Newswise — Researchers have determined that sewage plant effluents can affect not only individual aquatic organisms but also can have an impact on aquatic populations, reducing their long-term viability. The study is published in the latest issue of Environmental Toxicology and Chemistry.

It is well established that sewage treatment plants release estrogens that can cause changes in the gender of an individual fish or invertebrate, but the long-term consequences to populations were unclear. This study confirmed that such estrogens can have population-level impacts.

During experiments conducted as part of this study, effluent exposure reduced male fish aggressiveness and also their ability to successfully compete with unexposed males for a nest. The exposed fish were less able to attract a female and successfully reproduce. Their competitive reproductive fitness was reduced.

While estrogens had a negative impact on fathead minnow reproduction, exposure to a male hormone showed opposite results. Exposed fish were more aggressive than unexposed fish, and they competed better for nests, thus producing a greater number of eggs.

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Environmental Toxicology and Chemistry is the monthly journal of the Society of Environmental Toxicology and Chemistry (SETAC). For more information about the Society, visit

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Environmental Toxicology and Chemeistry (Vol. 26(2), 2007)