Use of benthic prey by salmonids under turbid conditions in a laboratory stream

Abstract

The negative effect of turbidity on the reactive distance of salmonids has been well established. However, determining the consequences of this relationship for overall feeding success remains problematic, as successful foraging by salmonids across a broad range in turbidity has been observed under a variety of conditions. Previous laboratory and field observations suggest that benthic feeding by salmonids in flowing water affects the turbidity dependence of foraging success. Two experiments were conducted in a laboratory stream to quantify benthic feeding success of salmonids across turbidity treatments ranging from 0 to 400 nephelometric turbidity units (NTU). In one experiment, cutthroat trout Oncorhynchus clarkii and coho salmon O. kisutch were offered the same nonliving prey in the drift and on the stream bottom; in the second experiment, cutthroat trout were offered only live oligochaetes moving along the stream bottom. In the first experiment, benthic feeding success of both cutthroat trout and coho salmon at 100 NTU was at least 70% of their feeding performance in clear water (i.e., 0 NTU), whereas neither species fed at 400 NTU. In the second experiment, benthic feeding success of cutthroat trout at 150 NTU was about 35% of their performance in clear water but dropped to near 0% at 200 NTU; no feeding was observed at 400 NTU. Passive integrated transponder tag detections suggested that cutthroat trout activity increased over the range of 0–150 NTU. Although drift and benthic foraging success decreased over the examined turbidity range, the results suggest that both cutthroat trout and coho salmon are capable of feeding from the benthos in relatively turbid, flowing water. The importance of this ability for salmonid populations will be influenced by the turbidity regime, food availability patterns, and hydraulic complexity of the systems they occupy.

Citation

Harvey, Bret C.; White, Jason L. 2008. Use of benthic prey by salmonids under turbid conditions in a laboratory stream. Transactions of the American Fisheries Society 137: 1756-1763.