Short-term cues used by foraging trout in a California stream

Stream salmonids choose foraging locations to maximize the energy benefit of foraging within the constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use a search process to monitor changing habitat conditions as a means of locating potentially-better foraging locations. I explored the cues used by the cutthroat trout, Oncorhynchus clarki clarki, when searching for food at the pool scale by artificially increasing prey availability at different locations by using special feeders and by manipulating pool velocities. Behavior of individually marked fish was monitored from stream bank platforms under unmanipulated control conditions and under seven experimental sets of conditions involving different combinations of feeder location and velocity manipulation. Under natural conditions fish elected to forage in the deepest (>50 cm), fastest (0.10–0.25 m s⁻¹) locations and within 1 m of structure cover, but would readily move to shallower (<30 cm) water away from cover if velocities were manipulated to be highest there. Although fish did not locate feeders unless they were placed in high-velocity areas, when high velocity was provided fish would move into very shallow water (<20 cm) if prey were delivered there. Responses of individual trout to manipulations indicated that water velocity was the main physical cue used by fish to decide where to forage, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicated fish were not “perfect searchers” that could quickly locate new food resources over short time scales, even when the new resources were within a few meters of the fish’s normal foraging location. When given the correct cues, however, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the ten-day observation period was common, consistent with the idea that trout used movement as a means of exploring and learning about habitat conditions at the reach scale.     

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

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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.     

Foraging modes of stream benthic fishes in relation to their predation effects on local prey density

Habitat use and foraging behavior of two benthic insectivorous gobies, Rhinogobius sp. CO (cobalt type) and Rhinogobius sp. DA (dark type), were examined in relation to their predation effects on local prey density in a small coastal stream in southwestern Shikoku, Japan. Correlations among the foraging range, frequency of foraging attempts and current velocity indicated that individuals using fast-current habitats had small foraging ranges and infrequently made foraging attempts while those in slow currents frequently foraged over large areas. The former and the latter were recognized as ambush and wandering foragers, respectively. Interspecific comparisons of habitat use, foraging behavior and prey preference suggested that Rhinogobius sp. CO selectively forage mobile prey by ambushing in fast currents, whereas Rhinogobius sp. DA randomly forage available prey by wandering in slow-current habitats. A cage experiment was conducted to assess prey immigration rate and the degree of predation effects on local prey density in relation to current velocity. The results of the experiment support, at least in part, our initial predictions: (1) prey immigration rates increase with current velocity and (2) the effects of fish predation on local prey density are reduced as current velocity increases. Overall results illustrated a link between the foraging modes of the stream gobies and their predation effects on local prey density: fish adopt ambush foraging in fast currents, where the decrease in prey density tends to be less, whereas fish actively forage over large areas in slow currents, where the decrease in prey is relatively large.     

Can replacement of native by non‐native trout alter stream‐riparian food webs?

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The effects of turbidity and vegetation on the risk of juvenile salmonids, Oncorhynchus spp., to predation by adult cutthroat trout, O. clarkii

Synopsis During their seaward migration, juvenile salmonids encounter structural and visual cover which varies between and within watersheds. In this study, the effects of two types of cover (turbidity and artificial vegetation) on the predation mortality of juvenile salmonids exposed to fish piscivores was investigated in outdoor concrete ponds. During experiments, adult coastal cutthroat trout, Oncorhynchus clarkii clarkii, were allowed to feed on juvenile salmonid prey — chinook salmon, O. tshawytscha, chum salmon, O. keta, sockeye salmon, O. nerka, and cutthroat trout — in separate trials. Daily instantaneous per capita predation rate was determined for each turbidity and vegetation treatment, within each trial. Mean predation rates varied between 1% and 76% daily. In the presence of cover, mean daily predation rates were 10–75% lower than those in controls (no vegetation and clear water), depending on prey species. Predation rates were significantly lower in the presence of vegetation cover and did not covary with prey size or species. The effects of turbidity were generally not significant and were not additive with the effects of vegetation. However, turbidity appeared to significantly reduce the effectiveness of vegetation as cover for juvenile chinook and sockeye salmon. We suggest that these two forms of cover do not affect risk of predation by fish piscivores to juvenile salmonids via the same mechanism.     

