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Maternal diet during oogenesis is the major source of methylmercury in fish embryos.

Development of the early life stages of fishes can be affected adversely by methylmercury (MeHg) transferred from the maternal parent to the developing egg. We examined maternal transfer of MeHg to eggs of fathead minnows Pimephales promelas and evaluated the role of maternal MeHg burden versus that in the maternal dietduring oogenesis on egg concentrations. Juvenile fish were fed one of four diets until sexual maturity (phase 1): A control diet (0.06 microg of Hg g(-1) dry weight) and three that were contaminated with MeHg at 0.88 (low), 4.11 (medium), and 8.46 microg of Hg g(-1) (high). At sexual maturity, female fish were paired with a male, again fed one of the four diets, and allowed to reproduce (phase 2). To assess the significance of female dietary exposure during oogenesis on MeHg in eggs, some fish were fed diets during phase 2 that differed from those during phase 1. Mean concentrations and burdens of MeHg in eggs from fish fed the same diet throughout the experiment varied with MeHg content of the maternal diet and were related positively to levels in the carcass of the maternal fish. However, MeHg in eggs was not proportional to that in carcasses among dietary treatments; MeHg in eggs from adults fed the control, low, medium, and high MeHg diets averaged 14%, 25%, 32%, and 35% of that in adults. For fish fed the control diet as juveniles and MeHg-contaminated diets after reaching sexual maturity, MeHg in eggs increased rapidly with duration of maternal dietary exposure prior to spawning. Moreover, concentrations of MeHg in eggs from fish fed the same contaminated diet as both juveniles and sexually mature adults were not related to the duration of adult exposure, and they were not appreciably greater than those from fish fed contaminated diets only just priorto spawning. These results indicate that the diet of the maternal adult during oogenesis, and not adult body burden, is the principal source of MeHg in fish eggs. Accordingly, the exposure of embryonic wild fishes to MeHg depends on levels of the contaminant in prey of the adult during oogenesis, which can vary intra- and interannually.