Seals and sea lions are eating more than eight times as much Chinook in Puget Sound these days as they did less than half a century ago, and that may be impacting the recovery of the inland sea’s ESA-listed salmon species as well as killer whales.
That’s according to a recently published study that estimated consumption of kings by the orcas themselves, California and Steller sea lions, as well as harbor seals — the population of which in Washington and British Columbia’s protected saltwaters is now believed to be among the densest in the world.
Thanks to the Marine Mammal Protection Act, pinniped numbers have surged in recent decades, a remarkable recovery for sure, but it’s also increasingly apparent that that’s affecting the status of other listed stocks, including economically and culturally important ones such as Chinook.
Anglers could long have told scientists about the increasing numbers of seals and sea lions and how they’ve become better and bolder at stealing salmon off our lines, but studies like this help flesh out the impact they’re having on the fish, providing quantifiable data to lawmakers and others.
Brandon Chasco is one of the university, private, state, tribal and federal researchers who worked on “Estimates of Chinook salmon consumption in Washington State inland waters by four marine mammal predators from 1970 – 2015,” which was published online earlier this month and soon in the Canadian Journal of Fisheries and Aquatic Sciences.
He estimates that as of two years ago, pinnipeds ate twice as much as resident orcas, and six times that of the combined catch of the recreational and commercial fleets.
(Chasco, who is a graduate student at Oregon State University, says an error in figures posted in the report’s online abstract will be corrected in the print version.)
The paper states that in 1970, sea lions and harbor seals ate 68 metric tons of Chinook and in 2015, they ate 628 metric tons.
Chasco says that harbor seals are estimated to have eaten 1 million individual Chinook in the former year and 8.6 million in the latter, including 8.5 million juveniles.
He says that assuming average mortality in the ocean, those young kings would have yielded 160,000 fish of varying ages, “essentially double of killer whale consumption.”
Now, it’s probable that many of those juveniles would have ended up eaten by something else before returning to their rivers, but the estimates help identify another potential source of the decline of the ESA-listed salmon beyond old hatchery and overharvest practices, and ongoing habitat destruction. It’s helping to show pinniped predation problems right here in the waters of Puget Sound, including on steelhead smolts.
Chasco says the idea with the study was not to make statements about what to do, but to use bioenergetics models to figure out “where is energy flowing in the system?”
“We went out of our way not to suggest targets for management,” he says.
The work suggests that old assumptions that the natural mortality rate during the ocean phase of the Chinook lifecycle was constant may not be so and what’s more it may have changed over time, says Chasco.
That, however, could have implications for salmon managers this year as they plug forecasts into FRAM, or the Fishery Regulation Assessment Model.
“As more protected species respond positively to recovery efforts, managers should attempt to evaluate trade-offs between these recovery efforts and the unintended ecosystem consequences of predation and competition on other protected species,” write the study’s authors.
Editor’s note: For a longer take on this, see Christopher Dunagan’s article on the Encyclopedia of Puget Sound.