR models to improve impact predictions.     

Diel patterns in the feeding and habitat use of O-group fishes in a regulated river: the River Great Ouse, England

Feeding and habitat use behaviour of juvenile O+ fishes, principally Rutilus rutilus (L.) and Leuciscus cephalus (L.) was analysed at 4-h intervals for a period of 24 h on 29 June, 27 July, and 13 September 1994 at a regulated site on the River Great Ouse, UK. Twenty-five random point samples were taken by electrofishing during each sampling period from a 250 m reach of the river. Catch per unit effort decreased at night for both R. rutilus and L. cephalus. The number of prey items in the gut and the number of taxa found also decreased at night except for R. rutilus on one occassion. Habitat use remained near constant, with only slight increases in depth and distance from the bank at night. Use of Nuphar lutea by roach increased at night. Chub tended to avoid Nuphar lutea and were positively associated with emergent macrophyte and algal cover both during the day and night.     

Introduced northern pike predation on salmonids in southcentral Alaska

Northern pike (Esox lucius) are opportunistic predators that can switch to alternative prey species after preferred prey have declined. This trophic adaptability allows invasive pike to have negative effects on aquatic food webs. In Southcentral Alaska, invasive pike are a substantial concern because they have spread to important spawning and rearing habitat for salmonids and are hypothesised to be responsible for recent salmonid declines. We described the relative importance of salmonids and other prey species to pike diets in the Deshka River and Alexander Creek in Southcentral Alaska. Salmonids were once abundant in both rivers, but they are now rare in Alexander Creek. In the Deshka River, we found that juvenile Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) dominated pike diets and that small pike consumed more of these salmonids than large pike. In Alexander Creek, pike diets reflected the distribution of spawning salmonids, which decrease with distance upstream. Although salmonids dominated pike diets in the lowest reach of the stream, Arctic lamprey (Lampetra camtschatica) and slimy sculpin (Cottus cognatus) dominated pike diets in the middle and upper reaches. In both rivers, pike density did not influence diet and pike consumed smaller prey items than predicted by their gape‐width. Our data suggest that (1) juvenile salmonids are a dominant prey item for pike, (2) small pike are the primary consumers of juvenile salmonids and (3) pike consume other native fish species when juvenile salmonids are less abundant. Implications of this trophic adaptability are that invasive pike can continue to increase while driving multiple species to low abundance.     

Trophic plasticity of invasive juvenile largemouth bass Micropterus salmoides in Iberian streams

Biological invasions are a major factor for biodiversity loss, particularly in freshwater environments. Largemouth bass Micropterus salmoides is native to North America and is invasive on the Iberian Peninsula, primarily to provide angling opportunities in reservoirs. However, this species is a threat to the endemic Iberian fauna via predation and competition. Currently, there is little information on largemouth bass in European streams. Thus, we assessed the trophic plasticity and body condition of young largemouth bass in both invasive (the regulated Bullaque River) and native (Murray Creek) streams. Abundance of juvenile largemouth bass, percentage of full stomachs and body condition were higher in Bullaque River. Largemouth bass preyed on benthic invertebrates much more heavily in the Bullaque River, whereas fishes were the most important prey in Murray Creek. Prey richness, diet diversity and trophic niche breadth were higher in the Bullaque River population. Largemouth bass preferred water-column fishes as prey and avoided consuming benthic fishes in Murray Creek, whereas water-column fishes were avoided in Bullaque River. These results demonstrate that largemouth bass display substantial trophic plasticity which possibly facilitates its success as invasive species. Regulated Iberian streams may provide both suitable food and habitat resources with minimal predation pressure, and hence may serve as recruitment sources for this invasive fish.     

