Amelia Hesketh remembers the sound of mass marine death as much as the smell. It was July 7, 2021—the last day of the heat dome—and she’d gone down to the rocky shoreline of Oak Bay’s Chinese Cemetery to check on an experiment. For much of the two weeks prior, the whole Pacific Northwest had been baking under a relentless sun. She could feel it—not just in the air, but in the crinkle and crunch of dried-out and empty barnacle shells underfoot. Oysters and mussels cooked. Seaweed shriveled. Whole patches of thatched barnacles lay barren, their empty centres looking like mouths gasping for life. 

It was an eerie sight for a marine biologist. 

“You knew that there was a slow bomb going off on all of the beaches around you,” the Victoria-born Hesketh told Capital Daily.

A PhD candidate and Vanier Scholar in the University of British Columbia’s department of zoology, Hesketh had already been studying the effects of heat-related stress on the intertidal critters that call the Salish Sea home. Two-and-a-half months before temperature records were shattered across British Columbia, she had installed 24 testing sites on the south-facing rocks of Harling Point. But she couldn’t have drawn up an experiment that would illuminate the matter as starkly as what she saw when she arrived.

The plan had been to track how thatched barnacles, a common species on the South Island, fared under thermal stress. The test was straightforward: Hesketh would compare twelve artificially shaded patches of barnacle beds with twelve exposed sections of rock. They were a perfect test subject. Since barnacles can’t regulate their own body temperature, they are more susceptible to temperature extremes—a handy indicator species when considering climate change. 

She had expected that a share of barnacles exposed to the sun would die. (A five to 10 percent death rate is normal under typical conditions, Hesketh says.) But then the heat dome came—and for two weeks, Victoria sweltered. The number of dead barnacles Hesketh tallied on exposed rock neared 100 percent.

“It was one of the clearest results I've ever seen as an ecologist,” she says.

More than a billion seashore animals were estimated to have died on British Columbia’s coast during the two weeks of the summer heat wave—a figure that Chris Harley, Hesketh’s supervisor at UBC and the foremost authority on the heat dome’s effects on marine life, would eventually revise to 10 billion in the months that followed. (Capital Daily reached out to Harley, but could not arrange an interview before publication.) More died across the Juan de Fuca Strait in Washington, and down the Pacific Coast in Oregon.

As mass casualties go, it was utterly unprecedented in scale in the Pacific Northwest.

And now Hesketh and her colleagues are trying to figure out what comes next.

‘Never seen a mass mortality event like that’

The Salish Sea was already a climate change hotspot before the heat dome. Since the early 1970s, sea surface temperatures in the Strait of Georgia have warmed at four times the rate of global ocean averages. BC summers bring low tides during the hottest time of the day. It’s also more acidic than other bodies of water, due in part to the way the Pacific coastal wind pushes surface water farther from shore, leaving room for cold, nutrient-rich (and naturally acidic) water to well up in its place.

But what happened last summer was a “perfect storm,” according to a recent study published in Ecology: a confluence of the hottest temperatures felt in years (and in some parts of BC, the hottest temperatures ever recorded), combined with the lowest tides seen in nearly a generation.

Species that were used to complete submersion suddenly found themselves exposed to air and sun. Clams, mussels, oysters, and snails that might have barely survived 10 minutes of extreme heat were subjected to 30 minutes—or an hour, or many more. Cockles lay with their shells opened, too weakened to close.

“You really couldn’t have come up with a worse scenario for intertidal organisms,” says University of Washington marine scientist and lead author Wendel Raymond.

Based across the Juan de Fuca Strait in Port Angeles, WA, Raymond has spent his career studying seagrasses, sea otters, kelp canopies, and shellfish along the Pacific Coast from Oregon to Alaska. He’d been on the shorelines of Freshwater Bay and Crescent Beach during the last days of June 2021, when the weather was hottest, looking across the Juan de Fuca Strait at East Sooke. There, too, the death toll was obvious.

