ANCHORAGE, Alaska (CBS Local) — Scientists are blaming unprecedented heat wave this summer for reduced salmon populations in parts of Alaska.

Researchers say they found more than 800 dead salmon at the end of July during an expedition along Alaska’s Koyokuk River. They estimated the total was likely four to 10 times larger.

When they inspected the fish, they couldn’t find any signs of parasites or infection. Many of the salmon were also carrying healthy eggs.

Because the die-off coincided with a heat wave and record high temperatures in the water, they concluded that heat stress was responsible for the mass deaths.

Stream temperatures near Anchorage surpassed 76 degrees this year for the first time since record keeping began in 2002, CNN reported. On July 7, a major salmon stream on the west side of the Cook Inlet registered 81.7 degrees.

Warmer water makes it difficult for salmon to absorb the oxygen they need to breathe.

Sue Mauger, the science director for the Cook Inletkeeper said she and her team published a study in 2016, creating models outlining moderate and pessimistic projections for how climate change would drive temperatures in Alaska’s streams.

“2019 exceeded the value we expected for the worst-case scenario in 2069,” she told CNN.

Ecologists worry reduced populations of several varieties of Alaskan salmon, including sockeye, chum and pink salmon, will work their way up the food chain and impact animals that eat them, including orca whales.

In the meantime, things are looking up in other parts of the state. The largest sock-eye salmon fishery in the world is reporting a boom in the number of fish it is seeing return at Bristol Bay.