SEXUAL DIFFERENCES IN MORTALITY AND SUBLETHAL STRESS IN CHANNEL CATFISH FOLLOWING A 10 WEEK EXPOSURE TO COPPER-SULFATE

Copper sulfate is regularly used in the aquaculture industry as an alg icide and for the treatment of various ectoparasitic infections. In th is study, male and female channel catfish (Ictalurus punctatus) (100 f ish per group) were treated for 10 weeks with three levels of copper s ulfate, giving water concentrations of 220, 354, and 465 mu g Cu l(-1) . Six fish were sampled at 2-week intervals and copper levels in muscl e and hepatic tissue were measured. No copper accumulation was detecte d in muscle tissue during the exposure period, but hepatic accumulatio n began to occur within the first 2 weeks of treatment, with a peak at 4-6 weeks and subsequent decrease before the end of exposure. No mort ality was seen in controls or at 220 mg Cu l(-1). However, lethality w as seen within the first week in the 354 and 465 mu g Cu l(-1) exposur es, with males being more susceptible than females. Metallothionein (M T) levels showed an increasing trend through the 6-week time point, fo llowed by a general decrease toward control levels for the remainder o f the exposure. The pattern of hepatic MT content was similar to that of hepatic copper accumulation, with a correlation coefficient (r) of 0.767 (P less than or equal to 0.001). Measures of whole-body weight, body length, liver weight, liver somatic index, and condition factor w ere recorded as indicators of sublethal stress. No substantial change was seen in any of these measures in females; however, a decreasing te ndency was seen in males for body weight, body length, and condition f actor. However, correlation analysis of hepatic copper with somatic me asures showed a significant relationships in both sexes. When hepatic MT was compared to somatic indices, significant correlations were obse rved in males, but not females. The results of this study indicate an association between MT expression and sublethal stress in males as wel l as a potential protective mechanism other than MT in pre-spawn femal e channel catfish exposed to copper.