MICROHABITAT-SPECIFIC FORAGING BEHAVIOR, DIET, AND GROWTH OF JUVENILE COHO SALMON

Observations of behavior were made on marked juvenile coho salmon Oncorhynchus kisutch in pools in Huckleberry Creek, Washington. Fish exhibited two distinct types of forage behavior during observations made from May to August 1988 and from June to August 1989. Some coho salmon formed dominance hierarchies wherein fish defended forage stations in ranks determined by an individual's ability or willingness to defend access to drifting food. Other coho salmon foraged as nonhierarchical floaters. Eight percent of marked individuals changed their foraging behavior class during their first summer. Focal point velocity (FPV) differentiated specific microhabitats used by these two foraging behavior classes. Coho salmon in dominance hierarchies (FPV > 0.06 m/s) fed most frequently on drifting invertebrates, which contributed 81% of their energy intake. Floaters (FPV less-than-or-equal-to 0.06 m/s) fed by patrolling large forage arenas, where they fed opportunistically on items delivered by aerial drop and instream drift. Energy contribution from large aerially delivered food items represented 69% of floaters' total energy intake. Dominant fish in the hierarchy grew faster than subdominants and subdominants grew faster than floaters. Growth rates in specific microhabitats were directly related to relative food availabilities in the different foraging behavior classes.