“I’ve never seen a mass mortality event like that before,” he told Capital Daily.

He wasn’t alone among colleagues, either. His email inbox and text messages were filled with reports from other marine scientists sharing pictures of empty-shelled beaches from the Strait of Georgia to Puget Sound.

“It was like, ‘Whoa… Something’s going on here,” he recalls. 

Then it dawned on him: The best evidence of the heat dome’s effects would soon disappear. The tides would come in. Scavengers would flock. Other shells would scatter and wash away. He needed to act if he wanted any chance at documenting what was happening—and he had to be quick. 

Soon, along with shellfish biologist Julie Barber, he was coordinating a cross-border survey of 24 species spanning 108 sites along Vancouver Island, the Lower Mainland, the Sunshine Coast, the Olympic Peninsula, and Puget Sound. It would become the first comprehensive study of the heat wave’s effect on shellfish in the weeks and months that followed.

Some species harder hit than others

The heat dome did not affect all intertidal species equally, Raymond and his fellow biologists learned. As they surveyed beaches up and down the coast, several trends emerged: outer coastal organisms fared better than their invertebrate peers on the inner coast; species on steeper, rockier shores outlived those on flatter beaches; and—in a reversal of real-estate logic—those with north-facing views were much happier than their south-facing neighbours.

There are reasons for this. For one, low tides arrived earlier on the outer coast, when the weather was milder. Bamfield’s lowest tide came at 10:15 am on June 28, 2021, whereas Vancouver saw its lowest tide at 3:10 pm—near the hottest part of the day. (At the time, temperatures were flirting with 30 degrees Celsius, and felt closer to 37 with humidity.)

Rockier shores provide refuge for mussels, barnacles, oysters, sea snails, and crabs that live in the cracks and crevices between seaweed-slicked boulders, away from the harshest glare of the sun—and, conveniently, the seagulls, bald eagles, and herons that would sooner turn them into lunch. Daylight is also mildest on the north-facing beaches that soak up fewer hours of sun.

“We had one beach at Penn Cove that had super high mortality, and then a different beach facing a different direction was fine,” Barber says.

The water temperature itself might have made a difference, too. The Juan de Fuca Strait is cooler than the Georgia Strait, by as much as three to four degrees Celsius in late June, according to Environment Canada.

For the crustaceans living along its shoreline, marine biologist Fiona Beaty explains, the Juan de Fuca serves as a “tongue of cold water.” It bathes the intertidal, cooling it. And in a matter of margins, where mere minutes exposed to extreme heat can prove fatal, it might well have offered a fighting chance to a number of species.

‘It’s not like everything comes back at once’

A PhD candidate in Harley’s UBC zoology lab, Beaty has spent her doctoral studies researching the impact of climate change on marine ecosystems, from the threats of ocean acidification to the resilience of nearshore networks in the Howe Sound. She knows the Salish Sea well; she grew up on its beaches, between Vancouver and Bowen Island. 

And while she worries about the effects of human-caused climate change, she also has hope.

The ocean is a resilient place, she told Capital Daily. “It’s used to being hit by severe environmental stressors and diseases and [so on]—the same as our communities on land.”

Given time, it can recover.

The first signs of life have already returned a year after the heat dome. Baby barnacles have flourished; find a rocky beach at low tide on the coast, and you’ll see them.

“There are more than I’ve ever seen,” Hesketh says.

The barnacles’ return is a good sign; their presence can help to kick-start the return of other species—from the seaweed beds that anchor to their shells, to the sea slugs that seek shelter in their nooks and crannies. 

In some ways, this is expected, Beaty says—the first stage of ecological succession.

“You'll see [species] come back, but they do so in stages. It's not like everything comes back at once,” she says.

The clincher for Beaty and her peers is whether we’ll see more extreme heat events like last summer’s—ones that would disrupt the window these battered marine ecosystems need to recover. There’s good reason to believe we will.

A 2013 study from the Potsdam Institute for Climate Research suggested heat waves could quadruple in frequency by 2040.

