(CN) – In the ocean near Seattle, three families of orca are starving. Known as the Southern Resident killer whales, their decline is mostly due to the decreasing numbers of the Chinook salmon they eat. But further north, fish-eating orcas are thriving. Resident orcas near Alaska have tripled, and a study published Tuesday says their continual consumption of only the largest Chinook prevents those big fish from reproducing and has forced an evolution toward smaller fish – more bad news for Southern Residents.
Chinook salmon in the Pacific Northwest spend up to five years in the ocean before returning to the rivers of their birth to spawn and die. In the ocean, they swim north past Canada and Alaska, sometimes as far as the Bering Sea. It’s a journey from which many don’t return. Along the way they feed whales, sea lions and the nets of human fishermen. But one predator is having an outsize effect, according to a study published Monday in Proceedings of the National Academy of Sciences.
The largest Chinook salmon are 10% shorter today than in 1971, the study says. It’s a change that translates to a 30% reduction in overall body mass and could be contributing to the decline of endangered killer whales – even as it appears to be caused by the whales’ northern relatives.
Fishermen have noticed the changes for decades. So a team of researchers with the University of Washington and the National Oceanic Atmospheric Administration set out to verify those anecdotes, and figure out why such an important fish appeared to be shrinking.
Unlike most orca, Resident killer whales don’t eat seals and other sea mammals. Instead, their diet is focused on just one fish – Chinook salmon. And they choose the biggest, most calorie-dense Chinook. While Southern Resident killer whales have declined to just 73, Northern Residents off the coast of Canada have increased to over 200. And Alaskan Resident killer whales have nearly tripled.
As Chinook salmon swim back toward their home river each year, they must first get past three hungry populations of killer whales, each culling the largest Chinook from passing schools. That dynamic helps to explain why Southern Residents are starving, according to Schindler.
“The mystery is, why are those big fish no longer showing up to feed the Southern Residents?” Schindler said. “And an explanation that really hasn’t been widely appreciated is that Southern Residents have to compete with Northern Residents and Alaskan Residents, even though they don’t necessarily overlap at the same time. The fish are moving through this gauntlet of predators and Southern Residents are last in line at the cafeteria.”
The change can’t be explained by human fishing, according to Jan Ohlberger, lead author of the study and research scientist with the University of Washington. During the same years when the populations of Northern and Alaskan Resident killer whales were exploding, human fishing has been increasingly regulated.
The study examines changes in Chinook size over the last 50 years, because those are the years for which the best data are available. But human fishing had a heavier toll before then, when regulators hadn’t yet been scaled back to minimize harm to fish. But even back then, there was “some indication” that Chinook sizes had already decreased, Ohlberger said.
Nor can the decrease in size be explained by hatcheries, even though hatchery fish tend to be smaller and mature at younger ages than wild fish. The changes in Chinook size are happening even among fish in western Alaska, where wild runs have no freshwater interaction with hatchery salmon.
Climate change, dam construction and human fishing have all impacted salmon. And within that altered environment, a natural predator can have an outsize effect on its prey, according to Daniel Schindler, an aquatic ecologist at the University of Washington and study co-author.
“One mystery is, why were Chinook salmon ever big?” Schindler said over the phone. “Because presumably there were even more whales in the past. The most logical explanation is there used to be a lot more Chinook salmon. And if there were a lot more, they may have been able to withstand higher predation by killer whales. It’s only now that we have expanding numbers of killer whales at the same time as abundance of Chinook are low enough to cause this decline in size.”