Effects of dietary methylmercury on reproductive endocrinology of fathead minnows.

Recent laboratory studies have demonstrated that environmentally realistic concentrations of dietary methylmercury can impair reproduction of fish. To evaluate relations between reproductive success and biomarkers of methylmercury exposure, we fed juvenile fathead minnows (Pimephales promelas) one of three diets contaminated with methylmercury: 0.06 (control), 0.87 (low), and 3.93 (medium) microg of Hg g(-1) dry weight. At sexual maturity, fish were paired, allowed to reproduce, and then analyzed for total mercury, plasma testosterone (T), and 17beta-estradiol (E2). Diets did not affect survival or growth of fathead minnows. Methylmercury suppressed levels of T in males and E2 in females. Male fathead minnows fed the control diet had mean T concentrations 20% and 106% greater than those fed the low and medium diets; control females had mean E2 concentrations 149% and 402% greater than those fed the low and medium diets. Methylmercury also inhibited gonadal development of females; the gonadosomatic index (GSI) of females fed the medium diet was 40% less than that of females fed control or low diets. Plasma levels of T in males and E2 in females were positively related to GSI. Methylmercury reduced the reproductive success of fathead minnows. Spawning success was 32% for pairs fed the control diet, 12% for pairs fed the low diet, and 0% for pairs fed the medium diet. Pairs fed the low diet required, on average, 5 d longer to spawn a clutch of eggs than the controls. Concentrations of methylmercury fed to fathead minnows in this study are also encountered by invertivorous and piscivorous fish in some methylmercury-contaminated aquatic ecosystems. This suggests that reproduction of wild fishes may be adversely affected by methylmercury and that suppressed hormone levels may be used to indicate diminished reproduction of fish.