Effects of prey abundance,distribution, visual contrast and morphology on selection by a pelagic piscivore

相似文献   

A comparison of Visual Prey Detection Among Species of Piscivorous Salmonids: Effects of Light and Low Turbidities

Differences in reaction distance to prey fish by piscivorous salmonids can alter predator–prey interactions under different visual conditions. We compared reaction distances of three piscivorous salmonids commonly found in western lakes: cutthroat trout, Oncorhynchus clarki utah, rainbow trout, O. mykiss, and the nonnative lake char, Salvelinus namaycush. Reaction distances to salmonid prey were measured as functions of light and turbidity in a controlled laboratory setting. In addition, predation rates and swimming speeds of lake char preying on juvenile cutthroat trout were measured experimentally under a range of light levels. Reaction distances for cutthroat trout and rainbow trout increased rapidly as light levels increased, reaching relatively constant reaction distances at higher light levels. Reaction distances for lake char were similar to cutthroat trout and rainbow trout at the lower light levels; however, lake char reaction distances continued to increase with increasing light intensity to asymptote at distances 65% higher than those for both cutthroat and rainbow trout. Predation rates by lake char were low for the darkest light levels, increased rapidly under low light levels (0.50–0.75lx), and then declined to an intermediate rate at all higher light levels. Swimming speeds by lake char also increased rapidly from extremely low light conditions to a peak and declined to an intermediate level at light levels above 1.00lx. These results suggest that, above the saturation intensity threshold, piscivorous lake char react to fish prey at greater distances than do cutthroat trout and rainbow trout. These differences may help explain the decline of native trout following the introductions of nonnative lake char in lakes and reservoirs of western North America.     

Comparison of allopatric cutthroat trout stocks with those sympatric with coho salmon and sculpins in small streams

Synopsis Juvenile stocks of allopatric (upstream of barrier falls) cutthroat troutSalmo clarki and those sympatric (downstream of barrier falls) with coho salmonOncorhynchus kisutch and sculpinsCottus spp., were sampled during the late summer period of low flows in six small coastal streams in British Columbia. The objective was to obtain and compare information on pattern of habitat use and fish size distribution of these two trout types. In most instances, density (n m^–2; g m^–2) of cutthroat trout was considerably greater in pools and glides in the allopatric than in the sympatric stocks. The sympatric salmonids were dominated by juvenile coho salmon in pools and cutthroat trout in riffles. Sympatric cutthroat trout constituted from 7 to 45 % of the salmonids present in pools and from 50 to 90% in riffles. Glides were areas of intermediate densities for both salmonids, although coho salmon was the more abundant species in most instances. The density of sculpins was high in all three habitat types, and frequently it exceeded that of coho salmon and cutthroat trout combined. Sympatric cutthroat trout consisted primarily of underyearling fish, whereas allopatric cutthroat trout consisted primarily of two or more age classes with a large proportion of them living in pools. When tested in a laboratory stream both types of cutthroat trout had similar habitat preferences and agonistic behaviours, with the exception that allopatric trout made greater use of cover and defended pools more intensely than sympatric trout when the flow was increased. The results of this study provide insight of potential impact of coho salmon juvenile transplants into stream segments supporting allopatric cutthroat trout.     

Vulnerability to predation and physiological stress responses of experimentally descaled juvenile chinook salmon,Oncorhynchus tshawytscha

Synopsis Juvenile salmonids,Oncorhynchus spp., commonly encounter conditions (e.g., during hatchery release and dam passage) that result in damage to the skin, scale, and slime complex. We conducted laboratory experiments to determine if descaling of juvenile chinook salmon,O. tshawytscha, increased their vulnerability to predation, and to assess the physiological stress responses elicited by descaling. Salmon were experimentally descaled on either 10% or 20% of their total body area. When offered equal numbers of control and descaled juvenile chinook salmon, northern squawfish,Ptychocheilus oregonensis, did not consume significantly more of either prey type (48–60% of consumed prey were descaled). Juvenile chinook salmon descaled on 10% of their body area did show significant physiological stress responses, however. Mean concentrations of plasma cortisol peaked 1 h after descaling, and returned to control levels by 12 h. Plasma glucose peaked 3 h post-treatment and remained elevated for 24 h. Plasma lactate increased immediately following treatment and returned to undisturbed control levels by 3 h. The osmoregulatory response of plasma potassium was highly variable, but plasma sodium decreased immediately and remained low for 24 h. The observed physiological responses suggest that descaling of juvenile chinook salmon could result in decreased resistance to disease and other stressors encountered in the field, possibly leading to reduced performance capacity and lowered survival.     

