No clean bill of health for Yukon salmon
Chinook salmon in the Yukon River have always had one distinction to their credit; if they make it back to the upper reaches of the Yukon River system to spawn, they have migrated further than any other chinook salmon in the world.
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Chinook salmon in the Yukon River are carrying a parasite never before detected in wild salmon.
(photo: E.R. Keeley)
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But this same stock of salmon is now notable for another, more troubling reason. It is the first wild population of salmon documented to have the parasite Ichthyophonus hoferi, a distinction the fish most certainly could do without.
Six or seven years ago, people fishing on the Alaskan portion of the river began catching salmon with little white spots in the heart and liver, and large grayish white lesions in the meat.
While eating infected fish is not thought to be a human health concern, fish processors did not want to buy salmon showing signs of the disease. People also reported that meat from infected fish tasted odd.
A 1999 pilot study sampled fish in the U.S. portion of the river, and found that the number of diseased fish increased between the mouth of the river and sites further upstream. Last summer, salmon were sampled along the length of the river, and those tests produced some puzzling results.
At the mouth of the river, 25 percent of the fish carried the parasite, but only 2 percent showed signs of the disease. Fish can carry the parasite, and never show actual signs of the disease itself.
Moving up the river, the percentage of fish carrying the parasite stayed about the same, but more of these infected fish showed clinical signs of the disease, meaning the telltale little white spots.
All of the infected salmon sampled near Dawson City had clinical signs of the disease. But at the Whitehorse Fishway, only 12 percent of the salmon that were sampled had the parasite, and 10 percent had the disease.
Paul Hershberger, a faculty research associate in the School of Aquatic and Fishery Sciences at the University of Washington, has been working on this study. He says that researchers cannot yet explain what happened to the missing diseased fish.
One theory is that the fish somehow cleared themselves of the infection between Dawson and Whitehorse, but Hershberger does not think this is very likely. "Once fish get to the state where you can see the disease, I have never yet seen a case where they have recovered," he says.
Other theories are that the diseased fish were lagging behind the main pulse of salmon and were not sampled, or that they died upstream of Dawson, never making it as far as Whitehorse.
Researchers are also trying to figure out how the salmon might have picked up the disease in the first place. Ichthyophonus hoferi is very common in Pacific herring, and the salmon might have picked up the parasite by eating infected herring. Another theory is that warmer-than-normal water is stressing the fish.
Salmon are cold water fish, and during the summer of 1999, the water in the Yukon River might have been too warm for their liking. If the disease is present at low levels all the time, the warm water could stress the salmon to the point that they cannot fight off the disease.
"We do not know whether it had an effect or not, but we were looking at water temperatures of 18 degrees Celsius at the mouth of the river. It was really warm," says Herschberger.
This parasite creates even more problems for Fisheries and Oceans Canada, as biologists there are already trying to cope with lower than normal salmon runs in the Yukon River. In 2000 only about 12,000 chinooks made it back to their spawning grounds, a number far below the department's goal of 28,600 fish.
Pat Milligan, a stock assessment biologist with Fisheries and Oceans Canada, says that the department has a good track record on predicting returns to the spawning grounds. The numbers of salmon passing through fish wheels at the international border and at Dawson City are used to make these estimates.
Last summer the low returns might have been caused by high water levels, which make it harder even for healthy fish to swim upstream. The high water might have made it impossible for fish weakened by disease to complete the long trip upstream.
"It could have been a combination of factors. We obviously need to understand more about it," says Milligan.
This year biologists may take samples from juvenile fish still living in fresh water. These tests would tell them whether the salmon are carrying the infection before they ever reach salt water.
Chinook runs have been low for three years in a row, which could mean that numbers might stay low for some time. Milligan says that there have been cases in which salmon stocks have taken 20 years to recover after years in which few fish returned to the spawning grounds.
"We might have to take a more precautionary approach in management, ensuring that more fish make it to the spawning grounds," he says.
For more information on chinook salmon in the Yukon, contact Fisheries and Oceans Canada at (867) 393-6722. Paul Hershburger can be contacted at paulh@fish.washington.edu or (206) 685-4864.
