SEATTLE — Like mariners scanning the horizon from the crow’s nest, scientists have for years been on the lookout in the Pacific Northwest for signs that a dreaded salmon-killing disease, scourge to farmed salmon in other parts of the world, has arrived here, threatening some of the world’s richest wild salmon habitats. Most say there is no evidence.

But for years, a biologist in Canada named Alexandra Morton — regarded by some as a visionary Cassandra, by others as a misguided prophet of doom — has said definitively and unquestionably that they are wrong. Wild Pacific salmon, she has said, are testing positive for a European strain of the virus that causes the disease, infectious salmon anemia, or I.S.A.

The virus, which has struck farmed salmon populations in Chile, among other places, is not harmful to humans who eat the fish, but could potentially pose grave threats in a part of the world where salmon plays a huge role in local economies and ecosystems. If the virus, which is in the influenza family, mutates into a virulent Pacific strain in the crowded fish farms in British Columbia, where wild and farmed salmon are sometimes in proximity, fish populations on both sides of the farm/wild divide, Ms. Morton believes, could be devastated.

“It’s an uncomfortable truth,” she said.

But scientists and government testing groups in Canada and the United States have said repeatedly over several years that Ms. Morton’s findings were not sufficient to sound an alarm, and that the risks to wild salmon, even in the event of a fish-farm outbreak, are unclear. After rounds of government hearings and millions of dollars spent on research, the two sides are in an increasingly bitter standoff.