How do chum salmon spend the winter?
Winter is known to be an important part of the Pacific salmon life cycle—and a critical period for their survival. Scientists believe that a third of all Pacific salmon, including Western Alaska stocks, overwinter in the Gulf of Alaska.
But winter salmon ecology in the region is poorly understood because it’s hard to get out and do surveys in the open ocean during that season, said researcher Ed Farley.
Farley is the program manager for the Ecosystem Monitoring and Assessment Program at the Alaska Fisheries Science Center, Auke Bay Labs, in Juneau.
He gave a Strait Science presentation last week to talk about the results of recent winter surveys of Pacific chum stocks.
Members of the North Pacific Anadromous Fish Commission started a project to survey Pacific salmon wintering grounds as part of the International Year of the Salmon initiative. This initiative, which lasted for several years between 2018 and 2022, was intended to figure out ways to support the resilience of salmon in both the Atlantic and the Pacific. The Pacific surveys involved researchers from Canada, Japan, Korea, Russia and the United States, which all host salmon populations that intermingle in the Gulf of Alaska.
The group surveyed the Gulf of Alaska between February and March 2019 and again in 2020. A larger survey began in February 2022 with four vessels, two Canadian, one American and one Russian. But within weeks of the survey’s start, Russia invaded Ukraine. After Russia called its vessel back, the team was able to quickly charter another American fishing vessel, the Northwest Explorer, to participate.
Farley explained that the researchers saw some differences in the condition of salmon they sampled in 2019—when a severe marine heatwave hit the Gulf of Alaska— and 2022, when cooler conditions returned.
For example, one-year-old chum salmon in their first winter at sea had much lower fat content than in 2022,
according to the samples.
A group of Japanese researchers involved in the project also looked at older surveys from 1996 and 2006 to compare with the 2019 results. They found that the condition of two-yearold chum salmon was worse in 2019, and the proportion of skinny fish they found in 2019 was almost 40 percent, compared to about 5 percent in 1996 and 10 percent in 2006.
“These are older age chums,” Farley said. “Typically, after the first year in the ocean, the older ages are more capable of handling some of the changes in their environment.” But that wasn’t the case in 2019. “What this really indicates is this marine heatwave was very stressful for chum salmon.”
Other researchers involved in the project looked at what chum salmon were eating in these winter habitats.
Once again, conditions in 2019 were rough for chums. A much larger proportion of their diet during that anomalously warm year came from very poor prey items like jellyfish, Farley said.
“In the 20 years of data that we’ve seen, it seems like things have been shifting a little bit, but it shifted a lot in ’17 through ’19 in terms of the types of prey and the amount of poor-quality prey that was available,” Farley said. “I know that there were a lot of things that changed in that ecosystem—unusual mortality events, seabird die-offs, you name it, there were a lot of impacts.”
In 2022, when it was cooler, chums were still eating some jellyfish but a bigger proportion of their diet was coming from fish, shrimp and krill.
“These are much better prey in terms of quality,” Farley said.
Farley saw these various results as signs that extreme events like heatwaves could negatively impact chums and lead to high mortality rates.
“These extreme events have negative impacts on the condition of chum salmon,” Farley said. “This stress likely increased over-winter mortality and could have contributed to these really poor adult returns that have occurred over the last few years to Western Alaska.”