Skewed embryonic sex ratios in a viviparous fish: a result of endocrine disruption?

Sex ratios in catches of wild adult fish are often affected by sex related differences in growth, behaviour and survival. Therefore, such sex ratios may not reflect the relative number of males and females that are recruited to the population. However, this can be circumvented by studying unborn fish embryos. We have found skewed sex ratios among embryos from the viviparous eelpout (Zoarces viviparus) which could indicate exposure to endocrine disrupting chemicals. The sex ratio was close to 50/50 at two presumably clean sites from Kattegat/Skagerrak and two from the Baltic Sea. On the Swedish Baltic coast near a large pulp mill the relative number of female embryos was significantly lower (42%; P=0.006) but approached 50% further south in the discharge gradient. Treatment of females during early pregnancy with methyltestosterone inhibited oocyte development in all embryos, and instead testis-like tissue was formed. Thus, masculinization found in the field could be caused by exposure to androgen mimics or substances interfering with steroid-synthesis, activity or excretion. This may arise from exposure to the pulp mill effluents, since endocrine disruption in fish has been reported earlier near North American mills. Irrespectively of the cause, reduced numbers of female offspring may have negative impacts on the recruitment capacity of a population.