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Science Summary—Indicators of Reproductive Endocrine Disruption in Fish in the Chesapeake Bay Watershed

Prepared by Vicki Blazer, U.S. Geological Survey
(Released August 2012) (PDF Version)


As the largest and most productive estuary in North America, Chesapeake Bay is a vital ecological and economic resource. The bay and its tributaries have been degraded in recent decades, however, as a result of excessive inputs of nutrients and sediment and by the impacts of toxic contaminants on the health of fish and wildlife. Although a Total Maximum Daily Load (TMDL) has been established to reduce nutrient and sediment inputs to the bay (U.S. Environmental Protection Agency, 2011), scientists need to improve the current understanding of the extent and severity of the effects of toxic contaminants (see inset box below) and other factors on the health of fish and wildlife.

U.S. Geological Survey Studies of Fish Health in the Chesapeake Bay Watershed

Figure 1 image and link to PDF of Figure 1

Figure 1. Microscopic appearance of smallmouth bass testes containing (A) a single oocyte (severity rating 1); and (B) multiple oocytes, commonly in clusters (severity rating 4) (modified from Blazer and others, 2007) PDF.

In 2002, the U.S. Geological Survey (USGS) began comprehensive fish health assessments to determine the cause(s) of fish mortality in the South Branch Potomac River. During these investigations, a high prevalence of intersex was observed in smallmouth bass (Micropterus dolomieu). Intersex is the presence of characteristics of both sexes in an organism that normally exhibits the characteristics of only one sex throughout its life. In the case of smallmouth bass, the intersex condition is present in the male fish, which have oocytes (immature eggs) in the testes (fig. 1). Additional monitoring indicated that the prevalence of testicular oocytes ranged from 100 percent at some sites in the Shenandoah River to low or background levels (10-14 percent) at selected out-of-basin sites such as the Gauley and Cheat Rivers. Prevalence in the South Branch Potomac River in West Virginia was intermediate, with 50 to 75 percent of the male bass affected (Blazer and others, 2007). Since the initial observation, intersex has been found throughout the Potomac River watershed (fig. 2).

Natural estrogens, as well as synthetic chemicals with estrogenic activity, are endocrine-disrupting chemicals that affect the hormonal system of fishes when present at sufficiently high concentrations. The presence of testicular oocytes in fish is one indicator of exposure to these chemicals; another is the presence of vitellogenin in the circulating blood of male fishes. Vitellogenin is a precursor of egg yolk that normally is found only in the blood of egg-laying female fish.

Map of Potomac River Watershed
Figure 2. Potomac River watershed, selected watersheds in which intersex has been
studied by the U.S. Geological Survey, and generalized locations of sampling sites (modified from Blazer and others, 2011). PDF.

Aquatic organisms in the Chesapeake Bay watershed are exposed to complex mixtures of chemicals that can have additive, synergistic, or antagonistic effects. The USGS, working with the U.S. Fish and Wildlife Service and State partners (Pennsylvania, Maryland, Virginia, and West Virginia), is documenting the biological effects of these chemicals on fish, conducting chemical monitoring to determine the extent and potential causes of reproductive endocrine disruption throughout the Potomac River and nearby watersheds, and identifying potential sources (point and nonpoint) of relevant chemicals.

This Science Summary is one in a series that is designed to facilitate the understanding and application of results of relevant USGS studies by Chesapeake Bay resource managers and policy makers. It provides a brief overview of the most recent published work by the USGS and collaborators on indicators of reproductive endocrine disruption in fish in the Chesapeake Bay watershed, an understanding of how this information can be used to develop effective management policies and practices, and a list of references for additional information.

The KEY FINDINGS and the IMPLICATIONS FOR MANAGEMENT POLICIES AND PRACTICES AND NEXT STEPS listed below are from Blazer and others (2007; 2010; 2011), Alvarez and others (2009), and Iwanowicz and others (2009).

bulletKey Findings

bulletImplications for Management Policies and Practices and Next Steps

The term "toxic contaminants" includes legacy contaminants such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), legacy pesticides (DDT, dieldrin), and heavy metals; emerging contaminants such as currently used pesticides, flame retardants, human and animal hormones, antibiotics, other pharmaceuticals, and personal care products; and other substances that have adverse effects on the health of fish and wildlife.