Ontogenetic variations in the type and size of prey consumed by juvenile coho,Oncorhynchus kisutch,and chinook,O. tshawytscha,salmon

Synopsis Stomach contents of juvenile coho,Oncorhynchus kisutch, and chinook,O. tshawytscha, salmon collected in purse seines off the coast of Washington and Oregon were examined for variations related to predator size. There was a general trend toward increasing consumption of fish with increasing body size, due mainly to the increase in northern anchovy biomass consumed by the larger salmon. Most of the major prey taxa showed significant differences among the size classes examined for both salmon species. There was a direct relationship between predator and prey size for both coho and chinook, but considerable variation was found in prey length consumed within each size class. Prey width did not provide as good a fit as prey length for either species. In general, coho consumed larger fish prey in relation to their body length than chinook but there were substantial differences by month or year of collection.     

Resource utilization by bull char and cutthroat trout in a mountain stream in Montana,U.S.A.

Resource utilization of sympatric populations of bull char,Salvelinus confluentus, and west-slope cutthroat trout,Oncorhynchus clarki lewisi, were studied by underwater observations of foraging behaviour and microhabitat use, and dietary analysis in a mountain stream of the Flathead River Basin, northwest Montana, U.S.A. Nearly 70% of bull char were categorized as benthic foragers, which moved constantly and captured prey primarily from the streambed, while all cutthroat trout were drift foragers, which held relatively fixed focal points in the midwater layers of pools during foraging. The composition of stomach contents was markedly different between the two species. Bull char fed primarily on baetid mayflies captured from the benthos or drift, whereas cutthroat trout ate primarily terrestrial invertebrates. The species also used different microhabitats. Bull char held positions close to the streambed and rarely strayed far from overhead cover, whereas cutthroat trout held focal points farther above the bed and far from overhead cover. Dietary segregation between these two salmonids appeared to result not only from differences in foraging tactics but also in the foraging microhabitats. Resource partitioning is considered to be one of important mechanisms allowing coexistence of these two stream salmonids.     

Annual changes in the population size of the salmon louse Lepeophtheirus salmonis (Copepoda: Caligidae) on high-seas Pacific Salmon (Oncorhynchus spp.), and relationship to host abundance

The population size of the salmon louse, Lepeophtheirus salmonis, was monitored annually in the summers of 1991–1997 by examining six species of Pacific salmon (Oncorhynchus spp.) caught by surface long-lines in oceanic offshore waters of the North Pacific Ocean and Bering Sea. The annual copepod population size on all salmonids caught was estimated by combining the calculated number of copepods carrying on each salmonid species. The copepod population fluctuated markedly from year to year, which resulted largely from marked annual changes in abundance of pink salmon (O. gorbuscha). Since pink salmon were most frequently and heavily infected and since their abundance changed every year, the copepod population was high in the years when this salmonid species was abundant, but low when it was rare. On the contrary, chum salmon (O. keta) did not show high prevalence and intensity of infection, but the annual abundance of this host species was consistently high, i.e. chum salmon carried many copepods every year. Copepods on other salmonid species (sockeye salmon O. nerka, coho salmon O. kisutch, chinook salmon O. tshawytscha, and steelhead trout O. mykiss) constantly formed a small percentage of the total copepod population. Both chum and pink salmon are the most important hosts in terms of their substantial contribution to support the copepod population, but the importance as hosts of each species is definitely different between the species. Chum salmon is a stable important host supporting the copepod population at a relatively high level every year, while the number of copepods on pink salmon annually exhibits marked fluctuations, and this salmonid species is regarded as an unstable important host.     

