Behavioural interaction and habitat use in juvenile chinook, Oncorhynchus tshawytscha, and coho, O. kissutch, salmon

Abstract

Young anadromous chinook salmon and coho salmon reside sympatrically in many streams and rivers tributary to the North Pacific Ocean. This study tested the hypothesis that behavioural domination by coho salmon, which reside in freshwater for at least 1 year before seaward migration, promotes variability in habitat use and in the duration of freshwater residence by chinook salmon, which migrate seaward in their first year of life (‘ocean-type’) or after a year or more in freshwater (‘stream-type’ chinook). In laboratory stream channels coho behaviourally dominated chinook; they spent more time attacking chinook than vice versa and numerically dominated upstream sections of the channels where food was introduced. Principal components analysis was used to study habitat use by coho and chinook in four natural streams. In two streams where they were sympatric, coho and chinook used different habitats: coho used slow water, deep ‘pool’ areas while chinook used faster water, shallow ‘riffle’ areas. In two streams where chinook were allopatric, however, they made greater use of pool habitats than when in sympatry. Coho and chinook introduced into a controlled-flow stream channel also preferred pool and riffle habitats respectively. The differences in habitat use in the stream channel were probably the result of species-specific differences in habitat preference and not behavioural interaction because chinook made greater use of riffle habitats whether they were introduced alone or with coho. Co-introduction with coho had no significant effect on the degree of emigration by chinook from the stream channel compared to chinook introduced alone at the same density. While coho may socially dominate chinook in simple poollike habitats, differences in habitat preference between the species that have developed during sympatric evolution probably minimize the extent to which coho influence the duration of freshwater residence by chinook.