Glochidial infestation of fish by the endangered thick‐shelled river mussel Unio crassus

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Individual movements,home ranges and habitat use by native rheophilic cyprinids and non‐native catfish in a large regulated river

The mobility patterns of two native species, barbel, Barbus barbus (L.) and chub, Squalius cephalus (L.), and of one non‐native fish species, the catfish Silurus glanis (L.), were assessed on a 35.5‐km reach of the Upper Rhône River, a strong flowing river with notable thermal regime alterations. An active acoustic tracking technique adapted to large rivers allowed (1) the identification of longitudinal home ranges, movements and preferred habitat at large scale, and (2) the analysis of the influence of discharge and water temperature on the movement patterns of the fish. The active fish‐tracking system recorded 1,572 fish localisations over 7 months on a weekly basis for 80% of the tagged fish (37 barbel, 23 chub and 13 catfish). Compared with the catfish, barbel and chub showed wider longitudinal home ranges, more movements >1 km and higher interindividual variability. The catfish preferred artificially heated habitats with less morphological diversity. The three species were more often localised in river sections with high density of woody debris. The results suggest that habitat degradation is more damaging for cyprinids in large modified rivers, while the catfish seemed less, impacted.     

Glochidial infestation by the endangered mollusc Unio crassus in rivers of north‐eastern France: Phoxinus phoxinus and Cottus gobio as primary fish hosts

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Environmental DNA as a non‐invasive sampling tool to detect the spawning distribution of European anadromous shads (Alosa spp.)

1. Populations of the European shads Alosa alosa (Linnaeus, 1758) and Alosa fallax Lacépède, 1800 (Alosa spp.) are protected under legislation because of their vulnerability to human disturbances. In particular, river impoundments block their upstream migration, preventing access to spawning areas. Knowledge on the spatial extent of their spawning is important for informing conservation and river management plans.
2. Determining the spatial extent of Alosa spp. spawning is challenging. They enter rivers over a 2‐3‐month period and the species potentially migrate different distances upstream. Capture and handling can be problematic, spawning events generally occur at night, and kick‐sampling for eggs is limited to shallow water. Assessing their spatial extent of spawning could, however, incorporate non‐invasive sampling tools, such as environmental DNA (eDNA).
3. An eDNA assay for Alosa spp. was successfully developed, based on the cytochrome c oxidase subunit I gene segment and quantitative polymerase chain reaction (qPCR). Application in spring 2017 to the River Teme (River Severn catchment, western England) revealed high sensitivity in both laboratory and field trials. Field data indicated Alosa spp. spawning between May and June, with migrants mainly restricted to areas downstream of the final impoundment.
4. eDNA can thus be used as a non‐invasive sampling tool to determine the freshwater distribution of these fishes in Europe, enhancing their conservation at local and regional scales.

     

Diet and trophic niche overlap of native and nonnative fishes in the Gila River,USA: implications for native fish conservation

Pilger TJ, Gido KB, Propst DL. Diet and trophic niche overlap of native and nonnative fishes in the Gila River, USA: implications for native fish conservation. Ecology of Freshwater Fish 2010: 19: 300–321. © 2010 John Wiley & Sons A/S Abstract –  The upper Gila River basin is one of the few unimpounded drainage basins west of the Continental Divide, and as such is a stronghold for endemic fishes in the region. Nevertheless, multiple nonindigenous fishes potentially threaten the persistence of native fishes, and little is known of the trophic ecology of either native or nonnative fishes in this system. Gut contents and stable isotopes (¹³C and ¹⁵N) were used to identify trophic relationships, trophic niche overlap and evaluate potential interactions among native and nonnative fishes. Both native and nonnative fishes fed across multiple trophic levels. In general, adult native suckers had lower ¹⁵N signatures and consumed more algae and detritus than smaller native fish, including juvenile suckers. Adult nonnative smallmouth bass (Micropterus dolomieu), yellow bullhead (Ameiurus natalis) and two species of trout preyed on small‐bodied fishes and predaceous aquatic invertebrates leading to significantly higher trophic positions than small and large‐bodied native fishes. Thus, the presence of these nonnative fishes extended community food‐chain lengths by foraging at higher trophic levels. Although predation on juvenile native fishes might threaten persistence of native fishes, the high degree of omnivory suggests that impacts of nonnative predators may be lessened and dependent on environmental variability.     

