Effects of turbidity on the ability of juvenile rainbow trout, <i>Oncorhynchus mykiss</i> , to feed on limnetic and benthic prey in laboratory tanks

Abstract The feeding rate of juvenile rainbow trout, Oncorhynchus mykiss Richardson, 1836, on small (1–2 mm diam.) Daphnia spp. in laboratory tanks was not reduced by turbidity levels up to 160 Nephelometric Turbidity Units (NTU). Furthermore, the high feeding rates on larger, benthic prey (viz. Deleatidium spp. and chironomid larvae) in clear water (0 NTU) were maintained at turbidities up to 160 NTU. Therefore, any impairment of vision by increased turbidity did not affect the ability of trout to feed on these prey, and non‐visual senses may be used for the capture of such prey in turbid waters. Although trout were strongly size‐selective for both large chironomid and Deleatidium larvae in clear water, turbidities over 20 and 160 NTU, respectively, reduced size‐selection. Vision is therefore needed by trout to select larger prey. However, chironomid larvae were consumed by trout in the complete absence of light, so their ability to capture these prey in tanks was not dependent on visual cues. It is apparent that trout do use other senses such as the lateral line system to detect and capture such prey when turbidity levels are high and when light levels or water clarity are low. This ability is expected to offset any reduction in visual feeding caused by increased turbidity, and it helps explain the increased emphasis on epibenthic feeding by trout in turbid waters.