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

Abstract

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.