Chinook salmon – the Northwest’s largest and most iconic fish species – are shrinking.
Researchers have documented that adult kings returning from the North Pacific are on average 10 percent shorter and as much as 30 percent lighter than 40 years ago.
“If you catch a 30-pound Chinook these days, you’re pretty happy with that,” said Daniel Schindler, a professor in the School of Aquatic and Fisheries Sciences at the University of Washington. “It used to be relatively common to catch 50-pound Chinook salmon. And those size fish are almost unheard of these days.”
Schindler is part of a team of UW scientists who collaborated with federal researchers at NOAA Fisheries to query data sets over the past four decades, looking for the reason why the endangered fish are getting smaller.
UW researcher Jan Ohlberger is the lead author of the study, which was published Monday in the Proceedings of the National Academy of Sciences.
He says they were able to rule out ocean conditions and commercial fishing as causes for the decline in large Chinook. Fisheries managers have reduced the catch quotas allowed over the past 40 years.
But at the same time, there has been a threefold increase in the populations of fish-eating orcas on the West Coast and in Alaska. Northern Residents in the Salish Sea now number more than 200 and in Alaska, resident whales are in the thousands.
“The main point is that over the last four to five decades, harvest rates of Chinook have been stable or declining,” Ohlberger said. “But the predation pressure has increased – so, has taken over – as an important driver of these declines.”
He says fish-eating orcas in Alaska and the northern Salish Sea are taking a much larger percentage of the Chinook now. And they're picky eaters that favor the largest salmon. This means far fewer of the largest Chinook make it home, which also is cause for concern among fisheries experts, because smaller female salmon are less fertile.
Every year, the state spends tens of millions of dollars on salmon restoration – in part to provide more food for the endangered Southern Resident killer whales, by investing in habitat and hatcheries. The new study finds boosting production of the Chinook salmon they prefer may actually feed booming populations of other orcas.
“One possibility is that enhancing salmon down here does nothing more than feed more Northern Residents – because a lot of those salmon head north up along the coast and are certainly intercepted by Northern Resident killer whales,” Schindler said.
He adds that the new findings leave many questions unanswered. The biggest takeaway from this study is that the fate of the Southern Residents needs to be considered in a broader context that includes whales up and down the entire West Coast.