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Some factors affecting rainbow trout Ginetz, R. M. J.

Abstract

Various aspects of rainbow trout (Salmo gairdneri) predation on migrating sockeye salmon (Oncorhynchus nerka) fry and rainbow trout eggs were studied during 1970-71 in specially-constructed artificial streams, and in the laboratory. Tests involving sockeye fry as prey were conducted near Babine Lake, British Columbia, while those using rainbow trout eggs were done near Abbotsford, British Columbia. Examination of the effects of physical factors such as water velocity, water turbidity, and light intensity on predation on migrating sockeye fry showed mortality to be inversely related to water velocity and water turbidity; inversely related to light intensity at low light levels but directly related at very low levels. Other stream tests showed mortality to be inversely related to the amount of exposure of predators to fry, before the beginning of a nightly fry migration. Exposing predators to abnormal light for varying periods of time, immediately prior to fry migration, reduced over-all mortality during fry migration. Mortality was not proportional to the length of exposure of predators to abnormal light. Fry experience with predators was shown to increase the ability of fry to escape or avoid predation on subsequent predator encounters. Additional experience served to further increase their ability to escape or avoid predators. Conclusions drawn from stream tests and a behavioral study are that experienced fry migrate in a manner rendering them less susceptible to predation and the migration pattern (compact and in mass) is influenced in part, or completely, by encounter and escape from predator-prey interactions experienced earlier. "Handling" or other fright-evoking stimuli appeared to have similar effects. Rainbow trout feeding on colored fish eggs indicated color preference patterns which are influenced by background coloration (color contrast between food and background), and light intensity. Preference was for colors showing the most contrast with the background at a particular light intensity. At low light levels, on a pale-blue background, preference was for lighter colors, while it was for darker colors at high light levels. Mortality differences increased proportionately with contrast between colors. Finally, trout displayed what appears to be a behavioral preference for red, and possibly blue, regardless of surrounding environmental conditions. In a food deprivation study rainbow trout displayed an S-shaped hunger response curve when fed on eggs. Indications were that rainbow trout will feed to gut capacity when given the opportunity. Finally, beyond an upper limit of food deprivation, the amount of food eaten by an individual remains fairly constant.

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