Effects of Suspended Sediment on the Escape Behavior and Cover‐Seeking Response of Juvenile Chinook Salmon in Freshwater

Abstract

AbstractIn laboratory experiments, we quantified the innate adaptive escape behavior and cover‐seeking response of juvenile Chinook salmon Oncorhynchus tshawytscha exposed to elevated levels of turbidity and suspended sediments for a duration predicted to elicit sublethal effects. Treatment fish were held for 48 h at 10°C in a sediment slurry (30,000 mg/L; 50,000 nephelometric turbidity units [NTU]) made up of about 46% clay (<0.002 mm), 46% silt (<0.053 and >0.002 mm), and 8% fine sand (<2.00 and >0.053 mm). The behavioral response of individual control and treatment fish was then examined in clear water (1.9 NTU) through the use of a performance challenge test. Challenge tests can determine the sublethal effects of contaminants on an organism's ability to adapt and survive in its environment. The behavioral avoidance response was characterized as the amount of time in which individual fish swam out of the field of view and sought cover. The tendency of fish to escape to shelter, the time to swim from an open, well‐lit space into shade beneath overhead cover, and the trajectory of each escape response were recorded by a video camera coupled to a digital recorder. The ability of sediment‐exposed Chinook salmon to escape to cover was impaired relative to that of control fish: There was a significant increase in stuporous behavior and a significant reduction in cover‐seeking response in the sediment‐exposed fish. Treatment fish were slower to seek cover from intense light and displayed erratic swimming behavior. These results suggest that even a relatively brief (48‐h) exposure to elevated levels of suspended sediment could indirectly jeopardize survival in the wild, as such overt performance and behavioral changes would probably render juvenile Chinook salmon more conspicuous and therefore more susceptible to avian and aquatic predators.