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Scientists Mull Implications of Salmon Sex Change

by Mike O'Bryant
Columbia Basin Bulletin
- December 15, 2000

Scientists have reason to believe that a contributing factor in the decline of Northwest salmon is that many females could have male chromosomes. Samples of fall chinook salmon taken in 1999 from the Hanford Reach on the Columbia River found that 80 percent of the females spawning probably began life as males.

"We have found that a majority of the female chinook salmon sampled carry a genetic marker that is found only in male salmon," said James Nagler, assistant professor of zoology at the University of Idaho. "The best explanation for these results is that these females have been ‘sex reversed' and are in fact male."

The team of scientists working on the project said the cause of the sex reversal is unknown, but that it could be due to pesticides and other chemicals that mimic hormones and disrupt the normal development of salmon, or it could be due to changes in water temperature. Both conditions are present in the river at Hanford Reach, although the presence of contaminants is very low.

Scientists have reversed the sex of salmon in the laboratory using hormones and through changing temperature, but this is the first indication of it having happened to wild fish.

"These results may explain in part the difficulties some salmon have had reproducing in the Columbia River basin," Nagler said. "How far can we take this? Not very far. It's an interesting observation, but it will need further study."

The study was published in the Environmental Health Perspectives, a journal of the National Institute Environmental Health Sciences. The researchers are Nagler, Gary Thorgaard, of Washington State University's School of Biological Sciences, Jerry Bourma, a University of Idaho graduate student, and Dennis Dauble of Pacific Northwest National Laboratory.

According to the researchers, the genetically-altered females from Hanford Reach carry one X and one Y chromosome, which is normally that of a male. Like females, they produce eggs, spawn and then die like all other Pacific salmon, but they still have the genetic markers of males. "These female fish have a Y chromosome and that is not normal," Nagler said.

If a genetically-altered female mates with a normal male, some of the offspring will be males with two Y chromosomes. Nagler called that fish a super male and predicted it could increase the male population. Their offspring can only be male, something that would cause an imbalance in the sex ratio of spawning fish.

He said the researchers could not determine whether there were any of the super males in the study sample because, while they can determine if a fish has a Y chromosome, they have yet to create a way to determine how many Y chromosomes are present. The study sample came from a Washington Department of Fish and Wildlife carcass survey that the agency has performed for the past few years.

Nagler cautioned that the study is very preliminary, very interesting, but that a lot still needs to be done.

"Is this a one-off?" he asked. "Will we see this every year or in other rivers? We have no clue. We can make some educated guesses, but it's all speculative at this point."

Hanford Reach has the healthiest wild run of fall chinook salmon in the Columbia River mainstem, so the findings surprised Nagler. They also looked at fish from the Priest Rapids and Dworshak hatcheries as reference fish and did not find the presence of females with Y chromosomes. It appears that the phenomenon is confined to wild fish, Nagler said.

A collection of samples this year from the lower Columbia River below Bonneville Dam and from the Yakima River will give the researchers further information. Those fish have yet to be analyzed.

Link information:
Pacific Northwest National Laboratory:

An abstract of the article can be found at:

Mike O'Bryant
Scientists Mull Implications of Salmon Sex Change
Columbia Basin Bulletin, December 15, 2000

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