Champaign, Ill. – Largemouth bass males swimming in the lower Des Plaines River below the Chicago metropolitan area showed female characteristics at a high rate, according to a new study at the Illinois Natural History Survey (INHS), Prairie Research Institute (PRI), University of Illinois.

Scientists caught 51 male largemouth bass in spring, just before spawning begins when intersex conditions, or the presence of eggs in male testicular tissue, would be most observable. Of the 51 fish, 21, or 41 percent, showed intersex conditions. 

This study may have implications for fishermen who enjoy the sport and for those who consume fish from rivers with a history of severe pollution, said Jason DeBoer, large river fisheries ecologist at INHS.

Although the prevalence of intersex condition does not necessarily indicate decreased fertility and reduced populations of largemouth bass, it might foretell some negative effects.  Males with intersex conditions may have less mobile sperm, fertilization of the eggs can be inhibited, or they are less likely to protect their nest from predators.

“The presence of eggs in male testicular tissue can be caused by chronic exposure to chemicals that are known to disrupt the endocrine systems of animals among a number of other physiological explanations, DeBoer said.  “I don’t believe that the reproductive success of the largemouth bass population has been disrupted; there’s a very robust population in the river. However, they are just not necessarily healthy.”

Findings also showed that 41 percent of the bass sampled suffered from other physical impairments associated with contaminated waters, such as parasites, fin erosion, black spot disease, or infection from grubs in the internal organs.

These impairments may not be directly linked with the intersex condition and some conditions may occur in habitats not affected by pollution.

Exhaustive chemical analyses of surface waters and animal tissues can be expensive and may not screen for all compounds in a river. Therefore, intersex conditions in fish are a biomarker, or an indication of contaminant exposure and potential adverse effects.

The Des Plaines River, where the fish were captured, is downstream of the greater Chicago Metropolitan area where discharge from sewer overflow outlets, water from Lake Michigan, and effluents from seven large water reclamation plants flow.  Intersex conditions may be caused by endocrine-disrupting chemicals from domestic, industrial, and agricultural wastes, particularly from natural and synthetic steroidal sex hormones.

Results from this study are comparable to other regional and national studies of intersex conditions in fishes, with implications for the management of commercially and recreationally valued fish populations.

Additionally, many of the endocrine-disrupting chemicals could have reproductive consequences in those that consume water or fishes from regional rivers.

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Contact: Jason DeBoer
(309) 543-6000
jadeboer@illinois.edu 

Editor: Lisa Sheppard
(217) 244-7270
sheppard@illinois.edu

The American Midland Naturalist, 176(1):158-165.

Survey of Intersex Occurrence in Largemouth Bass (Micropterus salmoides) from the Upper Illinois River Waterway

Mark W. Fritts, Jason A. Deboer, Andrea K. Fritts, Kristen A. Kellock, Robert B. Bringolf, Andrew F. Casper

http://dx.doi.org/10.1674/0003-0031-176.1.158