Investigation of Potential Factors for High Intersex Rates of Largemouth Bass in Small Impoundments

Recently we reported that a high proportion (65%) of male largemouth bass collected from small impoundments (<200 acres) were intersex. These small impoundments had no input from wastewater facilities and were located in largely forested areas with minimal agriculture or livestock, suggesting that factors other than previously described endocrine disrupting chemicals were involved in the intersex condition. Small impoundments have several unique biotic and abiotic conditions that may contribute to the occurrence of intersex such as high nitrate levels, cyanobacteria blooms and sediment bound estrogenic substances. Our objectives were to determine reproductive and developmental effects of nitrate exposure in a fish full-lifecycle study, and determine reproductive and developmental effects in fish exposed to sediments from small impoundments where intersex bass were collected. Fathead minnows were exposed to nitrate (0, 50 and 250 mg/L) or sediment from <24 hr post hatch to sexual maturity. Growth rate, gonadal somatic index, body condition, sex hormone levels, vitellogenin induction and incidence of intersex were determined in the mature fish. Findings from this study suggest that factors involved in the high rate of intersex in small impoundments are different from those associated with intersex fish in river habitats and may have implications for pond management.