Species replacement by a nonnative salmonid alters ecosystem function by reducing prey subsidies that support riparian spiders

Replacement of a native species by a nonnative can have strong effects on ecosystem function, such as altering nutrient cycling or disturbance frequency. Replacements may cause shifts in ecosystem function because nonnatives establish at different biomass, or because they differ from native species in traits like foraging behavior. However, no studies have compared effects of wholesale replacement of a native by a nonnative species on subsidies that support consumers in adjacent habitats, nor quantified the magnitude of these effects. We examined whether streams invaded by nonnative brook trout (Salvelinus fontinalis) in two regions of the Rocky Mountains, USA, produced fewer emerging adult aquatic insects compared to paired streams with native cutthroat trout (Oncorhynchus clarkii), and whether riparian spiders that depend on these prey were less abundant along streams with lower total insect emergence. As predicted, emergence density was 36% lower from streams with the nonnative fish. Biomass of brook trout was higher than the cutthroat trout they replaced, but even after accounting for this difference, emergence was 24% lower from brook trout streams. More riparian spiders were counted along streams with greater total emergence across the water surface. Based on these results, we predicted that brook trout replacement would result in 6–20% fewer spiders in the two regions. When brook trout replace cutthroat trout, they reduce cross-habitat resource subsidies and alter ecosystem function in stream-riparian food webs, not only owing to increased biomass but also because traits apparently differ from native cutthroat trout.     

Prey selection by experienced and naive juvenile Atlantic salmon

Both in foraging groups and in a sequential prey encounter context, learning had a visible effect on the pattern of selection for three live prey types ( Ecdyonurus larvae, Hydropsyche larvae, and Gammarus ) by juvenile Atlantic salmon Salmo salar . Compared to wild-caught fish, naive, hatchery-reared fish that had not been exposed to natural prey ate Hydropsyche larvae in a remarkably low proportion, and consumed a higher proportion of Gammarus. Ecdyonurus experienced a high and rather steady predation rate across the experience gradient, but after a short period of experience with live prey the consumption rate for Hydropsyche increased drastically, and that of Gammarus decreased, matching the selection pattern exhibited by wild fish. Individual fish offered prey in a sequential encounter context increased consumption rates of all the prey types as they gained experience, but the improvement was higher for the prey that were less consumed initially. Fish became more selective as they approached satiation, conforming to the prediction of optimal foraging theory that higher predator's energy requirements, as well as low food availability, result in reduced selectivity. The results also suggest that fish from distinct populations can differ in the degree of diet selectivity according to their energetic requirements for growth. The fast learning response of Atlantic salmon parr towards novel prey probably allows fish to maintain a high foraging efficiency when faced with frequent changes in the availability of different prey types.     

The effects of prey relative abundance and chemical cues on prey selection in rainbow trout

An optimal foraging model was used to predict prey selection based on both energy maximization and number maximization strategies. The influence of chemical cues and relative abundance on rainbow trout diet selection was examined under laboratory conditions.
In most fish, diet composition was strongly influenced by chemical cues. No fish followed an energy maximization strategy, and selection of prey based on taste persisted despite large caloric penalties associated with these choices. In the absence of chemical cues, diet composition was based on relative abundance of prey (a number maximization strategy). Within the feasible constraints of the optimal foraging model lie a large number of possible diet combinations which would provide sufficient energy for growth and reproduction. This provides a wide scope for feeding flexibility. Response to prey chemical cues may be the basis for observations of individual diet variability in trout.     

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.     

Effects of biotic and abiotic factors on the distribution of trout and salmon along a longitudinal stream gradient

Synopsis We examined the influence of biotic and abiotic factors on the distribution, abundance, and condition of salmonid fishes along a stream gradient. We observed a longitudinal change in fish distribution with native cutthroat trout, Oncorhynchus clarki utah, and introduced brown trout, Salmo trutta, demonstrating a distinct pattern of allopatry. Cutthroat trout dominated high elevation reaches, while reaches at lower elevations were dominated by brown trout. A transition zone between these populations was associated with lower total trout abundance, consistent changes in temperature and discharge, and differences in dietary preference. Variation in cutthroat trout abundance was best explained by a model including the abundance of brown trout and diel temperature, whereas variation in brown trout abundance was best explained by a model including the abundance of cutthroat trout and discharge. These results suggest the potential for condition-mediated competition between the two species. The results from our study can aid biologists in prioritizing conservation activities and in developing robust management strategies for cutthroat trout.     

Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone

The loss of aquatic subsidies such as spawning salmonids is known to threaten a number of terrestrial predators, but the effects on alternative prey species are poorly understood. At the heart of the Greater Yellowstone ecosystem, an invasion of lake trout has driven a dramatic decline of native cutthroat trout that migrate up the shallow tributaries of Yellowstone Lake to spawn each spring. We explore whether this decline has amplified the effect of a generalist consumer, the grizzly bear, on populations of migratory elk that summer inside Yellowstone National Park (YNP). Recent studies of bear diets and elk populations indicate that the decline in cutthroat trout has contributed to increased predation by grizzly bears on the calves of migratory elk. Additionally, a demographic model that incorporates the increase in predation suggests that the magnitude of this diet shift has been sufficient to reduce elk calf recruitment (4–16%) and population growth (2–11%). The disruption of this aquatic–terrestrial linkage could permanently alter native species interactions in YNP. Although many recent ecological changes in YNP have been attributed to the recovery of large carnivores—particularly wolves—our work highlights a growing role of human impacts on the foraging behaviour of grizzly bears.     

Effects of prey density and spatial distribution on prey consumption of the adult predatory ladybird beetle

The effects of prey density and spatial distribution on prey consumption of the adult predatory ladybird, Harmonia axyridis , were investigated by using a 2 × 2 factorial design in large scale cages. Prey density influenced prey consumption of the ladybirds, and the frequency with which predation occurred was quite different between the prey distributions. The ladybirds consumed a relatively constant and small number of aphids when the prey were uniformly distributed, whereas the number of prey consumed per day when predation occurred was large and much more variable when the prey were contagiously distributed. At high prey density, the number of prey consumed was highest during the first day of the experiment; thereafter, only 10–20 aphids were consumed during the following 3 days. However, these patterns of prey consumption were not observed at low prey density. The percentage of aphids that remained on the host plants when the experiments were terminated was higher at low prey density than at high prey density, suggesting that predator foraging efficiency at low prey density was lower than at high prey density. Ladybirds foraging for high prey density were more frequently observed on the plants with aphids than ladybirds foraging for low prey density. Prey distribution also influenced the frequency of residence of ladybirds on the plants. The different predation patterns observed in the two spatial distributions, in which prey consumption was much more variable for the contagious distribution, might be explained by the difference in prey encounter rate of the predator between the distributions. This study indicated that the ladybirds had limited ability to search out prey over large spatial scales.     

A new look at optimal foraging behaviour; rule of thumb in the rainbow trout

A realistic model of foraging behaviour in stream-living rainbow trout was developed to address some shortcomings of previous tests of optimal foraging theory. The model predicts prey selection based upon both energy-maximization and number-maximization strategies. Trout diet selection on the basis of prey size and prey calorific content was examined under laboratory conditions.
Diet composition was strongly influenced by relative abundance of prey types under all feeding regimes, as predicted by the model. Additional important factors include fish stomach capacity and daily digestive capacity. Experimental results strongly suggest that prey selection in rainbow trout is not based on calorific content. Examination of energy (kJ) obtained from observed diets showed no energy penalties associated with 'suboptimal' food choices, since all fish (regardless of their choices) consumed far more energy than required for their basic metabolic needs.     

