THE DIEL DRIFT OF CHIRONOMIDAE IN AN ARTIFICIAL STREAM AND ITS RELATION TO THE DIET OF COHO SALMON FRY, ONCORHYNCHUS KISUTCH (WAULBAUM)

Abstract

The diel pattern of drifting adults, pupae, pupal exuviae, and larvae of Chironomidae in a stream was established. Emergence was trimodal for the common species (Cricotopus sp. and Psectrocladius sp.), and larval drift-rates increased almost two-fold in darkness. The concurrent diel consumption of Chironomidae by coho fry was examined. The fish appeared not to make use of the peak drift-rates of larvae. Closest temporal correspondence between drift and diet was shown by adults. Forage ratios demonstrated greater consumption of chironomids than would be expected from their proportions in the drift. The order of acceptance was pupae, adults, larvae, and exuviae. High degree of utilization of an item is attributed to flotation and visibility. The major determinants of how much food is taken from the stream are identified as: marginal residence of the fish, the occupation by the fish of slow-water feeding stations, and the visibility of food items.