SURFACE, PLANKTONIC, AND BENTHIC FORAGING BY JUVENILE CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) IN TURBID LABORATORY CONDITIONS

We investigated the effect of turbidity on the foraging behaviour of juvenile chinook salmon (Oncorhynchus tshawytscha) in the laboratory. We assessed visual ability by measuring the reaction distance of juvenile chinook to planktonic adult Artemia prey. Chinook exhibited a log-linear decline in reaction distance with increasing turbidity. These results were similar to those obtained by workers for other species. We also determined the effect of turbidity on the foraging rate of juvenile chinook for surface (Drosophila), planktonic (Artemia), and benthic (Tubifex) prey across a range of turbidity levels (<1, 18, 35, 70, 150, 370, 810 NTU). Foraging rates were reduced at higher turbidities for all three prey. However, for surface and benthic prey, foraging rates were also low in clear water, while highest rates were attained at intermediate turbidity levels (35-150 NTU). We suggest that increased feeding rate in turbid conditions reflects a reduced potential risk to predators.