Like in ‘Postapocalyptic Movies’: Heat Wave Killed Marine Wildlife en Masse

Dead mussels and clams coated rocks in the Pacific Northwest, their shells gaping open as if they’d been boiled. Sea stars were baked to death. Sockeye salmon swam sluggishly in an overheated Washington river, prompting wildlife officials to truck them to cooler areas.

The combination of extraordinary heat and drought that hit the Western United States and Canada over the past two weeks has killed hundreds of millions of marine animals and continues to threaten untold species in freshwater, according to a preliminary estimate and interviews with scientists.

“It just feels like one of those postapocalyptic movies,” said Christopher Harley, a marine biologist at the University of British Columbia who studies the effects of climate change on coastal marine ecosystems.

To calculate the death toll, Dr. Harley first looked at how many blue mussels live on a particular shoreline, how much of the area is good habitat for mussels and what fraction of the mussels he observed died. He estimated losses for the mussels alone in the hundreds of millions. Factoring in the other creatures that live in the mussel beds and on the shore — barnacles, hermit crabs and other crustaceans, various worms, tiny sea cucumbers — puts the deaths at easily over a billion, he said.

Dr. Harley continues to study the damage and plans to publish a series of papers.

Such extreme weather conditions will become more frequent and intense, scientists say, as climate change, driven by humans burning fossil fuels, wreaks havoc on animals and humans alike. Hundreds of people died last week when the heat wave parked over the Pacific Northwest. A study by an international team of climate researchers found it would have been virtually impossible for such extremes to occur without global warming.

Just before the heat wave, when Dr. Harley took in the eye-popping weather forecasts, he thought about how low the tide would be at midday, baking the exposed mussels, sea stars and barnacles. When he walked to the beach last week on one of the hottest days, the smell of decay struck him immediately. The scientist in him was excited, he admitted, to see the real-life effect of something he’d been studying for so long.

But his mood quickly changed.

“The more I walked and the more I saw, the more sobering it all became,” Dr. Harley said. “It just went on and on and on.”

The dead sea stars, usually the most eye-catching creatures in tidal pools, hit him particularly hard. But the obvious mass victims were blue mussels, an ecologically important species that feeds sea stars and sea ducks and creates habitat for other animals.

Scientists have only begun to consider the domino effects. One concern is whether the sea ducks, which feast on mussels in the winter before migrating to their summer breeding grounds in the Arctic, will have enough food to survive the journey.

“It’s at least something that we’re starting to think about,” he said.

Species that live in intertidal zones are resilient, he noted, and the mussels on the shady north side of boulders seem to have survived. But if these extreme heat waves become too frequent, species won’t have time to recover.

While the heat wave over the Pacific Northwest has eased, punishing temperatures have persisted across much of the American West. Now, another heat wave appears to be building, only worsening the ongoing drought.

That means biologists are watching river temperatures with alarm. Salmon make an extraordinary migration, often hundreds of miles, from the inland rivers and lakes where they are born, out to sea, and then back again to spawn the next generation. A network of longstanding dams in western states already makes the journey perilous. Now, with climate change worsening heat waves and droughts, scientists say the conditions look grim without intense intervention, which comes with its own risks.

“We are already at critical temperatures three weeks before the most serious heating occurs,” said Don Chapman, a retired fisheries biologist who specialized in salmon and steelhead trout, talking about conditions along the Snake River in Washington, where four dams are the subject of longstanding controversy. “I think we’re headed for disaster.”

The plight of the salmon illustrates a broader danger facing all kinds of species as climate change worsens. Many animals were already struggling to survive because of human activity degrading their habitats. Throw in extreme heat and drought, and their odds of survival diminish.

As an emergency measure, workers with the Idaho Fish and Game agency have begun capturing a variety of endangered sockeye salmon at the Lower Granite dam, putting them into a truck and driving them to hatcheries as a stopgap measure to decide what to do next. (Idaho game officials first tried trucking the adult fish during a heat wave in 2015. It has been done for juvenile salmon on a variety of runs for a variety of reasons.)

In California’s central valley, Jonathan Ambrose, a biologist with the National Oceanic and Atmospheric Administration, said he wished he could do something similar. The chinook salmon he monitors historically spawned in the mountains. But since the Shasta Dam was built more than three quarters of a century ago, they have adapted by breeding just in front of the mammoth structure, which they cannot cross. The critical problem this year is that the water there is expected to grow too warm for the eggs and juveniles. Previous efforts to secure state or federal funding to transport them past the dam have failed.

“We’re looking at maybe 90 percent mortality, maybe even higher this year,” Mr. Ambrose said.

Elsewhere in California, for the first time since the state built the Iron Gate Fish Hatchery on the Klamath River in 1962 to make up for lost spawning habitat, state biologists will not release the young salmon they have nurtured back into the wild, because they would likely die. Instead, they are spreading a million young salmon among other area hatcheries that could host them until conditions improve.

“I want to find the positives and there are some, but it’s pretty overwhelming right now,” said Dr. Harley, the University of British Columbia marine biologist. “Because if we become too depressed or too overwhelmed, we won’t keep trying. And we need to keep trying.”