Chinook salmon are shrinking, and Alaska’s killer whales may be among the culprits

A Chinook salmon frequents Oregon’s McKenzie River. (Morgan Bond Photo via UW)

King salmon, the big fish that are famous in Alaska and the Pacific Northwest, are shrinking — not only in numbers, but in size as well.

A study published today in the journal Fish and Fisheries has found that the largest and oldest Chinook salmon (also known as king salmon or Oncorhynchus tshawytscha) have mostly disappeared along the West Coast.

“Chinook are known for being the largest Pacific salmon, and they are highly valued because they are so large,” lead author Jan Ohlberger, a research scientist in the UW’s School of Aquatic and Fishery Sciences, said in a news release. “The largest fish are disappearing, and that affects subsistence and recreational fisheries that target these individuals.”

The study is based on 40 years’ worth of data focusing on populations of hatchery fish and wild Chinook populations from California to Alaska. The fish are born in freshwater rivers and streams, spend most of their lives in the Pacific Ocean, then return to their home waters to spawn and die.

For most of the populations studied, the average length of the returning fish has declined by as much as 10 percent over the decades, Ohlberger and his colleagues found.

Ohlberger said the widespread shrinkage points to a trend that goes beyond regional fishing practices, animal behaviors or inland ecosystems.

“This suggests that there is something about the larger ocean environment that is driving these patterns,” Ohlberger said. “I think fishing is part of the story, but it’s definitely not sufficient to explain all of the patterns we see. Many populations are exploited [by fishing] at lower rates than they were 20 to 30 years ago.”

The preferences of marine mammals such as killer whales may play a part.

“We know that resident killer whales have a very strong preference for eating the largest fish, and this selectivity is far greater than fisheries ever were,” said senior author Daniel Schindler, a UW professor of aquatic and fishery sciences.

The population of southern resident killer whales that frequent Washington state’s Puget Sound may be in decline, but the numbers of northern resident killer whales that hang out around Alaska appear to be growing at an extremely fast rate. The UW researchers suggest that the burgeoning northern orca population may be a significant factor in the plight of the Chinook salmon.

Other scientists and fishing industry sources have noted a rapid decline in the numbers of Chinook salmon returning to the Southeast Alaska coastline — which has led to tighter restrictions on commercial fishing. And last month, yet another research group from Washington State University reported a dramatic long-term decline in genetic diversity among Columbia River Chinook salmon.

In addition to Ohlberger and Schindler, the authors of “Demographic Changes in Chinook Salmon Across the Northeast Pacific Ocean” include Eric Ward and Bert Lewis.