Image: University of Idaho/BILL LOFTUS

For years salmon populations in the northwestern U.S. have been dwindling. As a result, a number of them are now endangered or extinct. Researchers have long suspected that degradation of the river ecosystems might play a leading role in the plight of these fish, but the idea had not been fully explored. New findings, though, suggest that in at least one case, what's going on is stranger than anyone had imagined. According to a report published today in the journal Environmental Health Perspectives, tissue samples collected last year from fall chinook salmon spawning in the Columbia River (right) show that 84 percent of fish that appeared to be female carried male-specific genetic markers. "The best explanation for these results," says lead author James J. Nagler of the University of Idaho, "is that these females have been 'sex-reversed' and are in fact male."

Such reversals are not unheard of, as salmon sex can be reversed experimentally in the lab, Nagler reports. But these new findings document the phenomenon for the first time in a wild fish population. Earlier studies have shown that hormones can induce sex reversal in trout embryos, and young sockeye salmon will make the switch if exposed to different temperatures. Both factors might be affecting the Columbia river chinooks: Nagler and his colleagues have detected pesticides and other chemicals that could mimic estrogen in the Columbia's water, and the river's temperature regularly changes with the hydroelectricity-related fluctuating water flows.

Intriguingly, such a reversal does not appear to render the fish reproductively impaired. The altered females produced eggs, spawned and then died--the normal Pacific salmon life cycle. But frequent matings between altered females and normal males, both of which carry an X chromosome and a Y chromosome, can yield males with two Y chromosomes. If, as in the case of coho salmon and rainbow trout, these males are sexually viable, they can only produce male offspring. This reduces the number of genetic females in the population with each successive generation, and leads to an imbalance in the sex ratio of spawners. Indeed, the team concludes, "this may be occurring in the present population, where 92 percent of the wild fish sampled carry a DNA marker found on the Y chromosome."