Marine mammals on the U.S. West Coast are now so numerous they are taking more chinook salmon than fishermen, posing a new threat for Puget Sound’s critically endangered killer whales.

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Booming populations of seals, sea lions and other marine mammals are eating so much chinook salmon, they may pose a bigger challenge to the survival of hungry local orca whales than fishermen do, a new study has found.

The findings also helped researchers quantify yet another pressure on protected chinook- salmon runs: the voracious appetites of recovering populations of predators.

Consumption of chinook by protected marine mammals other than southern-resident killer whales jumped 150 percent from 1975 to 2015, researchers found. That’s such an increase that commercial and recreational fishermen, facing tighter and tighter catch limits, have watched their share of chinook crater by 41 percent during the same time period, while seals, sea lions and northern populations of killer whales feast.

By now, the combined harvest of chinook by commercial and recreational fishermen in Central and Northern California, Oregon, the Columbia River, the Salish Sea and Gulf of Alaska is probably less of a competition problem for the southern residents than other marine mammals beating them out for a meal, researchers found.

For managers, the situation poses a conundrum. Nine West Coast populations of chinook salmon are protected under the Endangered Species Act but getting chomped by other protected species.

How do you protect one species of marine mammal while not hurting another?

And what about cutting back human harvest to protect chinook — only to feed them to predators?

“The trick is, now recovery of fish species is no longer just a fish problem, or a fishing problem,” said Isaac Kaplan, a research fishery biologist at NOAA’s Northwest Science Center in Seattle and an author on the paper. “It is now an ecosystem problem, involving trade-offs with predators, all of which are protected, and some of which are in critical condition.”

The findings are a result of a modeling exercise that looked at predators’ recovery in the ocean, and the effect it’s having on salmon populations.

The results were surprising, revealing new aspects of a feast-and-famine reality with a dynamic that had not been previously understood.

While the southern residents’ population declines, all other marine mammals tracked in the research study — seals, sea lions, and the killer whales in Alaskan and Canadian waters — are booming.

That is thanks in part to 40 years of protection from hunting, and a feast of a far wider range of chinook salmon species headed north to Alaska, as well as the chinook predators pick off within the southern residents’ feeding grounds.

Meanwhile, southern- resident killer whales, the J, K, and L pods that frequent Washington’s interior waters, are critically endangered, with a total population down to just 76 animals, a 30-year low.

It’s not just the overall number that matters, said Ken Balcomb, director of the Center for Whale Research. It is also the downward trend in the number of breeding-age females and longer intervals between births.

“We are killing the goose, and we are killing it by not feeding it,” Balcomb said.

The local orcas’ preference for chinook salmon in their home range within the interior waters of Washington is hurting them in two ways, according to the paper, published Monday in the journal Scientific Reports.

Not only are they losing out on the fish they want to eat, but they also aren’t eating the other animals that are their direct competition for those fish.

It’s unusual in nature to see an animal lower on the food chain beating out a top predator for its meal.

In the summer, the southern residents eat chinook salmon exclusively as the whales forage in the Salish Sea, and earlier in the year they will linger for months off the mouth of the Columbia River, cruising for early spring kings, said Mike Ford, another author on the paper and director of conservation biology at the Northwest Fisheries Science Center.

Southern residents will eat other salmon the rest of the year — for instance coho, which make up more than 40 percent of their diet in late summer, according to orca fecal analysis published by researchers last year. The southern residents will take other fish, too, such as lingcod, but they are small print on their menu.

Just what they are eating in winter is under active study, with results not yet published, Ford said.

The chinook runs targeted by the southern residents give the local orcas a narrower range of species to choose from, among runs that are in low abundance, researchers found. The amount of chinook the southern residents have been consuming hasn’t grown much in 40 years, and lack of food is even emerging in other research as the biggest block to a rebound in the local killer-whale populations, behind vessel noise, contamination and other threats.

The whales are on a path to extinction within a century if they can’t get significantly more chinook to eat, those researchers found.

Yet the local orcas will not eat the seals beating them out for their meal, unlike orcas in Canadian and Alaskan waters.

“It’s bizarre,” said Ford, who added that the southern residents certainly kill marine mammals, such as porpoises. They just don’t eat them. “It is something about their behavior and culture, they want to eat salmon.”

The role of other factors affecting the local orcas, including noise and contamination, can’t be ruled out as driving their decline, Ford said, even as nearby orcas are thriving. “But they seem to be food-limited. Their body condition has been so poor, it is not as good as the northern residents that are also increasing. It is possible they are out-competed for food.”

There’s no doubt the seal boom has benefited the killer whales in Alaskan and Canadian waters, which savor pinnipeds. Those orcas gorge on seals and other marine mammals — and also ate more chinook than any of the other marine mammals studied.

The marine-mammal conservation success story is due to a range of protections, from bans on commercial whaling to the Marine Mammal Protection Act and Endangered Species Act.

In addition to Oregon State University, collaborators in the study include NOAA Fisheries’ Northwest Fisheries Science Center in Seattle, Western Washington University, Makah Nation Fisheries Management, and the Washington Department of Fish and Wildlife.

The image of the orca chasing a salmon was obtained under NMFS permit #19091, courtesy of John Durban (NOAA Fisheries/Southwest Fisheries Science Center), Holly Fearnbach (SR3: SeaLife Response, Rehabilitation and Research) and Lance Barrett-Lennard (Vancouver Aquarium’s Coastal Ocean Research Institute).