Eighty five percent of male smallmouth bass and 27 percent of male largemouth bass tested in waters in or near 19 National Wildlife Refuges in the Northeast U.S. were intersex, according to a new study by U.S. Geological Survey and U.S. Fish and Wildlife Service researchers.

Intersex is when one sex develops characteristics of the opposite sex. It is tied to the exposure of fish to endocrine-disrupting chemicals that can affect the reproductive system and cause the development of characteristics of the opposite sex, such as immature eggs in the testes of male fish. Intersex is a global issue, as wild-caught fish affected by endocrine-disrupting chemicals have been found in locations across the world.

Estrogenic endocrine-disrupting chemicals are derived from a variety of sources, from natural estrogens to synthetic pharmaceuticals and agrochemicals that enter the waterways. Examples include some types of birth control pills, natural sex hormones in livestock manures, herbicides and pesticides. 

“It is not clear what the specific cause of intersex is in these fish,” said Luke Iwanowicz, a USGS research biologist and lead author of the paper. “This study was designed to identify locations that may warrant further investigation.  Chemical analyses of fish or water samples at collection sites were not conducted, so we cannot attribute the observation of intersex to specific, known estrogenic endocrine—disrupting chemicals.”

This prevalence of intersex fish in this study is much higher than that found in a similar USGS study that evaluated intersex in black basses in nine river basins in the United States. That study did not include river basins in the Northeast.

"The results of this new study show the extent of endocrine disrupting chemicals on refuge lands using bass as an indicator for exposures that may affect fish and other aquatic species," said Fred Pinkney, a USFWS contaminants biologist and study coauthor. "To help address this issue, the U.S. Fish and Wildlife Service encourages management actions that reduce runoff into streams, ponds and lakes -- both on and off of refuge lands.”

The journal article, Evidence of estrogenic endocrine disruption in smallmouth and largemouth bass inhabiting Northeast U.S. National Wildlife Refuge waters: a reconnaissance study,” by L.R. Iwanowicz, V.S. Blazer, A.E. Pinkney, C.P. Guy, A.M. Major, K. Munney, S. Mierzykowski, S. Lingenfelser, A. Secord, K. Patnode, T.J. Kubiak, C. Stern, C.M. Hahn, D.D. Iwanowicz, H.L. Walsh, and A. Sperry is available online in Ecotoxicology and Environmental Safety.