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 i ncreasing, raising the possibility that this highly toxic heavy metal might alter fish populations. Previous investigations have demonstrate d 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 t o investigate the effects of dietary methylmercury on growth, gonadal development, and plasma cortisol levels in juvenile walleye (Stizosted ion vitreum). For a period of 6 months, two groups of walleye were rea red on untainted catfish fillets, while two test groups were fed fille ts 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 si gnificantly impaired both growth and gonadal development in males, whi ch was apparent as reduced fish length, weight, and gonadosomatic inde x. Testicular atrophy was observed in fish fed the mercury-tainted fil lets, but was nonexistent in control animals. Mercury also suppressed plasma cortisol in juveniles (sexes combined). The findings of this st udy suggest that dietary methylmercury, at levels currently found in t he aquatic environment, might reduce juvenile survival by impairing gr owth and immune function. Furthermore, these results suggest that meth ylmercury might also affect reproductive potential of teleosts by impa iring testicular development in young.