Foraging time and spatial patterns of predation in experimental populations

Summary Responses of the predaceous mites Phytoseiulus persimilis, Typhlodromus (=Metaseiulus) occidentalis, and Amblyseius andersoni to spatial variation in egg density of the phytophagous mite, Tetranychus urticae, were studied in the laboratory.The oligophagous predator P. persimilis showed initially a direct density dependent foraging time allocation and variation in foraging time increased with prey density. With changes in prey density due to predation, predator foraging rates (per hour) decreased with time and density dependent foraging gradually became density independence, because P. persimilis continued to respond to initial prey density, instead of the changing prey density and distribution. The consequent spatial pattern of predation by P. persimilis was density independent, although slopes of predation rate-prey density regressions increased with time.Compared with P. persimilis, the narrowly polyphagous predator T. occidentalis responded relatively slowly to the the presence or absence of prey eggs but not to prey density: the mean and variation of foraging time spent in patches with prey did not differ with prey density, but was significantly greater in patches with prey eggs than in patches without eggs. Prey density and distribution changed only slightly due to predation and overall foraging rates remained more or less constant. The consequent spatial pattern of predation by T. occidentalis was inversely density dependent. As with P. persimilis, slopes of predation rate-prey density regressions increased with time (i.e. the inverse density dependence in T. occidentalis became weaker through time).The broadly polyphagous predator A. andersoni showed density independent foraging time allocation with variation independent of prey density. With changes in prey density over time due to prey depletion, overall foraging rates decreased. The consequent spatial pattern of predation by A. andersoni also changed through time; it initially was inversely density dependent, but soon became density independent.Overall, P. persimilis and T. occidentalis spent more time in prey patches than A. andersoni, suggesting that A. andersoni tended to spend more time moving outside patches. The overall predation rates and searching efficiency were higher in P. persimilis than in A. andersoni and T. occidentalis. Predator reproduction was highest in P. persimilis, lower in T. occidentalis and the lowest A. andersoni.The differences in response to prey distribution among the three predaceous species probably reflect the evolution of these species in environments with different patterns of prey distribution. The degree of polyphagy is a major determinant of the aggregative response, but other attributes such as handling time are also important in other aspects of phytoseiid foraging behavior (e.g. searching efficiency or predation rate).     

Modelling competition and hybridization between native cutthroat trout and nonnative rainbow and hybrid trout

Native salmonid fish have been displaced worldwide by nonnatives through hybridization, competition, and predation, but the dynamics of these factors are poorly understood. We apply stochastic Lotka-Volterra models to the displacement of cutthroat trout by rainbow/hybrid trout in the Snake River, Idaho, USA. Cutthroat trout are susceptible to hybridization in the river but are reproductively isolated in tributaries via removal of migratory rainbow/hybrid spawners at weirs. Based on information-theoretic analysis, population data provide evidence that hybridization was the primary mechanism for cutthroat trout displacement in the first 17 years of the invasion. However, under some parameter values, the data provide evidence for a model in which interaction occurs among fish from both river and tributary subpopulations. This situation is likely to occur when tributary-spawned cutthroat trout out-migrate to the river as fry. The resulting competition with rainbow/hybrid trout can result in the extinction of cutthroat trout even when reproductive segregation is maintained.     

Modelling competition and hybridization between native cutthroat trout and nonnative rainbow and hybrid trout

Native salmonid fish have been displaced worldwide by nonnatives through hybridization, competition, and predation, but the dynamics of these factors are poorly understood. We apply stochastic Lotka–Volterra models to the displacement of cutthroat trout by rainbow/hybrid trout in the Snake River, Idaho, USA. Cutthroat trout are susceptible to hybridization in the river but are reproductively isolated in tributaries via removal of migratory rainbow/hybrid spawners at weirs. Based on information-theoretic analysis, population data provide evidence that hybridization was the primary mechanism for cutthroat trout displacement in the first 17 years of the invasion. However, under some parameter values, the data provide evidence for a model in which interaction occurs among fish from both river and tributary subpopulations. This situation is likely to occur when tributary-spawned cutthroat trout out-migrate to the river as fry. The resulting competition with rainbow/hybrid trout can result in the extinction of cutthroat trout even when reproductive segregation is maintained.     

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.     

Why don't predators have positive effects on prey populations?

Summary Three mechanisms by which increasing predation can increase prey population density are discussed: (1) Additional predation on species which have negative effects on the prey; (2) Predation on consumer species whose relationship with their own prey is characterized by a unimodal prey isocline; (3) Predation on species which adaptively balance predation risk and food intake while foraging. Possible reasons are discussed for the rarity of positive effects in previous predator-manipulation studies; these include the short-term nature of experiments, the large magnitudes of predator density manipulation, and various sources of bias in choice of system and interpretation of results.     

Effects of ice cover on the diel behaviour and ventilation rate of juvenile brown trout

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