Intersex in Male Smallmouth Bass in the Missisquoi River in Vermont: Understanding Factors that Can Lead to Endocrine Disruption in Field Settings

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The presence of testicular oocytes (intersex) in male smallmouth bass (Micropterus dolomieu) in the Missisquoi River in Vermont varied over the period of the study and was not related to concentrations of known endocrine disrupting chemicals in the River. Although previous studies have shown linkages between endocrine disrupting chemical exposures and intersex in fish, these results indicate that unidentified stressors may also be associated with the development of intersex in smallmouth bass and are the subject of continued study.

USGS scientist dissecting a fish to determine possible effects from exposure to endocrine disrupting contaminants

USGS scientist dissecting a fish to determine possible effects from exposure to endocrine disrupting contaminants.

(Credit: Vicki S. Blazer, USGS Leetown Center. Public domain.)

Endocrine disruption is a form of sublethal toxicity in which natural or anthropogenic chemicals, defined as endocrine disruptors, can lead to changes in normal endocrine system function. In laboratory and natural environments, estrogenic endocrine disrupting chemicals (EEDCs) have been reported to interfere with development, reproduction, immunocompetence, and behavior in fish and other organisms.

The presence of testicular oocytes (intersex) in male fish is often used as a biological indicator of exposure to EEDCs and has been documented globally and throughout the United States. For example, previous U.S. Geological Survey (USGS) research has documented intersex in male bass in the Chesapeake Bay Watershed. USGS and U.S. Fish and Wildlife Service (USFWS) scientists also have documented the presence of intersex and other indicators of EEDC exposure in male bass in northeastern USFWS National Wildlife Refuges.

Although intersex has been documented in wild fish, it is often unclear what initiates intersex in a field setting. Scientists took steps to answer this question through a study in the Missisquoi River, which runs through the Missisquoi National Wildlife Refuge in Vermont. This river was selected because of known presence of biological indicators of endocrine disruption in male bass including intersex.

The scientists collected samples four times from September 2012 to June 2014. Smallmouth bass (Micropterus dolomieu) were collected for measurement of biomarkers of endocrine disruption (severity of intersex, plasma vitellogenin concentrations, and the expression of estrogen sensitive genes in liver tissues). In addition, polar organic chemical integrative samplers were deployed and analyzed to identify EEDCs present in the river during the time of fish sampling.

Micrograph of testes from male smallmouth bass

Micrograph of testes from male smallmouth bass that contain immature oocytes distinctive of intersex in this species of fish.

(Credit: Vicki Blazer, U.S. Geological Survey, Leetown Science Center. Public domain.)

The scientists reported that intersex prevalence and severity in male smallmouth bass decreased over the period of the study from 93 percent to 28 percent. Intersex severity was also reported to be significantly different between late summer and early spring collections. There was generally an absence of known EEDCs detected at the site during the study period. Other measures such as the presence of plasma vitellogenin and liver vitellogenin abundance in males did not indicate exposure to EEDCs at any of the four sample collections.

Although this study did not determine the cause of intersex decline or seasonal variability, the results augment the understanding of intersex prevalence in male smallmouth bass and nudge the science forward regarding the dynamic nature of intersex prevalence in male fish. The absence of known endocrine disruptors in this study indicates that unidentified chemicals or other stressors may be associated with the development of intersex in male smallmouth bass.

The USGS Fishing and Hunting Science Team and the Endocrine Disrupting Compounds in the Chesapeake Bay Watershed Science Team of the USGS Contaminant Biology and Toxic Substances Hydrology combined programs continues to work to identify the factors that initiate intersex and to determine if intersex presence has any demonstrable adverse effects on fish reproduction or results in population declines in a field setting. Multiyear data sets are currently being generated for smallmouth bass at numerous sites within the Chesapeake Bay watershed to address these questions.

The USFWS and the USGS Contaminant Biology and Toxic Substances Hydrology combined programs funded this study.