Behavioral avoidance: Possible mechanism for explaining abundance and distribution of trout species in a metal-impacted river

Behavioral avoidance of metal mixtures by rainbow trout (Oncorhynchus mykis s) was determined in the laboratory under water quality conditions that sim ulated the upper Clark Fork River, Montana, USA. A metal mixture with a fix ed ratio of observed ambient metal concentrations (12 mu g/L Cu:1.1 mu g/L Cd:3.2 mu g/L Pb:50 mu g/L Zn) was used to determine avoidance in a counter current avoidance chamber. Rainbow trout avoided all metal concentrations t ested from 10 to 1,000% of the simulated ambient metal mixture. The behavio ral response of rainbow trout to the metal mixture was more sensitive than the response of brown trout (Salmo trutta) previously reported from the sam e laboratory under the same experimental conditions. Additionally, rainbow trout that were acclimated to the simulated ambient metal mixture for 45 d preferred clean water and avoided higher metal concentrations. Therefore, o ur laboratory experiments on the behavioral avoidance responses of rainbow trout, as well as previously reported experiments on brown trout, show that both species will avoid typical metal concentrations observed on the Clark Fork River. And the greater sensitivity of rainbow trout to the metal mixt ure may explain, in part, why rainbow trout populations appear to be more s everely affected, compared to brown trout populations, in the upper Clark F ork River.