Testicular oocytes in smallmouth bass in Northeastern Minnesota in relation to varying levels of human activity

摘要

Testicular oocytes (TOs) have been found in black basses (Micropterus spp.) from many locations in North America. The presence of TOs is often assumed to imply exposure to estrogenic endocrine disrupting compounds (EDCs), however, a definitive causal relationship has yet to be established, and TO prevalence is not consistently low in fish from areas lacking evident EDC sources. This might indicate any of a number of situations: (a) unknown or unidentified EDCs or EDC sources, (b) induction of TOs by other stressors, or (c) TOs occurring spontaneously during normal development. In the present study we analyzed TO occurrence in smallmouth bass (M. dolomieu) from eight populations in Northeastern Minnesota watersheds with differing degrees of human development and hence, presumed likelihood of exposure to anthropogenic chemicals. Three watersheds were categorized as moderately developed, based on the presence of municipal wastewater discharges and higher human population density (4-81 per km²), and five watersheds were minimally developed, with very low human population density (0-1 per km²) and minimal built environment. Testicular tissues from mature fish were evaluated using a semi-quantitative method that estimated TO density, normalized by cross-sectional area. TO prevalence and density among populations from moderately developed watersheds was higher than in populations from minimally developed watersheds. However, TO prevalence was unexpectedly high and variable (7-43%) in some populations from minimally developed watersheds, and only weak evidence was found for a relationship between TO density and watershed development, suggesting alternative or more complex explanations for TO presence in smallmouth bass from this region.