Río de la Plata voyagers: Deciphering the migration ecology of a vulnerable marine catfish (Genidens barbus) in a large subtropical river (lower Uruguay River)

1. Genidens barbus is a vulnerable marine migrant catfish with low fecundity, a complex life cycle (i.e. mouth breeding), and is the target of industrial and artisanal fisheries of several countries. This species regularly migrates from marine to freshwater environments of the south-western Atlantic.
2. The aim of this work was to delve deeper into the migration ecology of G. barbus, characterizing both its timing and potential environmental drivers. Furthermore, aspects of the population structure and reproduction of migrants in the lower Uruguay River were studied and the presence of juveniles in the adjacent estuarine recruitment area was evaluated.
3. Data from 11 years (2008–2018) of records of adult G. barbus captured by artisanal fisheries were used alongside relevant environmental variables that were recorded monthly. Reproductive biology (i.e. sex ratio, gonadosomatic index, fecundity, and oocyte size) was analysed for a period of 3 years (2016–2018). The juvenile abundance in the Río de la Plata estuary was evaluated seasonally.
4. A total of 935 adult individuals of G. barbus were captured, representing a total biomass of 3,123 kg. The migration timing was from early spring to early summer. The abundance of migrants strongly increased with river discharge, suggesting that this variable regulates the upriver migration. Furthermore, pre- and post-spawn females and males displaying mouth breeding were recorded during the study period, confirming G. barbus reproduction in the lower Uruguay River.
5. The results obtained suggest that G. barbus ascend to spawn in the freshwater environments upstream from the mouth of the Uruguay River. Then, adult males incubate and carry the embryos downstream, releasing juveniles in the Río de la Plata estuary.
6. This relevant information will help with the implementation of effective management polices (e.g. fishing restrictions during the reproductive period) for the presently unregulated fishery of this vulnerable species in the lower Uruguay River.

     

Effects of predator species,vegetation and prey assemblage on prey preferences of predators with emphasis on vulnerability of age‐0 common carp

Abstract Predation is an important force structuring aquatic communities, but predator–prey interactions are complex and regulated by multiple factors. Invasive fishes may interact with native fishes to alter predator–prey preferences and community dynamics. For example, common carp, Cyprinus carpio L., is an invasive species that can become abundant and negatively affect aquatic ecosystems. Juvenile common carp are occasionally found in predator diets, but predator preferences for common carp compared with alternative prey remains unknown. Prey selection and feeding behaviour of five piscivores (flathead catfish, Pylodictis olivaris (Rafinesque); largemouth bass, Micropterus salmoides (Lacepède); smallmouth bass, M. dolomieu Lacepède; walleye, Sander vitreus (Mitchill); and northern pike, Esox lucius L.) foraging on juvenile common carp and two alternative prey (fathead minnow, Pimephales promelas Rafinesque, and yellow perch, Perca flavescens Mitchill) at variable densities and habitats were evaluated. Common carp and fathead minnow were generally selected for or neutrally selected across predator species, habitat types and prey assemblages. By contrast, yellow perch was generally selected against. Common carp were easily captured but difficult to manipulate and ingest compared with other prey. These results reveal that common carp are vulnerable to a variety of predators, suggesting control of this detrimental invader may be possible through biomanipulation.     

Stable isotopes and gut contents indicate differential resource use by coexisting asp (Leuciscus aspius) and pikeperch (Sander lucioperca)

Differential use of habitat and prey resources is an important mechanism that may allow coexistence of sympatric species. Unlike interactions between smaller cyprinid and percid fishes, the resource use by coexisting predatory asp (Leuciscus aspius) and pikeperch (Sander lucioperca) is relatively unknown. Here, gut content and stable isotope analyses were used to study ontogenetic dietary shifts and interspecific trophic niche overlap between asp and pikeperch coexisting in two reservoirs. The hypothesis that both species show an ontogenetic dietary shift from small invertebrates to large fish prey, but at the same time use different prey resources to reduce potential competitive interactions, was validated. The isotopic niches of the two predators showed no, or only a moderate, degree of overlap (0%–65%). The ontogenetic changes in the degree of interspecific isotopic niche overlap were different in the two reservoirs, suggesting that trophic segregation can be dynamic and variable among systems. Gut contents revealed that small (<100 mm standard length) asp consumed mostly terrestrial invertebrates and emerged aquatic insects, whereas small pikeperch foraged on zooplankton, larval and pupal stages of aquatic insects and fish. Larger individuals (>100 mm) of both species were predominantly piscivorous, with asp consuming more cyprinid prey and pikeperch more percid prey. Coexisting asp and pikeperch populations are able to utilise different prey resources, thereby reducing potential negative competitive interactions.     

