Why are Alaska’s salmon shrinking?

A fisherman sets a net on the Yukon River. Yukon River communities are heavily reliant on salmon for their food security. Photo by Peter Westley.

Alaskans who have fished for salmon consistently over the years know it: Alaska’s salmon, especially king salmon, are getting smaller. Now, a new study, published August 19 in the journal Nature Communications by lead author Krista Oke, a postdoctoral fellow with the College of Fisheries and Ocean Sciences at the University of Alaska Fairbanks; senior author Eric Palkovacs, Professor of Ecology and Evolutionary Biology at the University of California Santa Cruz; and an international team of co-authors, many of them also based in Alaska, delves into why that is — and what it means.

Salmon strips dry on a rack on the Yukon River, a place where the impacts of shrinking Chinook salmon have been among the most dramatic for subsistence users. Photo by Peter Westley.

Salmon are getting smaller

Over the last 60 years, the sizes of Alaska’s salmon have declined, though there were also periods of slight recovery. Around the year 2000, however, size declines intensified, and in 2010, they began accelerating. The size change was most extreme for Chinook salmon — most likely because they’re the largest and tend to stay out in the ocean the longest — and in regions that historically have older, larger Chinook salmon returning, like the Yukon and Kuskokwim Rivers. The average size of Chinook salmon in the Arctic, Yukon and Kuskokwim area, as well as Southcentral Alaska, was, on average, 10 percent smaller after 2010 than it was before 1990. Some specific populations declined as much as 20 percent on average.

Overall, the length of Chinook salmon has gone down 8 percent on average between 1990 and 2010; coho salmon length has gone down by 3.3 percent; chum salmon by 2.4 percent; and sockeye salmon by 2.1 percent.

That size decline was driven by fish spending less time in the ocean and returning younger. “Some populations lost multiple years, on average,” Palkovacs said.

There isn’t much size data out there for pink salmon, so that’s the one species they didn’t look into.

What does that mean?

The study identified several main consequences of smaller salmon. The first: smaller females mean fewer eggs.

“That means to get the same population productivity, you need more fish in the population,” Palkovacs said. “Fixed escapement policies assume a certain number of females is going to continue to produce the same number of juveniles. But if females are smaller, it means that same number of females is going to be producing a smaller number of eggs than in the past.”

From a subsistence perspective, “if you have a limited harvest opportunity and the fish you get are smaller than what you used to get, it really does mean fewer meals in your freezer for the winter,” Oke pointed out.

From a commercial perspective, smaller fish earn fishermen less money — because it takes more salmon to get the same amount of poundage, it takes more time to process a fish, and because larger fish can command more per pound.

From an ecosystem perspective, smaller salmon transport fewer marine-derived nutrients into the ecosystem.

From a resilience perspective, losing certain age classes means less resilience in the face of environmental change.

A female sockeye salmon that spent three years in the ocean (top) compared to a female that spent just two years in the ocean in Pick Creek, Alaska. One year in the ocean makes a big difference — which is why salmon returning younger has caused such a dramatic decline in body size. Photo by Andrew Hendry.

Why is this happening — and what can humans do to stop it?

“We didn’t find any single smoking gun that was able to explain these changes across species and across regions,” Oke said. “It really seems to be a cumulative effect of smaller impacts across lots of different factors. Different species respond differently to different specific drivers.”

That being said, climate change and ocean competition from high abundances of wild and hatchery-enhanced salmon contributed to salmon body size declines.

They also looked at size-selective fishing, but found that while it may impact certain populations, it likely can’t account for the larger trends across Alaska’s river systems and species.

Predation from marine mammals like killer whales is a potential cause that they didn’t have enough data to look at fully, but that “probably is not a broad-scale driver but may be having impacts on certain populations,” Palkovacs said. In Washington, for example, where salmon populations are a fraction of historic levels, killer whales selecting larger Chinook salmon would likely have a larger impact on the population now than it did when they engaged in the same practice 150 years ago.

“All of these factors matter to different degrees across different species, but they’re all generally pushing (salmon) to be smaller,” Palkovacs said. “That means there’s no magic lever we can pull to change the trends. On the other hand, it also means maybe the trends will be moderated by the fact that there’s this multitude of factors operating all the time. Everybody shares a little bit of the responsibility, so it almost suggests that it’s a collective problem that requires a collective solution.”

Read the study here: https://www.nature.com/articles/s41467-020-17726-z

For more information about the study and about SASAP, go to https://alaskasalmonandpeople.org/topics/the-declining-size-and-age-of-salmon/

About the study

The study, “Recent declines in salmon body size impact ecosystems and fisheries,” was made possible via a collaborate initiative through the National Center for Ecological Analysis & Synthesis (NCEAS). The initiative was called SASAP, the State of Alaska Salmon and People project. It brought people together from across multiple disciplines and backgrounds. One important part of the SASAP initiative was the inclusion and involvement of Indigenous voices and knowledge, which authors said added a lot to the study.

It was also only possible because Alaska Department of Fish & Game biologists made 60 years of detailed data on salmon size, age and sex across many different river systems available to the researchers — and because data scientists at NCEAS crunched that data into a huge dataset the authors could analyze. The bigger the data set — and this one was very big, with 12.5 million records of four species of salmon measured for age, sex and length, over 60 years across more than a thousand locations — the more difficult it is to work with.

That synthesized data set is now up online and available for anyone to use, analyze, and ask new questions.