Diel Habitat Partitioning by Bull Charr and Cutthroat Trout During Fall and Winter in Rocky Mountain Streams

We used underwater observation to determine diel habitat partitioning between bull charr, Salvelinus confluentus, and cutthroat trout, Oncorhynchus clarki, during fall and winter (0.1–8.3°C) in two Rocky Mountain streams that differed in habitat availability. The majority (>70%) of both species emerged from concealment cover at night, though bull charr exhibited a greater tendency for nocturnal behavior than cutthroat trout. Differences in day and night counts were most pronounced at temperatures <3°C, when very few fish of either species were observed in the water column during the day, but both species were common at night. Both species used concealment cover of large woody debris and boulder substrate crevices in deep pools during the day. At night, fish emerged from cover and habitat use shifted to shallow water with low cover. Microhabitat partitioning among species and size classes occurred at night, cutthroat trout moving into shallower, faster water that was farther from cover compared to bull charr. Smaller fish of both species occupied focal positions in slower, shallower water closer to the substrate than larger fish. Large, mixed-species aggregations also were common in beaver ponds both day and night. High variation in diel and site-specific winter habitat use suggests the need for caution in developing habitat suitability criteria for salmonids based solely on daytime observations or on observations from a few sites. Our results support the need to incorporate nocturnal habitat use and partitioning in studies of salmonid ecology.     

Summer and fall microhabitat utilization of juvenile bull trout and cutthroat trout in a wilderness stream, Idaho

Microhabitat use and availability were evaluated and compared between different size classes of juvenile resident bull trout (Salvelinus confluentus) and cutthroat trout (Oncorhynchus clarki) in a small wilderness stream within the South Fork Clearwater River basin, Idaho. The objective was to determine if utilization of measured habitat characteristics changed from summer to late fall. Sampling of fish was conducted with night snorkeling. During the summer, smaller juvenile bull trout (<66 mm) total length (TL) were associated with shallow stream margins over coarse substrates. In the fall, they moved to significantly deeper, lower velocity water, and closer to cover (p<0.05), but maintained their association with coarse substrates. During the summer, larger juvenile bull trout and larger juvenile cutthroat trout (66–130 mm TL) occupied significantly deeper water than smaller juvenile bull trout (p<0.05). Generally, larger juvenile bull trout were found closer to the bottom and in lower velocity water than larger juvenile cutthroat trout (p<0.05). In the fall, larger juvenile bull trout and larger juvenile cutthroat trout were associated with significantly deeper, lower velocity water located closer to cover than in summer (p<0.05). However, cutthroat trout occupied slightly deeper water over finer substrates than bull trout. Deep water with low velocities evidently provide important rearing areas for large bull trout and large cutthroat trout in the fall. Land management practices that maintain such environments will benefit these species.     

Size-structured Interactions between Native and Introduced Species: Can Intraguild Predation Facilitate Invasion by Stream Salmonids?

Dynamics of biological invasions may be complicated in size-structured animal populations. Differences in timing of life history events such as juvenile emergence create complex interaction webs where different life stages of native and non-native species act as predators, competitors, and prey. Stream salmonids are an ideal group for studying these phenomena because they display competition and predation in size-structured populations and have been introduced worldwide. For example, introduced rainbow trout (Oncorhynchus mykiss) are invading streams of Hokkaido Island, Japan and have caused declines in native masu salmon (O. masou) populations. However, age-0 rainbow trout emerge later than age-0 masu salmon and are smaller, which raises the question of why they are able to recruit and therefore invade in the face of a larger competitor. We conducted experiments in laboratory stream channels to test effects of increasing density of age-0 and age-1 rainbow trout on age-0 masu salmon. Age-1 rainbow trout dominated age-0 masu salmon by aggressive interference, relegating them to less favorable foraging positions downstream and reducing their foraging frequency and growth. The age-1 trout also reduced masu salmon survival by predation of about 40% of the individuals overall. In contrast, age-0 rainbow trout had little effect on age-0 masu salmon. Instead, the salmon dominated the age-0 trout by interference competition and reduced their survival by predation of 60% of the individuals. In each case, biotic interactions by the larger species on the smaller were strongly negative due to a combination of interspecific competition and intraguild predation. We predict that together these produce a positive indirect effect in the interaction chain that will allow the recruitment of rainbow trout in the face of competition and predation from age-0 masu salmon, and thereby facilitate their invasion in northern Japan.     

