RESISTANCE TO LEAD AND ZINC IN THE WESTERN MOSQUITOFISH GAMBUSIA-AFFINIS INHABITING CONTAMINATED BAYOU TREPAGNIER

This study investigated the occurrence of adaptation to lead and zinc in the western mosquitofish Gambusia affinis inhabiting contaminated B ayou Trepagnier (Louisiana, USA). This bayou has a long history of ind ustrial contamination. Present levels of lead and zinc in water and se diment were considerably higher in Bayou Trepagnier than in a nearby c ontrol stream. Tissue metal levels of mosquitofish were highly elevate d for lead and (to a lesser extent) for zinc. Fish collected from Bayo u Trepagnier and then exposed to zinc in a 96 h laboratory bioassay di d not differ in their sensitivity to zinc from conspecifics collected from a control site. In contrast, Bayou Trepagnier fish did exhibit an increased resistance to lead. This difference in resistance to lead b etween Bayou Trepagnier fish and control fish was no longer evident wh en both groups of fish were kept for 34 days in the laboratory under i dentical conditions (in clean water). While a genetic basis of the dif ference in resistance between the two populations cannot be fully excl uded, it does appear that the elevated lead resistance in Bayou Trepag nier mosquitofish is due to acclimation (physiological, individually-b ased) rather than adaptation at the population level.