Unusually warm weather could have led to the die-off of hundreds of chum salmon on the Kobuk River in Northwest Alaska, officials say.
A necropsy conducted by the Alaska Department of Fish and Game's pathology lab on a fish collected by the U.S. Fish and Wildlife Service indicated a normal, healthy salmon, though it had multiple types of algae stuck to its gills and skin. That indicates a high level of algae in the water, according to Fish and Game chief pathologist Tim Meyers, and that fish are dying from "low dissolved oxygen" in the water.
It appears that a stretch of warm, sunny weather in the region led to an increased algae bloom, wrote Susan Georgette, outreach specialist for the Selawik National Wildlife Refuge.
"Algae blooms consume lots of oxygen in the water," she wrote, "and combining that with a huge return of salmon could have led to enough oxygen depletion to kill salmon."
Hundreds of unspawned but seemingly otherwise healthy salmon began washing up dead on the shores of the Kobuk River this month, worrying residents, and some fish had white welts on their backs. Biologists suspect those welts were the result of sun exposure in shallow water along the river's edges.
The die-off was one of the largest ever in the Kobuk, according to locals. Georgette and a U.S. Fish and Wildlife Service colleague traveled by boat last week from Kiana to Coal Mine, about 25 miles upriver, to conduct fish and water sampling in the area. In an email, she said they counted 284 salmon carcasses in a little less than a half-mile stretch of river.
The deaths appear to be related in part to the massive chum salmon run in the region. The chum fishery is having one of its strongest years ever, with over 600,000 fish caught by commercial fishermen in Kotzebue this season alone. But the run appears to have clogged the Kobuk, pushing chums to the banks, where the water is shallower, warmer and contains less oxygen.
Further tests for diseases are being done on the fish, though Meyers said an infection is unlikely, based on the size of the die-off and the fact that pike and sheefish were also found dead.
"Mortality like that doesn't occur from an infectious agent," he said Monday from Juneau. "What causes that is environmental."