Résumé
Individual variation in sub-lethal sensitivity to the organophosphate pesticide trichlorfon was investigated in Nile tilapia, using critical swimming speed (U crit) as an indicator. Tilapia exposed for 96 h to 500 μg l −1 tri-chlorfon at 26 °C (Tcfn group, n = 27) showed a significant decline in mean U crit , compared to their own control (pre-exposure) performance in clean water (−14.5 ± 2.3%, mean ± SEM), but also compared to a Sham group (n = 10) maintained for 96 h in clean water. Individuals varied in their relative sensitivity to the pesticide , with the decline in U crit after exposure varying from 1 to 41%. The U crit of the Tcfn group did not recover completely after 96 h in clean water, remaining 9.4 ± 3.2% below their own control performance. The decline in performance was associated with a significant increase in net cost of aerobic swimming, of +28.4 ± 6.5% at a sustained speed of 2 bodylengths s −1 , which translated into a significant decline in swimming efficiency (E swim) of −17.6 ± 4.0% at that speed. Within the Tcfn group, individual E swim was a strong positive determinant of individual U crit across all trials, and a strong negative determinant of individual% decline in U crit after pesticide exposure (P < 0.001, linear mixed effect models). Trichlorfon had no effects on standard metabolic rate or active metabolic rate (AMR) but, nonetheless, individual U crit in all trials, and% decline in U crit after exposure, were strongly associated with individual AMR (positively and negatively, respectively, P < 0.001). Individual U crit under control conditions was also a strong positive determinant of U crit after tri-chlorfon exposure (P < 0.001), but not of the% decline in U crit performance. In conclusion, the OP pesticide impaired U crit performance by reducing E swim but individual tilapia varied widely in their relative sensitivity. Intrinsic individual physiology determined effects of the pesticide on performance and, in particular, good swimmers remained better swimmers after exposure.