Hatchery‐reared endangered Colorado pikeminnow (Ptychocheilus lucius) undergo a gradual transition to piscivory after introduction to the wild

1. Disruption of ecosystems by human activities has caused worldwide extinction threats, which has prompted conservationists to implement captive breeding programmes that aid the recovery of imperilled species. Understanding factors that limit the survival of hatchery‐spawned fishes after stocking is critical to future conservation efforts using captive populations.
2. As the size at which juvenile piscivorous fishes shift to consuming other fish can influence their survival, the transition to piscivory by hatchery‐spawned Ptychocheilus lucius Girard, 1856 (Colorado pikeminnow) was investigated after stocking the San Juan River in New Mexico, Colorado, and Utah, USA. The stable isotope ¹⁵N was used to track ontogenetic changes in the trophic position of individuals stocked at age 0, with the expectation that they would become fully piscivorous by age 2. By sampling the isotopic signatures of Colorado pikeminnow across multiple years, the ability of river discharge and densities of fish prey to explain interannual variation in trophic position was also explored.
3. Annual variation in river flow and the densities of fish prey had little predictive power in explaining variation in δ¹⁵N of age‐1 or age‐2 Colorado pikeminnow. After assessing the isotopic signatures of potential prey, a Bayesian isotopic mixing model suggested that invertebrates comprised nearly 25% of the diet of of both age‐1 and age‐2 individuals. The relationship between Colorado pikeminnow size and δ¹⁵N within stocked cohorts indicated that juveniles slowly transitioned to consuming fish prey as they grew, rather than abruptly switching, as indicated from a limited number of dietary studies using wild fish.
4. Together, these results suggest that Colorado pikeminnow stocked into the San Juan River slowly transition to consuming fish prey. If this pattern leads to poor survival and recruitment, the effectiveness of this management action for conservation could be compromised.

     

Rates of piscivory predict pharyngeal jaw morphology in a piscivorous lineage of cichlid fishes

The pharyngeal jaw apparatus is a key innovation hypothesised to increase foraging efficiency and facilitate utilisation of novel resources among teleost fishes. Here, we tested whether dietary characteristics could predict pharyngeal jaw morphology among eight species of Neotropical Crenicichla. Additionally, we tested the hypothesis that pharyngeal jaws may impose a functional constraint on piscivory via pharyngeal gape. We quantified the shape of the lower pharyngeal jaw (LPJ) using linear and geometric morphometrics and quantified diet using the relative volumetric proportions of prey items. We used principal component analysis to describe major axes of variation in LPJ shape and dietary patterns. The major axis of dietary variation significantly predicted LPJ morphology, which was driven by a significant relationship between LPJ shape and rates of piscivory. We also found that rates of piscivory predicted size‐corrected LPJ depth. Size‐corrected pharyngeal gape also significantly predicted rates of piscivory such that pharyngeal jaws may constrain piscivory by limiting pharyngeal gape. Strong form‐to‐function linkage between pharyngeal morphology and trophic patterns suggests an adaptive quality of the pharyngeal apparatus but may also impose functional constraints when consumers must switch prey or when prey availability is temporally or spatially unpredictable.     

Trophic interactions between introduced lake trout (Salvelinus namaycush) and native Arctic charr (S. alpinus) in a large Fennoscandian subarctic lake

Introduced fishes may have major impacts on community structure and ecosystem function due to competitive and predatory interactions with native species. For example, introduced lake trout (Salvelinus namaycush) has been shown to replace native salmonids and induce major trophic cascades in some North American lakes, but few studies have investigated trophic interactions between lake trout and closely related native Arctic charr (S. alpinus) outside the natural distribution of the former species. We used stomach content and stable isotope analyses to investigate trophic interactions between introduced lake trout and native Arctic charr in large subarctic Lake Inarijärvi in northern Finland. Both salmonids had predominantly piscivorous diets at >280 mm total length and were mainly caught from the deep profundal zone. However, lake trout had a more generalist diet and showed higher reliance on littoral prey fish than Arctic charr, whose diet consisted mainly of pelagic planktivorous coregonids. According to length at age and condition data, lake trout showed slightly faster growth but lower condition than Arctic charr. The results indicate that introduced lake trout may to some extent compete with and prey upon native Arctic charr, but currently have only a minor if any impact on native fishes and food web structure in Inarijärvi. Future monitoring is essential to observe potential changes in trophic interactions between lake trout and Arctic charr in Inarijärvi, as well as in other European lakes where the two salmonids currently coexist.     