Diet partitioning in sympatric Atlantic salmon and brown trout in streams with contrasting riparian vegetation

Prey intake by Atlantic salmon Salmo salar and brown trout Salmo trutta was measured across different riparian vegetation types: grassland, open canopy deciduous and closed canopy deciduous, in upland streams in County Mayo, Western Ireland. Fishes were collected by electrofishing while invertebrates were sampled from the benthos using a Surber sampler and drifting invertebrates collected in drift traps. Aquatic invertebrates dominated prey numbers in the diets of 0+ year Atlantic salmon and brown trout and 1+ year Atlantic salmon, whereas terrestrial invertebrates were of greater importance for diets of 1+ and 2+ year brown trout. Terrestrial prey biomass was generally greater than aquatic prey for 1+ and 2+ year brown trout across seasons and riparian types. Prey intake was greatest in spring and summer and least in autumn apart from 2+ year brown trout that sustained feeding into autumn. Total prey numbers captured tended to be greater for all age classes in streams with deciduous riparian canopy. Atlantic salmon consumed more aquatic prey and brown trout more terrestrial prey with an ontogenetic increase in prey species richness and diversity. Atlantic salmon and brown trout diets were most similar in summer. Terrestrial invertebrates provided an important energy subsidy particularly for brown trout. In grassland streams, each fish age class was strongly associated with aquatic, mainly benthic invertebrates. In streams with deciduous riparian canopy cover, diet composition partitioned between conspecifics with older brown trout associated with surface drifting terrestrial invertebrates and older Atlantic salmon associated with aquatic invertebrates with a high drift propensity in the water column and 0+ year fish feeding on benthic aquatic invertebrates. Deciduous riparian canopy cover may therefore facilitate vertical partitioning of feeding position within the water column between sympatric Atlantic salmon and brown trout. Implications for riparian management are discussed.     

Comparison of competitive ability between native and introduced salmonids: evidence from pairwise contests

Brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, have been introduced to freshwaters in Hokkaido, Japan. Today, it is recognized that these introduced salmonids have negative impacts on native salmonids such as white-spotted charr, Salvelinus leucomaenis, and masu salmon, O. masou. In particular, interspecific competition may be an important mechanism that could contribute to the exclusion for native salmonids. In this study, experimental pairwise contests were conducted to compare interference competitive ability between native and introduced salmonids. We demonstrated that brown trout were competitively superior to white-spotted charr and masu salmon whereas rainbow trout were superior to white-spotted charr. We suggest that introduced brown trout negatively impact both white-spotted charr and masu salmon, and introduced rainbow trout negatively impact white-spotted charr.     

The most variable vertebrate on Earth

In the search for the most variable non-human vertebrate on Earth, intraspecific variation of ten variable traits was compared among ten highly variable species. Mammals, birds and many reptiles, amphibians and fishes were excluded because most of the variation is among, and not within species. The focus was on northern fishes, where high intraspecific variation is well documented. The ten selected species were European whitefish Coregonus lavaretus, chinook salmon Oncorhyncus tshawytscha, sockeye salmon O. nerka, rainbow trout O. mykiss, atlantic salmon Salmo salar, brown trout S. trutta, arctic charr Salvelinus alpinus, brook charr S. fontinalis, dolly varden charr S. malma and threespine stickleback Gasterosteus aculeatus. Variation included not only size and phenotype, but also ecology, behaviour and life history. The traits were geographic range, migration, habitat, adult size, colour, body form, polymorphism, diet, reproduction and genetics. Arctic charr came on top in the final ranking, followed by dolly varden charr and rainbow trout. The two least variable were chinook salmon and threespine stickleback. It is proposed that arctic charr, which is also the northernmost fish on Earth, has evolved its unique variability in range, size, phenotype, ecology and life history by adapting to the extreme and highly unpredictable ecological conditions of arctic and other northern lakes for many glacial periods.     

Effect of differential gastric evacuation and multispecies prey items on estimates of daily energy intake in juvenile chinook salmon

Synopsis The caloric density of stomach contents in juvenile chinook salmon,Oncorhynchus tshawytscha, was not affected by gastric evacuation, suggesting a constant caloric density of stomach contents during evacuation. Differences in the caloric density of prey consumed did affect caloric density of stomach contents over a 24-h period. Consumption of the amphipodCorophium sp. was associated with reduced caloric densities of stomach contents. During periods whenCorophium contributed more than 4% of the stomach contents, average caloric density declined from 5.56 to 5.33 kcal g^–1. Despite this difference, estimates of daily energy intake of juvenile chinook salmon were only 3%, greater when developed from the mean caloric density of stomach contents excludingCorophium.     

