Salmon puzzle: Why did males turn female?

At Hanford Reach, Nagler takes samples from the fins of Chinook salmon that recently spawned. Bill Loftus/Univ. of Idaho

Every year, rivers of chinook—the Pacific’s largest salmon—leave the ocean for an upstream trek into the streams of their birth. When these 4-to-6-year-olds reach home, they spawn and die. Surprisingly, a new study finds, most of the moms in one of Washington State’s major spawning populations appear to have begun their lives as males.

“This is clearly abnormal,” notes James J. Nagler, a fish reproductive biologist at the University of Idaho in Moscow. The findings that he and his colleagues reported late last week worry several environmental scientists.

“Salmon are declining everywhere,” observes Kelly Munkittrick, an ecotoxicologist with Environment Canada who’s based at the University of New Brunswick in Fredericton. He says the discovery of sex-reversed male chinook (Oncorhynchus tshawytcha) provides “another potential piece of the puzzle as to why these animals are in so much trouble.”

During an analysis of adult salmon gonads performed last year, Nagler identified 50 males and 50 fish that appeared to be female at each of three spawning sites: hatcheries in Washington and Idaho and a river bend called Hanford Reach near Richland, Wash. The last of those sites is the most important fall spawning area for wild chinook in the main Columbia River.

Nagler took small fin samples and then assayed the DNA for a chemical normally associated with a male’s Y chromosome. Hatcheries use this assay to determine gender in young chinook.

No surprise, the males all tested positive. However, 84 percent of apparent females from the Hanford Reach also tested positive for the male-associated chemical, whereas none of the females spawning at the hatcheries did. In the January Environmental Health Perspectives, Nagler’s team argues that some pollutant probably converted wild male embryos or hatchlings into fish that look like and function as females.

Unfertilized fish eggs normally carry one of the mother’s two X chromosomes. Sperm can carry either the father’s X or his Y chromosome.

When hormone-mimicking chemicals create chromosomally male fish that lay viable eggs (SN: 2/5/00, p. 87: Available to subscribers at DDT treatment turns male fish into mothers), some of these eggs carry a Y chromosome instead of an X. That increases the odds that an offspring will get at least one Y chromosome and be male. Furthermore, some offspring may get a Y from both mother and father and become so-called supermales.

If supermales reproduce, Nagler says, any offspring would invariably be male—skewing the sex ratio in a population. Eventually, females could all but disappear. The scenario offers a new potential explanation for the decline of many salmon populations, says Nagler.

The trigger for the chinook’s sex reversal remains a mystery. Though estrogen-mimicking pesticides running off upstream croplands could play a role, Nagler has found no reports of high concentrations in the Columbia River near Hanford Reach. Another suspect, he notes, is the daily drop of several degrees in water temperature in response to a nightly release of cold water from hydroelectric dams. For several fish species, including another salmon, temperature variation has induced gender changes in laboratory studies.

By not releasing fish until many weeks after their gender becomes irreversible, hatcheries appear to protect fish from such environmental influences, Nagler says. But why weren’t all the Hanford Reach males feminized? It’s possible, Nagler says, that some came from a hatchery. “At Hanford,” he says, “hatchery fish could be masking the impact of what’s going on in wild fish.”

As the first report of environmental gender bending in wild Pacific salmon, “this is an extremely important paper,” notes Don Campton, a regional fish geneticist with the U.S. Fish & Wildlife Service in Longview, Wash. Further studies are needed to confirm the result and find out whether it represents a one-season fluke. Nevertheless, he worries it may also signal risks facing other fisheries.

Munkittrick agrees. He argues that whatever triggered the chinook’s sex reversal may threaten its beleaguered kin, including Atlantic salmon, which he studies. “If we can get access to similar tools,” such as an assay for the Y chromosome of the Atlantic species (Salmo salar), he told Science News, “we’re definitely going to see if similar things are going on here.”

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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