Low levels of dietary methylmercury inhibit growth and gonadal development in juvenile walleye (Stizostedion vitreum)

Mercury levels in the aquatic environment of North America have been increasing, raising the possibility that this highly toxic heavy metal might alter fish populations. Previous investigations have demonstrated toxic effects of mercury on teleost reproduction, but these findings were observed following unrealistically high exposures. In this study, we used concentrations frequently observed in North American lakes to investigate the effects of dietary methylmercury on growth, gonadal development, and plasma cortisol levels in juvenile walleye (Stizostedion vitreum). For a period of 6 months, two groups of walleye were reared on untainted catfish fillets, while two test groups were fed fillets injected with methylmercury, one group receiving 0.1 mu g Hg g(-1) food (low-mercury diet) and the other receiving 1.0 mu g Hg g(-1) food (high-mercury diet). After the exposure period, fish fed the low- and high-mercury diets had mean body burdens of 0.254 +/- 0.015 mu g Hg g(-1) and 2.37 +/- 0.09 mu g Hg g(-1), respectively. Dietary mercury significantly impaired both growth and gonadal development in males, which was apparent as reduced fish length, weight, and gonadosomatic index. Testicular atrophy was observed in fish fed the mercury-tainted fillets, but was nonexistent in control animals. Mercury also suppressed plasma cortisol in juveniles (sexes combined). The findings of this study suggest that dietary methylmercury, at levels currently found in the aquatic environment, might reduce juvenile survival by impairing growth and immune function. Furthermore, these results suggest that methylmercury might also affect reproductive potential of teleosts by impairing testicular development in young.