High Rates of Intersex in Bass from Small Impoundments

Recently, high incidences of intersex fish (males with testicular oocytes) were reported in some rivers in the southeastern U.S. To date, a systematic evaluation of the extent, severity, and causes of intersex fish has not been completed in Georgia.  Our objectives were to (1) assess intersex condition in black bass collected from rivers and impoundments across Georgia, and (2) determine estrogenic potency (a measure of the estrogens and estrogen-like substances) of surface waters where fish were collected. Fish (n=15-35) and water samples were collected from sites near municipal wastewater effluent discharges in each of four rivers and from a river that does not receive major wastewater effluent discharges.  Fish and water were also sampled from 12 impoundments without major wastewater inputs to determine if a natural ‘background’ rate of intersex could be established for fish from relatively unpolluted water bodies.  Gonads from all male fish were examined and the incidence and severity of the intersex condition were compared among sites.  Potency of estrogens in surface waters was determined by use of an in vitro yeast-based reporter gene (YES) assay.  The overall percentage of intersex for male bass collected in rivers was 30%, whereas 40% of male fish collected from impoundments were intersex.  Incidence of intersex varied substantially (0-88%) among impoundments, and surface area of the impoundment was a strongly negatively correlated with intersex incidence.  Severity of intersex was not predicted by impoundment surface area.  Causes of intersex remain unknown, but the high incidence of intersex males in some impoundments suggests that factors other than municipal wastewater are involved.