Survey of intersex occurrence in largemouth bass (Micropterus salmoides) from the Upper Illinois River Waterway

Intersex condition (ooctyes in testicular tissue) has been documented in many watersheds among a diverse variety of fishes worldwide. However, few studies have tested for the occurrence of the condition in fishes from rivers of the American Midwest. Midwestern watersheds, such as the Illinois River Waterway, U.S.A. may provide important new information about the effects of endocrine disrupting chemicals (EDCs) on fishes because of the variety of urban, industrial, and agricultural land uses within the watersheds. A first step in the study of EDCs in any ecosystem is a survey to document the symptoms of EDC exposure, such as intersex condition. Our objective was to test for intersex condition in male largemouth bass Micropterus salmoides in the lower Des Plaines River, an area directly affected by surface runoff and wastewater effluents from the Chicago Metropolitan Area. Histological analysis indicated that testicular oocytes were present in 21 of 51 (41%) largemouth bass sampled and oocyte numbers ranged from 1-25/thin section among intersex individuals. Our study details the severity of intersex in a population of largemouth bass near a major metropolitan area, which represents an important contribution to the understanding of fish reproductive ecology in ecosystems with a history of environmental disturbance and recovery such as the Illinois River Waterway.