The object of this study was to test the hypothesis that prey abundance in large part regulates dispersion, territory size and aggressive behavior of young rainbow trout during the stream rearing phase of their life history. In laboratory test channels, where age 0+ fry were introduced into channels receiving three different amounts of prey and permitted to emigrate voluntarily, density of fry remained highest at the highest prey level. Also, the distribution of fry was positively associated with a gradient in prey abundance. Both territory size and frequency of aggressive encounter varied inversely with prey level; the higher the prey level, the smaller the territory and the lower the frequency of aggressive encounter. Emigration from the test channels was neither as rapid nor as marked when prey level was abruptly reduced, compared to when fry were initially introduced to the different prey levels. However, frequency of aggressive encounter significantly increased when the prey level was decreased and significantly decreased when the prey was increased. In Loon Outlet Creek, the abundance of prey was positively associated with summer fish biomass in one study section, while a positive association was not apparent for a second study section. The second, located closer to Loon Lake, on the average tended to have throughout the summer a higher fish biomass and higher prey density. In addition, the spring lakeward migration of juvenile rainbow trout was negatively correlated with prey density and positively correlated with temperature. It is suggested that in the natural stream habitat, the density of fry and juvenile rainbow trout is strongly influenced by prey density especially when associated with the metabolic effects of temperature and fish size. / Science, Faculty of / Zoology, Department of / Graduate

The predatory behaviour of rainbow trout was studied to identify some of the major factors that influence their response to prey. Two benthic-living amphipods Cranqonyx sp. and Hyalella sp. were selected as representative prey. In some experiments, artificial food was utilized. Rainbow trout adopt a searching position some 10 to 15 cm from a substrate and locate food visually. As a result, they can detect only organisms that are exposed. In the presence of a complex substrate, trout were able to recognize moving prey with greater success (74%) than stationary targets (39%) with the same visual characteristics. The distance from which trout will react to food was shown to be dependent upon the size, inherent contrast and activity of the object as well as the ambient illumination, turbidity of the water and complexity of the substrate. After 6 to 7 days of experience with a new but palatable food, trout can increase their reactive distance through learning. A general system of equations was developed to describe the effect of each of these parameters on reactive distance. On the average, trout successfully capture 82% of the prey they attack. In the laboratory, the rate of capture reached a maximum level when the density of prey was increased to 240 animals per sq. m. Irrespective of the abundance of food, however, decreasing hunger motivation was found to depress the predator's rate of capture as was the presence of a substrate in which the prey could conceal themselves. The effect of water temperature on the vertical and horizontal movements of Cranqonyx and Hyalella was also examined. The vertical activity of both prey increased exponentially with a rise in temperature. In contrast, 10° C. was suggested to be the optimum temperature for the movement of exposed animals. A general simulation model was developed to test the hypothesis that the selective exploitation of 4 major invertebrate groups in Marion Lake, by trout, occurs at the perceptual level. The model considered the predatory behaviour of the fish as well as the density and physical characteristics of their prey, and was able to predict with some accuracy the occurrence of different foods in trout stomachs. The model was also able to account for the size -selective exploitation of Cranqonyx and Hyalella, the seasonal changes in the vulnerability of these species, and the fact that the less numerous Cranqonyx was captured just as frequently as Hyalella. Trout require a threshold rate of capture (about 2 captures / min.) to maintain a specific pattern of search. If they do not attain this threshold they will switch their attention to other hunting patterns. As a result, the population should converge, temporarily, into areas in which food is relatively more abundant. Since trout can also learn to increase their responsiveness to prey, both of these characteristics would improve their hunting efficiency. The results of this study indicate that visual predators will discover and, subsequently may exploit, large prey that tend to be exposed and active, with greater success than smaller, less active or less conspicuous species. Moreover, if a visual predator maintains a searching position, it may not detect benthic-living food organisms less than a critical size. The significance of these conclusions is discussed. / Science, Faculty of / Zoology, Department of / Graduate

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