Episodic predation of mammals by stream fishes in a boreal river basin

Stream‐dwelling fish typically feed on small prey items, such as benthic invertebrates, but maintain the capacity to opportunistically feed on rare, large‐bodied prey when available. However, consumption of particularly large prey is typically viewed as isolated events that are not organised spatially or temporally across watersheds. We assessed the occurrence of small mammals in the stomach contents of rainbow trout (Oncorhynchus mykiss) and Arctic grayling (Thymallus arcticus) over 13 years in the Wood River basin, Alaska (59°34′N, 158°48′W). Shrews (Sorex spp.) were the dominant mammals observed episodically every 2–3 years in the stomach contents of fish. Notably, shrew consumption was correlated in both Arctic grayling and rainbow trout within individual streams, and across the river basin in several subwatersheds. Predators of shrews were usually the largest individuals within each population, suggesting that smaller fish are gape‐limited and that dominant fish monopolised mammal prey. On average, 24% (11–38%) of Arctic grayling >298 mm (fork length) and rainbow trout >290 mm contained mammal prey during peak years. Small mammal populations often cycle every 2–5 years with well‐known functional and numerical effects for terrestrial predators, a dynamic that may be reflected in our 13 years data set of diet contents for aquatic predators. Although numerically infrequent when averaged over time, small mammal subsidies to streams may be episodically important to the energy budgets of long‐lived consumers in freshwater environments.     

Prey selectivity and ontogenetic diet shift of the globally invasive western mosquitofish (Gambusia affinis) in agriculturally impacted streams

The dietary breadth of invaders can influence their success, and having a wide dietary niche can facilitate the spread and survival of invaders under a variety of resource scenarios. The western mosquitofish (Gambusia affinis) is a globally distributed freshwater invasive fish. The spread of G. affinis is associated with agricultural land use, although the trophic role it plays in degraded systems is not well understood. We analysed the invertebrate community in 11 stream reaches in the North Island of New Zealand, in catchments spanning a range (45%–90%) of agricultural land use to determine how prey availability changes with land use. We then analysed the gut contents of 400 G. affinis from the 11 sites to determine how diet varied with prey availability and ontogeny. Invertebrate communities varied along the agricultural land‐use gradient, both in regard to taxonomic richness and community composition. G. affinis consumed a wide variety of food items with invertebrates being the most dominant, in particular Culicidae, Copepods and amphipods were the most commonly consumed invertebrates. There was also an ontogenetic diet shift from microinvertebrates (Cladocera, Copepods and diatoms) to larger invertebrates, including Culicidae, amphipods and terrestrial invertebrates. G. affinis are capable of consuming a wide variety of prey in agricultural streams; their preferred prey are generally pollution‐tolerant taxa commonly found in degraded streams. Having a large level of dietary plasticity coupled with preferring prey that are often associated with degraded systems likely facilitates to the spread of one of the most widely distributed freshwater invasive fish.     

Trophic ecology of speckled peacock bass Cichla temensis Humboldt 1821 in the middle Negro River,Amazon, Brazil

The aim of this study was to analyse the trophic ecology of speckled peacock bass Cichla temensis inhabiting two tributaries of the middle Negro River, the Aracá River and the Demeni River. Using an analysis of stomach contents and stable isotope composition (δ¹⁵N, δ¹³C) of scales, we describe the diet and evaluate the trophic position of subadult and adult individuals. We then test whether diet shifts and trophic positions occurred among successive size classes and among sample locations. The stomach content analysis confirmed the piscivorous feeding habit of the species and showed that the speckled peacock bass preyed on a variety species belonging to different trophic guilds. The length of the ingested prey increased with the size of the speckled peacock bass. Diet composition and trophic position were not different among size classes. δ¹³C values yielded significant shifts among the size classes: larger individuals displayed higher δ¹³C values than smaller individuals. Trophic position varied between locations, with lowest values observed in fish from the Aracá River. This study demonstrated that diet of C. temensis may vary according to the size of the fish, even at the subadult or adult stages, and according to the river/locality, even within a same basin. We then suggest that further studies take into account local availability of food resources to better explore C. temensis diet and tropic ecology.     

