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 US 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.
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 twelve percent of the salmon that were sampled had the parasite, and ten 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.