Read Supporting Press Releases

(February 2008) Intersex in Smallmouth Bass Coincident with Population and Agriculture in Potomac Watershed

(June 2009) One Step Closer to Understanding Fish Health in Potomac and Shenandoah Rivers—Estrogen Linked to Lowered Immunity in Fish

(September 2009) Widespread Occurrence of Intersex Bass Found in U.S. Rivers

References Cited

Alvarez, D.A., Cranor, W.L., Perkins, S.D., Schroeder, V.L., Iwanowicz, L.R., Clark, R.C., Guy, C.P., Pinkney, A.E., Blazer, V.S., and Mullican, J.E., 2009, Reproductive health of bass in the Potomac, USA, drainage: Part 2. Seasonal occurrence of persistent and emerging organic contaminants: Environmental Toxicology and Chemistry, v. 28, no. 5, p. 1,084-1,095, DOI:10.1897/08-417.1. (

Blazer, V.S., Iwanowicz, L.R., Henderson, Holly, Mazik, P.M., Jenkins, J.A., Alvarez, D.A., and Young, J.A., 2011, Reproductive endocrine disruption in smallmouth Bass (Micropterus dolomieu) in the Potomac River basin: spatial and temporal comparisons of biological effects: Environmental Monitoring and Assessment, v. 184, no. 7, p. 4,309-4,334, DOI 10.1007/s10661-011-2266-5. (

Blazer, V.S., Iwanowicz, L.R., Iwanowicz, D.D., Smith, D.R., Young, J.A., Hedrick, J.D., Foster, S.W., and Reeser, S.J., 2007, Intersex (testicular oocytes) in smallmouth Bass from the Potomac River and selected nearby drainages: Journal of Aquatic Animal Health, v. 19, no. 4, p. 242-253, DOI: 10.1577/H07-031.1.(

Blazer, V.S., Iwanowicz, L.R., Starliper, C.E., Iwanowicz, D.D., Barbash, P., Hedrick, J.D., Reeser, S.J., Mullican, J.E., Zaugg, S.D., Burkhardt, M.R., and Kelble, J., 2010, Mortality of centrarchid fishes in the Potomac drainage: Survey results and overview of potential contributing factors: Journal of Aquatic Animal Health, v. 22, no. 3, p. 190-218, DOI: 10.1577/H10-002.1. (

Harris, C.A., Hamilton, P.B., Runnalls, T.J., Vinciotti, Veronica, Henshaw, Alan, Hodgson, Dave, Coe, T.S., Jobling, Susan, Tyler, C.R., and Sumpter, J.P., 2011, The consequences of feminization in breeding groups of wild fish: Environmental Health Perspectives, v. 119, no. 3, p. 306-311,

Hinck, J.E., Blazer, V.S., Schmitt, C.J., Papoulias, D.M., and Tillitt, D.E., 2009, Widespread occurrence of intersex in black basses (Micropterus spp.) from U.S. rivers, 1995—2004: Aquatic Toxicology, v. 95, no. 1, p. 60-70, doi:10.1016/j.aquatox.2009.08.001.

Iwanowicz, L.R., Blazer, V.S., Guy, C.P., Pinkney, A.E., Mullican, J.E., and Alvarez, D.A., 2009, Reproductive health of bass in the Potomac, USA, drainage: Part 1. Exploring the effects of proximity to wastewater treatment plant discharge: Environmental Toxicology and Chemistry, v. 28, no. 5, p. 1,072-1,083, DOI: 10.1897/08-433.1. (

Kidd, K.A., Blanchfield, P.J., Mills, K.H., Palace, V.P., Evan, R.E., Lazorchak, J.M., and Flick, R.W., 2007, Collapse of a fish population after exposure to a synthetic estrogen: Proceedings of the National Academy of Sciences of the United States of America, v. 104, no. 21, p. 8,897-8,901. (

U.S. Census Bureau, 2002, United States Census 2000, Summary file 1, geographic comparison tables, accessed March 15, 2007, at

U.S. Department of Agriculture, 2004, 2002 Census of agriculture, State and county reports, accessed March 15, 2007, at

U.S. Environmental Protection Agency, 2011, Chesapeake Bay TMDL, Final Bay TMDL documents, Executive summary and appendices, accessed September 20, 2011, at

For further information contact Vicki Blazer (, 304-724-4434) or Scott Phillips (, 443-498-5552).

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