Global introductions of salmon and trout in the genus <Emphasis Type="Italic">Oncorhynchus</Emphasis>: 1870–2007

The purpose of this review is to provide a global perspective on Oncorhynchus salmonine introductions and put-and-take fisheries based on modern stocking programs, with special emphasis on freshwater ecosystems. We survey the global introductions of nine selected salmonines of the genus Oncorhynchus: golden trout, cutthroat trout, pink salmon, chum salmon, coho salmon, masu/cherry salmon, rainbow trout/steelhead, sockeye salmon/kokanee, and chinook salmon. The information is organized on a geographical basis by continent, and then by species and chronology. Two different objectives and associated definitions of ‘success’ for introductions are distinguished: (a) seed introduction: release of individuals with the purpose of creating a wild-reproducing, self-sustaining population; and (b) put-and-take introduction: release of individuals with the purpose of maintaining some level of wild population abundance, regardless of wild reproduction. We identify four major phenomena regarding global salmonine introductions: (1) general inadequacy of documentation regarding introductions; (2) a fundamental disconnect between management actions and ecological consequences of introductions; (3) the importance of global climate change on success of previous and future introductions; and (4) the significance of aquaculture as a key uncertainty in accidental introductions. We conclude this review with a recognition of the need to terminate ongoing stocking programs for introduced salmonines worldwide.     

Interaction for food and space between experimental populations of juvenile coho salmon (Oncorhynchus kisutch) and coastal cutthroat trout (Salmo clarki) in a laboratory stream

Populations of juvenile coho salmon and coastal cutthroat trout frequently cohabit small coastal streams in western North America. The pattern and mechanism of interactions for food and space between experimental populations of underyearlings of these two salmonids were examined at 3, 5 and 13 °C in a laboratory riffe/pool environment simulating winter and summer conditions. When tested separately at 13 °C, their habitat demands were similar and approximately 60–75% of either species occurred in pools. When tested together they segregated, with approximately 75% of coho in pools and up to 63% of cutthroat trout in riffles. In winter, at 3 °C, both species preferred pools and overhead cover, whether tested separately or together. At 5 °C, they partially segregated in a pattern similar to, but far less pronounced than, that in summer. Both species used similar forms of aggressive behaviour, although aggressive displays were more frequently used by coho, while nipping was more frequently used by cutthroat trout. Both salmonids were most aggressive when food was presented, irrespective of season, although coho responded with greater rapidity and intensity to feeding than did cutthroat trout. When tested together in summer, aggressiveness was high for both species, and agonistic interference by coho in pools and cutthroat trout in riffles appeared to largely account for their segregation. At 3 °C, aggression was low and both species weakly defended pools. At 5 °C, their aggression rose considerably and they partially segregated in a pattern resembling that at 13 °C. The mechanism of segregation between these two salmonids is clearly that of Nilsson's interactive type, which presumably functions to attenuate competition when streams are most likely to be resource limiting; typically, that is the late summer period of low flows and relatively high fish population densities.Based on a dissertation in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Institute of Animal Resource Ecology, University of British Columbia, Vancouver, B.C.Ministry of Agriculture and Fisheries, Fisheries Research Division, P.O. Box 8324, Riccarton Christchurch, New Zealand     

The potential influence of Atlantic salmon Salmo salar and brown trout Salmo trutta on density and breeding of the white‐throated dipper Cinclus cinclus

Interactions between birds and fish are often overlooked in aquatic ecosystems. We studied the influence of Atlantic salmon and brown trout on the breeding population size and reproductive output of the white‐throated dipper in a Norwegian river. Acidic precipitation led to the extinction of salmon, but salmon recolonized after liming was initiated in 1991. We compared the dipper population size and reproductive output before (1978–1992) and after (1993–2014) salmon recolonization. Despite a rapid and substantial increase in juvenile salmon, the breeding dipper population size and reproductive output were not influenced by juvenile salmon, trout, or total salmonid density. This might be due to different feeding strategies in salmonids and dippers, where salmonids are mainly feeding on drift, while the dipper is a benthic feeder. The correlation between the size of the dipper population upstream and downstream of a salmonid migratory barrier was similar before and after recolonization, indicating that the downstream territories were not less attractive after the recolonization of salmon. Upstream dipper breeding success rates declined before the recolonization event and increased after, indicating improved water quality due to liming, and increasing invertebrate prey abundances and biodiversity. Surprisingly, upstream the migratory barrier, juvenile trout had a weak positive effect on the dipper population size, indicating that dippers may prey upon small trout. It is possible that wider downstream reaches might have higher abundances of alternative food, rending juvenile trout unimportant as prey. Abiotic factors such as winter temperatures and acidic precipitation with subsequent liming, potentially mediated by prey abundance, seem to play the most important role in the life history of the dipper.     

