Measurement of sublethal metabolic stress in rainbow trout (Salmo gairdneri) using automated respirometry.

Sublethal effects of rotenone on the metabolic rates of rainbow trout (Salmo gairdneri Richardson) were determined. Metabolic measurements were made by the method of increasing velocity steps, using a miniature version of the Blazka respirometer with automatic dissolved oxygen measurement. Regressions of log metabolic rate versus swimming speed (expressed as relative performance) were compared for the toxicant levels tested. Results were interpreted in terms of Fry's classification of environmental factors using metabolic response categories ("Fry's paradigm"). At high rotenone dosages (96 hr. LC 50), reduction in the critical swimming speed and the active metabolic rate (limiting effect) were observed. At lower toxicant levels (0.20 x 96 hr. LC 50) the standard metabolic rate was elevated (masking effect) and the limiting effect disappeared. The elevation in metabolic rate decreased in competition with locomotor energy costs as they increased with swimming speed (modulated masking effect).