WESTERN CAPE, South Africa — The tourist, in her sandals, flower-print shirt and floppy sun hat, was concentrating on keeping a sure footing. The boulders on this desolately beautiful stretch of coastline were jagged and slippery.

Then, she looked up.

Before her was a breathtakingly grisly scene: six dead seals lined up in a row, one splayed open to reveal its inner workings, bright red blood collecting in pools and trickling down to the crashing waves below. A motley crew of younger women huddled over the carcasses. But the tourist didn’t seem fazed.

“What a shame,” she said. “Pardon me for being nosy, but do you know what’s killing them?”

Tess Gridley, a scientist who doesn’t study seals normally but has taken it upon herself to find out what’s been killing thousands of the animals along southern Africa’s Atlantic coast over the past six months, looked between the tourist and dead seals in front of her.

“That’s what we’re trying to figure out,” Gridley said. The seals were some of more than 50 she has necropsied on the region’s beaches.

Marine species on the coasts around Cape Town are facing multiple crises. A bird flu outbreak last year took out nearly a fifth of an endangered cormorant population in South Africa. Local penguin numbers are declining precipitously, in part because overfishing is depleting their sources of food. Further north, in Angola, fish stocks are plummeting as climate change rapidly warms the ocean.

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But seals? Who cares about seals? Most fishermen certainly don’t — the mischievous creatures snack on their catch. And while they may be cute, they aren’t endangered, not even close. Furthermore, seal mortality is famously high — as much as 40% of pups don’t survive. So who’s to say that the current die-off is even abnormal?

Gridley, 40, is convinced this is abnormal, and a mystery worth solving that has potential implications reaching far beyond seals.

“Seals are just gone from whole areas of coast, and no one has batted an eye,” she said. “I’m filling a gap because it seems nobody else will.”

It is understandable why endangered bird populations get attention. Bird flu can jump to mammal populations and, at worst, turn into a pandemic. More commonly, it can infect poultry and ostriches, both of which factor heavily into the South African diet and economy.

But Gridley’s leading theory for what’s killing the seals is also a toxin that in high concentrations can pose a threat to humans and their food.

Domoic acid, released in some algae blooms, is ingested by plankton and then moves up the food chain through shellfish and anchovies and so on. In humans, it can cause what is called amnesic shellfish poisoning, which, as the name suggests, primarily affects memory, but also balance, and can be fatal.

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Domoic acid poisoning has been linked in peer-reviewed studies to sea lion die-offs in California. In some instances, the animals were seen stumbling, bewildered, along coastal roads, their memory and balance seemingly gone.

“There are huge parallels” between California and South Africa, said Frances Gulland, commissioner of the U.S. Marine Mammal Commission. “And that’s concerning in part because the economic impacts were massive in California. When domoic acid spiked in samples, the whole shellfish industry shut down for months.”

In the late 1990s, Gulland and other researchers faced a similar mystery to Gridley’s, but publicity and public interest in the sea lion die-off led to a spate of funding for research that ultimately proved domoic acid’s treacherous but largely invisible threat to marine populations. It also helped describe how warming waters and increased agricultural runoff led to more harmful algal blooms.

Through a U.S. government-funded program called Mussel Watch, California now regularly tests ocean water and marine organisms for domoic acid and other toxins, and has tied the legality of seafood harvests and sales to toxin levels found in those samples.

South Africa lacks that kind of long-term study, said Grant Pitcher, a specialist scientist at the South African government’s fisheries, forests and environment department. The capacity to test for the toxin is also unavailable in South Africa.

Much of the suspicion that domoic acid is to blame in South Africa is based on odd behavior by South African seals similar to what was seen in California before they die, Pitcher said.

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“We know the same problematic species producing toxins predominate here as in California, and the timing of algae blooms has coincided with the seals dying,” he said. “As for the seals dying, we really don’t have much monitoring or sampling beyond what Tess is doing.”