Diet and trophic niche of the endangered fish Garra ghorensis in three Jordanian populations

Garra ghorensis is a small riverine cyprinid fish endemic to the southern Dead Sea that is endangered through habitat loss and invasive species. Here, their diet and trophic niche were assessed in three Jordanian populations: an allopatric population, a population sympatric with native Capoeta damascina and a population sympatric with invasive Oreochromis aureus. Stomach content analyses of samples collected between February 2011 and January 2012 revealed that detritus and algae were prominent food items in their diets, with low dietary contributions of animal material. The most frequent and abundant macro‐invertebrates in intestines were Odonata nymphs and gastropod species. The calculation of trophic niche size from the stomach content data revealed that the niche of G. ghorensis (0.10) was generally smaller than sympatric C. damascina (0.24), with an overlap of 72%, whereas they had a larger trophic niche than sympatric O. aureus (0.20–0.13), with a niche overlap of 54%. These outputs were generally supported by stable isotope analyses of δ¹³C and δ¹⁵N completed on samples collected at the end of the 2011 growth season, although these indicated a greater contribution of animal material to assimilated diet. They also indicated that the trophic niche breadth [as standard ellipse area (SEA)] of C. damascina (4.18‰²) was higher than G. ghorensis (2.48‰²) and overlapped by 26%. For G. ghorensis, their SEA was slightly larger than O. aureus (4.33–4.00‰²), with an overlap of 27%. Although both methods indicated some sharing of food resources between sympatric fishes, there was no evidence suggesting detrimental outcomes for G. ghorensis and thus was not considered as a constraint on the status of their populations.     

Ecological processes influencing mortality of juvenile pink salmon (Oncorhynchus gorbuscha) in Prince William Sound,Alaska

Our collaborative work focused on understanding the system of mechanisms influencing the mortality of juvenile pink salmon (Oncorhynchus gorbuscha) in Prince William Sound, Alaska. Coordinated field studies, data analysis and numerical modelling projects were used to identify and explain the mechanisms and their roles in juvenile mortality. In particular, project studies addressed the identification of major fish and bird predators consuming juvenile salmon and the evaluation of three hypotheses linking these losses to (i) alternative prey for predators (prey‐switching hypothesis); (ii) salmon foraging behaviour (refuge‐dispersion hypothesis); and (iii) salmon size and growth (size‐refuge hypothesis). Two facultative planktivorous fishes, Pacific herring (Clupea pallasi) and walleye pollock (Theragra chalcogramma), probably consumed the most juvenile pink salmon each year, although other gadids were also important. Our prey‐switching hypothesis was supported by data indicating that herring and pollock switched to alternative nekton prey, including juvenile salmon, when the biomass of large copepods declined below about 0.2 g m^?3. Model simulations were consistent with these findings, but simulations suggested that a June pteropod bloom also sheltered juvenile salmon from predation. Our refuge‐dispersion hypothesis was supported by data indicating a five‐fold increase in predation losses of juvenile salmon when salmon dispersed from nearshore habitats as the biomass of large copepods declined. Our size‐refuge hypothesis was supported by data indicating that size‐ and growth‐dependent vulnerabilities of salmon to predators were a function of predator and prey sizes and the timing of predation events. Our model simulations offered support for the efficacy of representing ecological processes affecting juvenile fishes as systems of coupled evolution equations representing both spatial distribution and physiological status. Simulations wherein model dimensionality was limited through construction of composite trophic groups reproduced the dominant patterns in salmon survival data. In our study, these composite trophic groups were six key zooplankton taxonomic groups, two categories of adult pelagic fishes, and from six to 12 groups for tagged hatchery‐reared juvenile salmon. Model simulations also suggested the importance of salmon density and predator size as important factors modifying the predation process.