A sea lion eating fish at Willamette FallsBryan Wright / Oregon Department of Fish and Wildlife

Let us first establish that sea lions are supposed to live in the sea.

Since the 1990s, however, male sea lions—a handful at first, now dozens—have been captivated by the attractions of the Willamette River. They travel all the way from Southern California to Oregon and then swim up 100 miles of river to arrive at an expansive waterfall, the largest in the region. Here, salmon returning to spawn have to make an exhausting journey up the fish ladders of the Willamette Falls. And here, the sea lions have found a veritable feast.

“They’re kind of sitting ducks,” the wildlife biologist Sheanna Steingass told me, describing the salmon. She paused to consider the metaphor. “Or sitting fish.” Every sea lion eats three to five fish a day.

In another world, this could just be a story about the intelligence of sea lions and their adaptability to river life. But in this world—where many salmon populations in North America have plummeted, and where the winter steelhead run at Willamette Falls has fallen from 25,000 fish in the 1970s to just hundreds in 2018—it’s a dire story for the fish. After spending years trying and failing to deter the sea lions by nonlethal means, the Oregon Department of Fish and Wildlife, where Steingass leads the marine-mammal program, started “lethal removal” of sea lions in December. As of mid-January, they have trapped and euthanized five sea lions at Willamette Falls.

Killing animals to save other animals is always controversial. Animal-rights groups like the Humane Society of the United States denounced the sea-lion killings, calling them a distraction from the salmon’s real problems. And it’s true that a long chain of human actions—overfishing, destruction of salmon habitats, dams blocking their migration, hatchery mistakes—have led to what everyone can admit is this nonoptimal situation.

“In a perfect world, in an unaltered world, this wasn’t a problem, because historically there were 16 million salmon in the Columbia River,” says Doug Hatch, a senior fisheries scientist at the Columbia River Inter-Tribal Fish Commission. The sea lion’s appetites would have barely made a dent. It’s only because humans have so unbalanced the natural world that as drastic an action as culling sea lions could appear to be the fix.

[Read: How climate change canceled the grizzly salmon run]

It’s not the first time this has happened, though. In the 1980s, a sea lion who earned the nickname Herschel began hanging out at the entrance to the Ballard Locks in Seattle. The locks forced all fish through a narrow channel, which was great for Herschel. He would linger by this stream of food, picking off steelheads at his leisure. (Steelheads are technically a type of rainbow trout, but they are similar enough to salmon to have been grouped with them in the past.) Soon, other sea lions started joining in.

The Marine Mammal Protection Act of 1972 protects sea lions from killing, capture, and harassment. But the sea lions at Ballard Locks were eating so many salmon and steelhead—whose own populations were falling—that in 1995 wildlife managers got a federal exemption to remove the three most problematic animals, Hondo, Big Frank, and Bob. (Herschel had stopped coming to the locks by then, either because he died or because he found other hunting grounds.) The three fish eaters were taken to SeaWorld, and wildlife managers deemed the problem solved.

[Read: The fantastical vision for the original SeaWorld]

In the early 2000s, sea lions started showing up en masse at the Bonneville Dam, which, like Willamette Falls, is in Oregon and more than 100 miles upriver. And as at Ballard Locks, the fish have to funnel through one of about 16 entrances to the fish ladders, making them easy pickings for sea lions. Watch the dam for just 15 minutes, says Hatch, and you might witness three to 10 fish kills.

Here too, it was just a few sea lions at first. Perhaps the first one chased a salmon upriver and—what luck!—stumbled upon a buffet. Then he returned with his buddies, and they with their buddies. One hundred to 200 sea lions now hunt at Bonneville Dam. Scientists have actually studied how this specific learned behavior spread through the male sea lions. They compared the diffusion of information through sea-lion social networks to the spread of a disease and recommended intervening early, before an outbreak becomes an epidemic.

This is why, said Steingass, it’s important to address the situation at Willamette Falls quickly. If more sea lions find out about the easy hunting grounds, the fish-and-wildlife department might end up with hundreds of sea lions they have to kill, rather than the 40-odd creatures that currently hunt there. “We’re going to have a better outcome for the salmon but also for the sea lions,” she says. It’s a justification grounded in cold, hard math, but it’s also grounded in the recognition that sea lions are intelligent and social creatures.

And also persistent ones. At Willamette Falls and Bonneville Dam, the wildlife managers tried a number of ways to scare the sea lions off. They set off “seal bombs”—basically firecrackers that sink and go off underwater. They chased sea lions in a boat. Hatch says they have even trapped sea lions at Bonneville Dam and dropped them off in the ocean as far as 500 miles away. The same sea lions were back to gorging on salmon within days. “The truth is that the positive incentive to eat these fish is so great, it’s very difficult to think of a negative conditioning that would be great enough,” Steingass said.

In December, Congress passed a bipartisan bill that streamlines the lethal removal of sea lions. It hasn’t completely gone into effect yet, but once it does, fisheries managers will no longer need to observe an individual sea lion hunting salmon five times before euthanizing it. Both state and tribal fisheries managers in the Pacific Northwest will be able to apply for the lethal-removal permits. The culling of the sea lions had been one of the most visible and controversial parts of salmon conservation, but Steingass and Hatch both said they see it as one small part of the larger effort that also includes habitat remediation and dam removals. If sea lions are gobbling up all the salmon, it negates everything else.

Sea-lion populations were once declining, too, but they have rebounded under the Marine Mammal Protection Act. Such is the challenge for humans trying to manage vast, interconnected ecosystems. Put a thumb on one part of the scale, and something somewhere else goes out of whack. Try to correct that, and you create another problem. Eventually, you end up with a policy of fisheries managers killing sea lions.

The sea lions feasting on salmon had found a clever way to thrive in the human-altered world—until the rules changed and they were on the wrong side.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.