Intersex (Testicular Oocytes) in Smallmouth Bass from the Potomac River and Selected Nearby Drainages V. S. BLAZER,* L. R. IWANOWICZ, AND D. D. IWANOWICZ U.S. Geological Survey, National Fish Health Research Laboratory, 11649 Leetown Road, Kearneysville, West Virginia 25430, USA D. R. SMITH AND J. A. YOUNG U.S. Geological Survey, Aquatic Ecology Branch, 11649 Leetown Road, Kearneysville, West Virginia 25430, USA J. D. HEDRICK West Virginia Division of Natural Resources, 1 Depot Street, Romney, West Virginia 26757, USA S. W. FOSTER U.S. Army Corps of Engineers, Huntington District, Post Office Box 9, Apple Grove, West Virginia 25502, USA S. J. REESER Virginia Department of Game and Inland Fisheries, 517 Lee Highway, Verona, Virginia 24482, USA Abstract.—Intersex, or the presence of characteristics of both sexes, in fishes that are normally gonochoristic has been used as an indicator of exposure to estrogenic compounds. In 2003, during health assessments conducted in response to kills and a high prevalence of skin lesions observed in smallmouth bass Micropterus dolomieu in the South Branch of the Potomac River, the presence of immature oocytes within testes was noted. To evaluate this condition, a severity index (0–4) was developed based on the distribution of oocytes within the testes. Using gonad samples collected from 2003 to 2005, the number of histologic sections needed to accurately detect the condition in mature smallmouth bass was statistically evaluated. The reliability of detection depended on the severity index and the number of sections examined. Examining five transverse sections taken along the length of the gonad resulted in a greater than 90% probability of detecting testicular oocytes when the severity index exceeded 0.5. Using the severity index we compared smallmouth bass collected at selected sites within the South Branch during three seasons in 2004. Seasonal differences in severity and prevalence were observed. The highest prevalence and severity were consistently noted during the prespawn–spawning season, when compared with the postspawn season. In 2005, smallmouth bass were collected at selected out-of-basin sites in West Virginia where fish kills and external skin lesions have not been reported, as well as at sites in the Shenandoah River, Virginia (part of the Potomac drainage), where kills and lesions occurred in 2004–2005. The prevalence of testicular oocytes is discussed in terms of human population and agricultural intensity. Intersex, also referred to as ovotestis, testis–ova or testicular oocytes (Hecker et al. 2006), has received considerable attention recently in both the public and scientific press. While intersex is most commonly described as the presence of female germ cells, or oocytes, within a predominantly male gonad (Nolan et al. 2001), the term is nonspecific and has been used for a range of gonadal abnormalities in which both male and female characteristics are present. In smallmouth bass Micropterus dolomieu the abnormality observed was the presence of oocytes in the male gonad and will be referred to as testicular oocytes (TO). Occasionally these abnormalities are visible macroscopically but most often the gonad must be examined microscopi- cally for detection. In fish that are normally gonochor- istic, the presence of TO has been used as an indicator of exposure to estrogenic compounds and has been documented in a variety of wild fish species in numerous geographic areas. Male roach Rutilus rutilis populations in UK rivers downstream from sewage treatment works have a high frequency of TO and vitellogenin induction (Jobling et al. 1998). Intersex frequency in roach was also correlated with exposure to domestic sewage effluents in Danish streams (Bjerre- gaard et al. 2006). Other freshwater fishes that have been observed with TO include male spottail shiners * Corresponding author: vblazer@usgs.gov Received April 21, 2007; accepted September 15, 2007 Published online December 31, 2007 242 Journal of Aquatic Animal Health 19:242–253, 2007 Ó Copyright by the American Fisheries Society 2007 DOI: 10:1577/H07-031.1 [Article]