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Study shows connection between recent marine heatwaves and Western Alaska chum salmon declines

Chum salmon swimming upriver.
Stori Oates
NOAA Fisheries
Chum salmon swimming upriver.

For newly hatched Western Alaska chum salmon, there is no time to waste when it comes to making their way to the open ocean. The tiny fry begin their journey from their natal streams just days or weeks after being born. When they finally reach the Bering Sea, sometime from mid-June to mid-July, their priority becomes consuming marine prey and building the energy reserves that will carry them through their first winter. Throughout their years in the ocean, the Western Alaska chum will travel extensively between the Bering Sea and the Gulf of Alaska.

Unfortunately, simultaneous warming trends in the Bering Sea and the gulf appear to have come as a double whammy for Western Alaska’s juvenile chum salmon. A new study by scientists from the National Oceanic and Atmospheric Administration (NOAA) and the Alaska Department of Fish and Game shows a possible link between a period of exceptionally warm ocean temperatures and chum crashes seen across Western Alaska.

“Loss of sea ice is having an impact on various ecosystems. And so with warming we're seeing a change in the food web,” said Ed Farley, lead author of the study and head of NOAA Fisheries Ecosystem Monitoring and Assessment Program. “That food web is less energetic. It's poorer-quality prey. And it's impacting juvenile salmon, especially juvenile chum salmon in the northern Bering Sea. It's impacting their fitness prior to winter.”

By “poorer-quality prey,” Farley primarily means jellyfish, also known as cnidaria. Jellyfish have been shown to proliferate when ocean temperatures warm.

“There are more cnidaria in the ecosystem of the Northern Bering Sea during warm years, but there was significantly more during this most recent anomalously warm period,” Farley said.

From 2017 to 2019, Bering Sea ice receded to record low levels thanks to persistent warm southerly winds.

In addition to bringing lower-quality prey, the study finds that a six-year period of warmer ocean temperatures in the Bering Sea that began in 2014 may have also increased the metabolic rates of chum, increasing their need to feed. While the chum grew in size, they remained skinny and poorly equipped for survival.

“Even though they were large and they were relatively abundant, they had very little fat,” Farley said.

Farley said that relative abundance of other juvenile salmon species in the Bering Sea, like chinook or pink salmon, usually correlates to how many will eventually swim back upstream. But that wasn’t the case with the lean chum observed in the study during the six-year warm period. Instead, record-low chum salmon returns have hit Alaska Native communities hard in recent years, especially those along the Yukon River.

A graphic shows changes in the stomach contents of Western Alaska juvenile chum salmon between warm and cold periods over a 17-year span (2003-2019).
NOAA Fisheries
A graphic shows changes in the stomach contents of Western Alaska juvenile chum salmon between warm and cold periods over a 17-year span (2003-2019).

While the new study focuses on the fitness of juvenile chum during their first summer in the Bering Sea, Farley and his colleagues have also looked into the fitness of juvenile chum wintering in the Gulf of Alaska. Farley said that waters in the gulf were unusually cool in 2022.

“We are seeing real differences in their energetic status and some of the other metrics that we're starting to look at now,” Farley said.

While cooler waters in the Gulf of Alaska may bode well for Western Alaska chum salmon returns in the short term, Farley said that long-term projections regarding ocean temperatures are less than rosy.

“By 2040, at least, the climate models suggest that the northern Bering Sea will not have sea ice in the winter,” Farley said. “And so I think that what we have just witnessed is some of what may be occurring in the future.”

Farley said that it’s hard to predict how ocean temperatures may change in the coming years.

“It could be that it warms up in the Bering Sea, but the Gulf of Alaska stays okay. But it's hard to say,” Farley said. “I think what this study does is it gives context to the impact of climate change and warming in these northern Arctic and sub-Arctic ecosystems, and the relative impact that that's going to have. And so it can be huge.”

Farley said that major recent advances in ocean modeling may provide a clearer look at chum migration patterns in the future, whichever way the temperatures trend. He also said that he is looking forward to the chance to present some of his team’s findings regarding the winter ecology of Western Alaska chum salmon at the upcoming Alaska Marine Science Symposium in Anchorage in late January 2024.

Evan Erickson is a reporter at KYUK who has previously worked as a copy editor, audio engineer and freelance journalist.
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