“Heat waves that used to occur as one-in-1,000-year events are becoming one-in-100-year events and one-in-100-year events are becoming one-in-20,” climate scientist Zeke Hausfather told The Washington Post last summer.

One thing that gives ecologists like Raymond further cause for concern is that this summer’s tides are even lower than last year’s. That’s due to a quirk in the moon’s orbital plane that makes it wobble, like a spinning top, on a roughly 18-year cycle. As the moon nears the Earth, its pull on the tides is at its strongest; this year marks the lowest tides until the cycle renews. 

And the next string of lower-than-usual tides falls next week, from July 12-15.

“If we had another heatwave this summer, it would be a problem,” Harley told the Canadian Press. “An ecosystem might be able to handle a big heat wave once every few decades—there’s enough time for recovery—but if it starts hitting every four or five years, the species that we’re used to just can no longer persist.”

A changing ocean

Little remains of the Greek philosopher Heraclitus’ writing from some 2,500 years prior, but what has lingered through centuries holds true: “Everything changes and nothing stands still.”

As oceans warm and the Salish Sea’s chemistry continues to change, it may one day prove inhospitable to the snails, clams, and mussels that currently call it home. But it may also become home to other species that take their place.

One theory Harley’s lab is exploring is whether the Salish Sea could, in time, more closely resemble the subtropical climes of East Asia. Pacific oysters, native to northeast Asia, have already begun to out-compete their invertebrate neighbours on BC’s coast.

“It could be that we see the transformation of marine life along our beaches,” Beaty says. “And that can be sad—it's kind of saying goodbye to this ecosystem that we grew up with—but it doesn't necessarily mean, like, total ecosystem collapse.” 

Transformation, she notes, is a part of ecology; it always has been—“It's just the rate of transformation is happening so quickly now that it can really destabilize these systems.”

Scientists are still learning what the long-term ripple effects may be from a die-off of any one species. Raymond’s cross-border study is a first step in that direction. Harley’s lab continues to explore the question, and others as well: Did enough animals reproduce before they died? And what might be learned from the pockets of life that survived? Could they hold the key to the Salish Sea’s resilience?

“It's kind of hard to imagine a future of going to any one of these beaches and there not being butter clams or cockles anymore,” Raymond says. “Our ecosystems are really connected, and a change in one part of the ecosystem is going to affect others; we [just] may not know what those [effects] are.”

‘Can the community recover?’

If there is cause for optimism surrounding the fate of Salish shellfish, it may well come from another, earlier experiment of Hesketh’s. In 2019, she went out to Salt Spring Island’s Ruckle Park with a stack of black and white plastic tiles. She wanted to see how barnacles would respond to artificially-increased temperatures.

There were 96 tiles—half of them black, half of them white. Each had a central square where barnacles could “cement” themselves and form a colony. She left those tiles in the intertidal zone for a year. The early results were predictable: the white tiles that reflected the sun teemed with healthy clusters of barnacles, while the black ones that absorbed it were largely devoid of colonies—even though the temperature difference was only about two degrees. 

“Barnacles like it better when it's cooler; that became clear,” she says.

She’d been planning to wrap the experiment and travel to Hong Kong, but then the pandemic hit, and those plans fell to pieces. So, she improvised. She changed half of the 48 white tiles to black with tape, and then did the reverse with her black tiles. And then she watched for another year.

“What happens if that stress goes away in the second year? Can the community recover?”

To her surprise, they could. The barnacles that spent the first year on black tiles began to thrive when the tiles were turned white. But the ones that stayed on black tiles under continued thermal stress stayed mostly bare.

“The community is a pretty resilient one,” she told Capital Daily. “It can come back… if you give it enough time.”

That “if” looms large for marine ecologists after the 2021 heat dome. The spawning season for a number of Pacific Northwest marine invertebrates, from Olympia oysters to gooseneck barnacles, falls roughly between April-May to September-October, and peaks in July.