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An experimental study of some visually released behaviour patterns in young coho salmon and Kamloops trout Stringer, George Everett

Abstract

Coho underyearlings settle toward the bottom when illumination decreases. The critical intensity for this response was found to be approximately 1 foot candle. A study of the nipping phenomenon in coho and kamloops trout revealed that coho nip more Intensively than trout in a homotypic group. However; in a heterotypic group of equal numbers, trout nip more readily. In a heterotypic group coho nip less frequently and show a preference to nip other coho. By comparison, the nipping Intensity of trout is not reduced and they nip either species equally. Factors affecting nipping are size, color and light intensity. In a group of coho or trout, there is a marked tendency for the larger members to nip the smaller. Red and orange colors are least effective in eliciting a nipping response. Light intensity changes between 4- and 12 foot candles have no significant effect; however, below k foot candles nipping declines rapidly as illumination is decreased. The social releaser for nipping is movement but size and color are important components of the releaser. Additional patterns of behavior have been described for trout, namely, "threatening" and fighting.

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