Acute and sublethal toxicities of rotenone in juvenile rainbow trout (Oncorhynchus mykiss): swimming performance and oxygen consumption.

Rotenone, a natural insecticide and piscicide, was shown to have an extremely small margin between no lethality (5.0 microg/L) and 100% mortality (6.6 microg/L) for static-renewal 96-hour toxicity tests with juvenile rainbow trout (Oncorhynchus mykiss). Dissolved organic carbon (DOC) at concentrations of 3.0 and 4.0 mg/L significantly increased the rotenone 96-hour LC(50) (median lethal concentration) from 5.80 microg/L (confidence interval (CI) 5.51 to 6.10) to 6.55 microg/L (CI 6.28 to 6.83) and 7.75 microg/L (CI 7.29 to 8.24), respectively, probably as a result of rotenone adsorption onto DOC, which decreased its bioavailability. Using concentrations of 0, 3.0, 4.0, and 5.0 microg/L rotenone and exposure periods of 2, 4, 6, 12, 16, 24, and 48 hours, the threshold concentration of rotenone for impairment of critical swimming performance (Ucrit) was 3.0 microg/L (P = 0.029), with no further impairment at higher concentrations and no time-dependent effect on Ucrit. Using continuous measures of oxygen uptake for 48 hours before and 48 hours during rotenone exposure (0, 1.5, 2.5, 3.0, and 3.5 microg/L), rotenone significantly decreased peak active oxygen uptake at all rotenone concentrations tested without affecting routine oxygen uptake. Fish were individually chased and then placed in rotenone concentrations of 0, 1.0, 3.0, 4.0, 5.0, and 6.0 microg/L to monitor initial postexercise oxygen uptake (Mo2Max) and excess postexercise oxygen consumption (EPOC) during a 40-minute recovery period. Rotenone significantly decreased Mo2Max (P = 0.002) after exposures to 4.0 and 5.0 microg/L, but not 6.0 microg/L, without affecting EPOC.