:quality(70)/cloudfront-us-east-1.images.arcpublishing.com/adn/CVUMHSPAJBBAZN6CGTQLLVXW2M.jpg?ssl=1 "Small fish size linked to poorer runs of chinook in Alaska’s biggest rivers")
The shrinking size of Alaska salmon, a decades-long trend linked in part to warming conditions in the ocean, is hampering the ability of chinook in Alaska’s two biggest rivers to produce new generations needed to maintain healthy populations, a new study shows.
The University of Alaska Fairbanks-led study shows how the body conditions of chinook salmon, combined with extreme heat and cold in the ocean and freshwater environments, have converged in the Yukon and Kuskokwim river systems to depress what is termed “productivity” — the successful reproduction that results in adult spawners returning to the same area.
The study examines 26 different populations of chinook in those two river systems in areas from Western Alaska to the Yukon River uplands in Canada. Chinook runs in those rivers have faltered in recent years, and the situation has been so dire on the Canadian part of the Yukon that U.S. and Canadian officials earlier this year suspended all harvests of Canadian-origin chinook for seven years.
The analysis of multiple factors and conditions revealed that fish size was a major factor that determined productivity, defined as adult salmon returning to spawning grounds successfully producing a next generation of adults to come back to the same spawning area.
There are explanations for the relationship of size to productivity success, said Megan Feddern, who led the study when she was a postdoctoral researcher at UAF’s College of Fisheries and Ocean Sciences. She is now a fish biologist with the National Oceanic and Atmospheric Administration.
Smaller female fish carry fewer eggs to spawning grounds, as has been shown by prior research. Additionally, those eggs are in poorer condition than eggs in big fish, Feddern said. And smaller fish, whether female or male, are less robust and less likely to make the full journey, especially up the full length of the nearly 2,000-mile Yukon River, she said.
“It carries over into that next generation,” Feddern said. “The females and males that are making the migration and have that better body condition are able to produce more, but then it also sets that next generation up for success.”
Other factors found to have hindered productivity for Yukon and Kuskokwim fish were warm summers at sea during the first year in the ocean for any given chinook salmon age group; unusually winter cold conditions encountered by fish in their first winters at sea; and unusually warm temperatures in freshwater river habitat.
The dangers of overly warm conditions for salmon have been documented before, as shown by past records of heat stress suffered by Yukon River salmon in 2016 and 2017. But just why cold winters in the Bering Sea are such problems is yet to be explained, Feddern said.
“I don’t think we have a good grasp on what exactly is happening when it comes to those cold winters,” she said. “We suspect it’s just, these young adults that are in the ocean are just not surviving through those really cold conditions.”
High runoff was another condition harming productivity for some populations, notably in the Canadian part of the Yukon River system, the study found.
The study evaluates conditions only up to 2016, so it misses events that happened during the recent and prolonged Bering Sea marine heatwaves, along with heat-related events in freshwater areas. For example, it does not include the ultra-warm river temperatures in 2019, a record-hot year in Alaska, that were blamed for a mass die-off of salmon in the Koyukuk River, a Yukon River tributary, and die-offs elsewhere.
Feddern said there are plans to update the study with data from those more recent years.
The study’s research questions were inspired by community concerns. Those were expressed at a 2022 workshop in Fairbanks that was hosted by UAF’s International Arctic Research Center.
There, as residents of river communities talked about their experiences and their relationships with fish, UAF researchers learned that the shrinking salmon size was a major worry, Feddern said.
“I would say that the size component that was included in this research was really driven by folks being concerned about that,” she said.
All species of Pacific salmon have been shrinking in size over the past decades, but the change has been most dramatic among chinook salmon, also known as king salmon.
The smaller size has negative impacts on the fishing industry. In the Bristol Bay region, the site of the world’s biggest sockeye salmon runs, the average sockeye salmon size this year was the smallest on record, according to the Alaska Department of Fish and Game. Even though this year’s total number of fish that returned was above the 20-year average, the total number of fish commercially harvested was below average, the department reported. The reason appears to be related to fish size, with many of the returning salmon being too small to be caught in fishers’ nets, a department biologist said in September.
Originally published by the Alaska Beacon, an independent, nonpartisan news organization that covers Alaska state government.
