The size of king salmon returning to Western Alaska rivers to spawn has been decreasing over the past few decades. Researchers at the University of Alaska Fairbanks think that they’re closer to understanding why.
Peter Westley and Andrew Seitz are fisheries scientists at the University of Alaska Fairbanks who helped publish new research on king salmon in February. To answer why these salmon are getting smaller, researchers attached tags to the fish that can record the depth and temperature of the water around them. Seitz said that many of the tags on the salmon were recording temperatures above 70 degrees Fahrenheit.
“And we thought, ‘Well, where is it, you know, in the high 70s in the winter in the Bering Sea, even at depth?’ And the only place that we can infer that is that warm is in the belly of a salmon shark,” Seitz said.
Westley and Seitz’s research indicates that returning king salmon are getting smaller because the bigger ones are getting eaten. They said that predators, like salmon sharks, may target the older, larger kings because they stand out. Seitz said that predators’ preference for larger fish may have always existed, but there could just be more predators now than in the past.
“There’s sort of circumstantial evidence that salmon shark numbers have increased since the 1980s since high seas drift netting was banned,” Seitz said.
Seitz is referring to the 1989 United Nations moratorium on high seas large-scale driftnets and the global ban in 1992. He said that the 1972 Marine Animal Protection Act could also have increased the number of other predators, like seals, that also prey on salmon. Seitz said that in addition to there being more predators, they may also be roaming around in more northern waters, which is an important habitat for king salmon.
“It was thought that salmon sharks largely hang out south of the Aleutian Islands and in the Gulf of Alaska, but it turns out that people are encountering salmon sharks as far north as the Bering Strait now,” Seitz said.
Westley said that smaller fish also means fewer king salmon in future runs.
“Larger fish have more eggs and tend to have bigger eggs. And so the large, old females are the ones that contribute most to the population,” Westley said. “Harvesters have to try to catch more fish to make up for the fact that they’re smaller, and there’s not that many extra fish to be caught these days. So it has profound consequences for people.”
It’s not yet clear how much the decline in king salmon numbers and size can be attributed to predators, as opposed to increased fishing pressure or other aspects of climate change. But Seitz and Westley’s research shows the need for more research into the non-human animals that have an impact on salmon sizes and numbers.