Physiological determinants of individual variation in sensitivity to an organophosphate pesticide in Nile tilapia Oreochromis niloticus.

Individual variation in sub-lethal sensitivity to the organophosphate pesticide trichlorfon was investigated in Nile tilapia, using critical swimming speed (U) as an indicator. Tilapia exposed for 96h to 500μgl trichlorfon at 26°C (Tcfn group, n=27) showed a significant decline in mean U, 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 96h in clean water. Individuals varied in their relative sensitivity to the pesticide, with the decline in U after exposure varying from 1 to 41%. The U of the Tcfn group did not recover completely after 96h 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 2bodylengthss, which translated into a significant decline in swimming efficiency (E) of -17.6±4.0% at that speed. Within the Tcfn group, individual E was a strong positive determinant of individual U across all trials, and a strong negative determinant of individual% decline in U 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 in all trials, and% decline in U after exposure, were strongly associated with individual AMR (positively and negatively, respectively, P<0.001). Individual U under control conditions was also a strong positive determinant of U after trichlorfon exposure (P<0.001), but not of the% decline in U performance. In conclusion, the OP pesticide impaired U performance by reducing E 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.