Salmon have been found dead in rivers across Western Alaska this summer. The largest die-off reported comes from the Koyukuk River, a tributary of the Yukon.

From the Koyukuk River, to the Kuskokwim, to Norton Sound, to Bristol Bay’s Igushik River, unusually warm temperatures across Alaska this summer led to die-offs of unspawned chum, sockeye and pink salmon. Warm waters also sometimes this summer acted as a “thermal block” — essentially a wall of heat salmon don’t swim past, delaying upriver migration.

Norton Sound residents have reported salmon die-offs in unusually large numbers during the last week. According to the Norton Sound Economic Development Corporation (NSEDC), dead pre-spawned pink salmon were found in multiple river systems over the weekend.

Dead salmon have shown up in river systems throughout Alaska, and the mortalities are probably connected to warm water or low river water levels, said a Fish and Game official.

Sockeye salmon are migrating up B.C.'s Fraser River right now, but the water is so warm the fish may die before they have the chance to spawn.