Gridley is systematically trying to get to the bottom of South Africa’s seal die-off, roping in veterinarians, algae experts and chemists who donate their spare time.

Gridley and her husband are whale and dolphin experts — Gridley focuses on bioacoustics, how those animals communicate — and are largely self-funding the seal work with the help of online donations. Gridley’s 77-year-old father takes quasi-forensic photos of the dead seals. Interns lug equipment and label jars of seal organs preserved in formaldehyde for testing later on. She’s largely taught herself how to cut open a seal, and brings some of them, kept cool on bags of ice, to her home outside Cape Town, where necropsies are easier than among the boulders.

“My kids are so tired of dead seals,” she said, wincing. “They are so over it.”

The die-off began last September. High tides coughed up dozens of dead seals per mile. Even if many people along the coast have mixed feelings about seals, it has been a shocking — and pungent — event. Concerned citizens eventually found Gridley, most via social media, where Gridley’s nonprofit, Sea Search, is active.

“I walk a lot along the beach with my dogs,” said Marianne Franck, 70, a recent retiree. “One day, I found 40 of the dead seals. They were on top of each other, just, like, you know, dumped there. Oh, the stench! And the maggots! And the pups who had survived crying like little lambs. And all you can do is just say goodbye.”

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A regular Monday morning for Gridley now includes receiving a stream of images of dead seals from vigilant volunteers up and down the coastline. She might then go survey one stretch or the other. Some days there are dozens of carcasses, some fresh, others picked up by beach cleaners and left in wheelbarrows with plastic bags over their heads, yet more presumably swept back into the sea by large swells.

The particular kind of seal that’s been dying — the Cape fur seal — is thought to number around 2 million. Their large population contributes to the lack of urgency in studying them. Funding around the cormorant die-off, for instance, has been easier to come by.

“We’ve been able to do PCR tests on hundreds of the birds,” said David Roberts, a clinical veterinarian at the Southern African Foundation for the Conservation of Coastal Birds. The government has also participated more actively. Thousands of sick cormorants were euthanized before they could pass along the flu, many of them by having their necks manually broken before being discarded in garbage bags. There were days last September when more than 500 were being found dead, Roberts said.

“Seals are much harder to handle. You need a big truck, multiple people to carry them, maybe even a stretcher,” he said. “If the seals had been dying of bird flu, it would have been emergency. But we don’t think that’s the case.”

Proving domoic acid poisoning is a particularly difficult endeavor anywhere, but especially in South Africa. The acid disappears from a dead animal within hours of death, so freshly dead specimens are essential. Even with volunteers sending images, luck plays a huge part in the process. The acid also causes the seals to vomit, which can decrease the presence of the acid in the stomach, making it harder to find.

Because there are no testing facilities in South Africa, Gridley has planned on sending some of her preserved samples to Britain or the United States. Brains and other organs require permits to ship internationally because they are animal parts, but seal feces can be sent without one. (“Sending poo, well, that’s easy!” she remarked, excitedly.)

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The likelihood that one factor — domoic acid, in this case — is solely to blame is unlikely. With climate change and overfishing dramatically reshaping marine ecosystems, the answer is almost always multifactorial.

“We have to be really sure its domoic acid before we say that publicly,” said Gridley. “It has the potential to cause real alarm, so we’re not rushing to any conclusions.”

Due diligence likely means at least dozens more necropsies.

It is not just grim work, but it is hard. On a recent, blazingly hot morning, Gridley clambered among the boulders bleached white by gull droppings toward two seals lying motionless. One was just in a deep slumber, and bolted awake, as if from a dream, to find an equally shocked human peering at it.

The other, however, was dead. Gridley and three interns dragged it toward the shade of a boulder, holding it by its fins and neck skin. They laid it next to five others.

“OK, ladies, let’s get systematic about this. Lay them out in order of freshness,” she said, gravely. And then, knife in hand, to herself: “Alright Tess, just get this done.”