Why are Alaska’s salmon shrinking?

by Mary Catharine Martin

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 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.

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, especially pink 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.”

Mary Catharine Martin is the communications director of SalmonState, an organization that works to ensure Alaska remains a place salmon and the people who depend on them thrive.