Sewage linked to fish-gender quirks.

During dry spells, the water in some streams can come mostly from municipal sewage-treatment plants. A new study finds reproductive impairments among fish residing in such waters.

Alan Vajda and his colleagues at the University of Colorado in Boulder sampled white suckers and flathead chubs upstream and downstream of waste-treatment plants on three Colorado rivers. He harvested the fish during last year's drought, when each stream's flow was dominated by sewage effluent.

Fish upstream of a Boulder treatment plant were fairly evenly divided between males and females. However, 93 percent of the 60 fish caught downstream in the same river were females. Ovaries in many of these fish were smaller than those in their upstream cousins, contained testicular tissue, bore an unusual shape, and held less-developed eggs.

Of 21 fish captured downstream of a Denver water-treatment plant, 81 percent were females and all of the males there had testes containing ovarian tissue.

The sex ratio downstream of the one Colorado Springs sewage-treatment plant was close to normal, but Vajda notes, "We still found intersex fish," which have gonads containing male and female tissue.

What's shocking, Vajda told Science News, is that the source of pollutants at the Colorado sites isn't industry, but flushed toilets.

Vajda and his colleagues are now looking to tie the gender bending of fish to particular pollutants and to find out whether the extra downstream females are the products of a selective die-off of males or a pollutant-triggered sex reversal.--J.R