Decline of the Migratory Form in Bull Charr, Salvelinus Confluentus, and Implications for Conservation

Large-bodied, migratory life history forms of bull charr, Salvelinus confluentus, were historically abundant in northwestern North America, but many remaining populations of this now-threatened species presently persist as small-bodied residents isolated in headwater streams. We examined whether the migratory form has been lost from headwater populations of bull charr and their potential for re-establishment. Upstream and downstream movement of bull charr and other salmonids from three tributary populations in the Bitterroot River drainage, Montana, was measured with weirs over a 17-month period. The migratory life history was rare or absent in two tributaries but still present at a low level in a third. In contrast, substantial numbers (n = 1745) of juvenile and adults of other salmonids (brown trout, Salmo trutta, cutthroat trout, Oncorhynchus clarki, and mountain whitefish, Prosopium williamsoni) were captured near tributary mouths, indicating a migratory life history was common in other species. Apparent decline of the migratory life history in bull charr was not directly related to damming suggesting other downstream mortality factors (predation, temperature) also are involved. Isolated, nonmigratory forms have increased risk of extinction, and restoration of the population connectivity via the re-establishment of migratory stocks is an important conservation goal for bull charr recovery. However, the factors governing migratory tendency remain unclear.     

A benthic predatory fish does not cause selection on armour traits in three‐spined stickleback Gasterosteus aculeatus (Gasterosteiformes: Gasterosteidae)

Predation can promote divergence between prey populations and contribute to ecological speciation. In theory, predators can also constrain prey population divergence. In coastal British Columbia, Canada, Gasterosteus aculeatus (three‐spined stickleback) species pairs only occur in lakes with a single species of predatory fish: Oncorhynchus clarkii (the cutthroat trout). Similar lakes containing additional predatory fish species (Cottus asper, prickly sculpins; Oncorhynchus mykiss, rainbow trout) contain only single species of morphologically intermediate stickleback, suggesting that these predators prevent the coexistence of stickleback species pairs. We conducted a mesocosm experiment to investigate how prickly sculpins might constrain divergence, by quantifying their impact on survival and natural selection on antipredator (armour) traits in F₂ stickleback from a cross between ecologically divergent populations. We tested three hypotheses: (1) sculpin predation on sticklebacks reduces survival in a way that could result in their exclusion from certain niches; (2) sculpins compete with stickleback; (3) sculpins respond to prey vulnerabilities in similar ways to cutthroat trout, tending to constrain rather than to enhance divergence. We found that sculpins significantly reduce stickleback survival, that their presence per se does not reduce growth in stickleback, and that predation did not result in selection on any of the armour traits measured, or on gill raker length, which is an important trophic trait. These results tend to refute hypotheses (2) and (3), while supporting hypothesis (1). © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 877–885.     

Linking piscivory to spatial–temporal distributions of pelagic prey fishes with a visual foraging model

A visual foraging model (VFM) used light-dependent reaction distance and capture success functions to link observed prey fish abundance and distribution to predation rates and the foraging performance of piscivorous cutthroat trout Oncorhynchus clarki in Lake Washington (WA, U.S.A.). Total prey density did not correlate with predation potential estimated by the foraging model for cutthroat trout because prey were rarely distributed in optically favourable conditions for detection. Predictions of the depth-specific distribution and timing of cutthroat trout foraging were qualitatively similar to diel stomach fullness patterns observed in field samples. Nocturnal foraging accounted for 34–64% of all prey fish consumption in simulations for 2002 and 2003. Urban light contamination increased the access of nocturnally foraging cutthroat trout to vertically migrating prey fishes. These results suggest that VFMs are useful tools for converting observed prey fish density into predictions of predator consumptions and behavioural responses of predators to environmental change.