A rebound in marine mammal populations on the West Coast has come with unintended consequences for salmon.
A new study found that a growing population of fish-eating killer whales, sea lions and harbor seals on the West Coast have feasted heavily on Chinook salmon runs in the last 40 years.
Their consumption of the fish — of which certain populations are listed as endangered and threatened — may now exceed the combined harvest by commercial and recreational fisheries, researchers say.
It’s a complex trade-off, fishery managers say. And many questions remain about what a growing predator population means for the fish and why, despite the feeding frenzy, the Southern Resident Killer Whale group in Washington state’s Puget Sound area continues to show few signs of recovery.
The study was a broad but “careful accounting exercise,” a first attempt to quantify marine mammal predation of Chinook salmon on the U.S. West Coast and up into British Columbia, Canada and Southeastern Alaska, said co-author Isaac Kaplan of the National Ocean and Atmospheric Administration’s Northwest Fisheries Science Center.
“The main story here is there are a lot of factors affecting salmon,” Kaplan said. “Those include dams and habitat (loss) and fishing and marine mammals. We know all of these things are a challenge to recovery for Chinook salmon populations.”
The researchers — a collaboration of federal, state and tribal scientists in the Pacific Northwest — used models to estimate that the yearly biomass of Chinook salmon consumed by sea lions, harbor seals and killer whales increased from 6,100 to 15,200 metric tons from 1975 to 2015, even while annual harvest by fisheries decreased from 16,400 to 9,600 metric tons.
While recovery efforts on the West Coast have boosted the numbers of wild salmon, researchers found the increased predation could be taking a toll and “masking the success of coast-wide recovery efforts.”
“We’re trying to understand all the threats that salmon face throughout their range,” said Eric Ward, a co-author and statistician (biology) with NOAA. “These fish have huge migrations. Fish from the Salish Sea or the Oregon Coast and Washington Coast migrate all the way up to Alaska and throughout that whole range they are vulnerable to predation.”
The study purposefully focused on predation by certain recovering marine mammal populations, said study lead Brandon Chasco, an Oregon State University post-doctoral student.
The study confirmed what communities near the mouth of the Columbia River already know — seals and sea lions eat a lot of salmon. The researchers estimated that California sea lions ate 46,000 adult Chinook salmon in 2015, while Stellar sea lions consumed 47,000. Harbor seals ate considerably less, an estimated 1,000 adult Chinook salmon.
“What we don’t know is if these marine mammals are effective and if they’ve taken the fish out of the mouths of other predators,” Chasco said. “Or, if it’s being stacked on top of bird consumption, stacked on top of fish consumption and the density of salmon overall is lower.”
“We just don’t know that yet,” he added, “and I don’t know when we’re going to know that.”
The question of what it all means for Southern Resident killer whales whose population numbers remain low is another gap, said Michael Ford, director of NOAA’s Conservation Biology Division and a co-author of the study. Are they being out-competed? What other stressors are at play?
But, he said, “I think there’s a lot of good news here.”
The recovery of marine mammal populations points toward the success of the Marine Mammal Protection Act, he said. The fact that these animals are eating so many salmon shows fishery managers have been successful at keeping Pacific salmon available — to some extent — to feed a growing number of predators.
Ford and others are in the middle of another study that would actually count how many salmon killer whales are eating over a certain period of time, moving beyond the theories proposed in the recent predation study. They hope to be able to make more direct comparisons between the healthy northern killer whale groups and the less-healthy southern killer whales.