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Scientists point to disease and warming waters to explain the decline of Chinook in the Yukon River


FAIRBANKS, Alaska (KTVF) – Research into declining chinook salmon populations in the Yukon River has led scientists to a theory that combines the stress of warm water and the damage of disease.

As salmon production on the Yukon River continues to decline, especially that of Chinook salmon, those who depend on the fish have faced “hardship (and) a lot of cultural loss,” said Keith Herron, a master’s student at the College of Fisheries and Ocean Sciences (CFOS) at UAF.

Although these fish have historically been a staple of the Yukon diet, fish numbers have declined due to the ocean, but researchers have also seen “a difference between estimates when they are counted as they enter the river and when they enter Canada .” Herron explained. “These estimates do not match and there may be as many as 40,000 fish missing and no harvest.”

Herron and others in the field have already noted the impact of warming waters on species in Alaskan waters. This especially includes crabs and other salmon species. Some have seen population declines, while some are simply declining and others may be moving north toward cooler waters.

However, Herron thinks disease plays a crucial role in the decline of chinook salmon in the Yukon River. Specifically a disease called ichthyophonus. “It’s a pathogen, they get it through food,” said Katherine Howard, chief scientist of the Salmon Ocean Ecology Program for the Alaska Dept. or Fish & Game (ADFG). This disease causes a lot of damage to the muscle tissue of salmon and has a huge impact on the heart.

One of the main theories behind the massive decline is that the combination of ichthyophonus and rising water temperatures have put greater pressure on the already stressed salmon, causing greater mortality during migration.

The disease is contracted while the chinook are at sea. They contract it by eating other fish infected with the disease. “But usually other fish can cure the disease over time,” says Howard. Chinook salmon, on the other hand, don’t seem to be as successful and are resisting it.

“One of the problems with maturing Chinook salmon is that as they move from this cooler ocean water to warmer fresh water, the warmer temperature allows the disease to develop in the Chinook salmon,” Howard said. Additionally, there is little to no relief once they enter the Yukon River. “In the lower 48 you have what they call a cold-water refuge, where the salmon can hang out for a while until the sun goes down in a shady spot and then move further up once it’s cooler.” The lack of these shady places on the Yukon makes these chinook more vulnerable to the elements and puts more stress on the ailing body.

How long this could last and whether there is a solution remains unknown, but for now the salmon and the people who depend on them will continue to face tougher times.

Although this disease and other environmental factors have hampered livelihood food security, ichthyophonus cannot be transmitted to humans by eating infected salmon. Unfortunately, however, most infected fish are not considered edible due to the poor quality of the meat.