For the first time, scientists have made a definite link between the toxin produced by algae that causes paralytic shellfish poisoning and marine mammal deaths.

Fur seals found dead last August on Alaska’s St. Paul Island had significant levels of saxitoxin in their bodies, as did several of the dead fish around them.

“I would say this is the absolute strongest case for saxitoxin poisoning in marine mammals anywhere,” said Kathi Lefebvre, a National Oceanic and Atmospheric Administration research biologist and the lead author of a study detailing the findings, published in the journal Marine Mammals Science in May.

The discovery, a warning for communities that rely on subsistence hunting, comes as long-term warming in the Bering and Chukchi seas is increasing the prevalence of a type of algae that produces saxitoxin is becoming more prevalent in these bodies of water.

Dangerous blooms of the Alexandrium have long been observed in more southern waters of Alaska, where paralytic shellfish poisoning is a well-recognized and sometimes deadly hazard.

Clams are notorious for accumulating saxitoxin, and there are frequent safety advisories that warn people of specific sites’ shellfish poisoning dangers, and two labs in the state — one in Anchorage operated by the Department of Environmental Conservation and one in Sitka operated by the Sitka Tribe of Alaska — test harvested shellfish to determine whether it is safe for human consumption.

Farther north, the emergence of large Alexandrium blooms and potentially dangerous saxitoxin levels is new. But, as Lefebvre explains it, the conditions to create those toxin hazards have been set up over decades and possibly even centuries.

Over time, ocean currents have carried bits of Alexandrium algae north, and over time, that algae has dropped to the bottom of the ocean. That created massive beds of dormant cysts, the equivalent of algal seeds.

The Bering, Chukchi and even Beaufort seas hold some of the highest concentrations of Alexandrium cysts ever found in the world, according to Don Anderson, a harmful algal specialist with the Woods Hole Oceanographic Institution. Anderson led research teams that mapped out Alaska cyst beds over several years, including a bed in the Chukchi that he said is the biggest ever discovered.

Those cyst beds were dormant until recently, said Lefebvre, who works closely with Anderson.

“They’ve just been building and building and building. And then in the last couple decades – actually, the last 10 years, maybe — the bottom temperature finally was warm enough for cyst germination,” Lefebvre said. That appears to be the cause of the large blooms now being found routinely in the Bering, Chukchi and Beaufort seas, she said.

In the case of the St. Paul fur seals, local beachcombers found 10 dead animals at a site on the northeastern side of the island. It is a catchment site where marine debris, kelp and other items are known to accumulate, said Lauren Divine, director of the island’s tribal ecosystem conservation office.

Tribal representatives managed to retrieve some of the dead seals, as well as some of the dead fish with them, said Divine, who works for the Aleut Community of St. Paul Island. They photographed the scene and gathered whatever information they could, and they contacted the NOAA-coordinated Alaska Marine Mammal Stranding Network and Anchorage-based veterinarian pathologist Kathy Burek-Huntington to continue the investigation, she said.

It was clearly a startling discovery, Divine said.

“This was something that everyone was quite alarmed (at) and noticed as something that was wrong in the ecosystem and something that really hadn’t ever been noticed or detected before,” she said.

In all five of the adult seals tested, saxitoxin was found in feces or urine, significant because they indicate higher exposure when tainted food was eaten. Two fish were sampled as well, and both turned up saxitoxin in their intestines.

At about the same time, samples being taken by researchers in the southeastern Bering Sea — the same area where the dead fur seals would have been foraging — revealed dense Alexandrium blooms, large cyst beds and extremely high prevalence of saxitoxin in fish, zooplankton, clams and worms.

“The seal mortality demonstrates that dangerous levels of toxin can accumulate there and that local communities need to be careful about what they consume during certain times of the year,” Anderson said by email.

For now, saxitoxin and the paralytic shellfish poisoning it causes remain the main algal toxin concern in Alaska.

Among people, there were 132 reports of paralytic shellfish poisoning between 1993 to 2021, with the highest prevalence in Southeast Alaska and the Kodiak Archipelago, according to a bulletin published in 2022 by the Department of Health’s epidemiology section. The last fatal case concerned a person who ate shellfish at Unalaska Island.

St. Paul residents and others in the Pribilof Islands harvest various types of animals beyond the clams and mussels that are routinely screened for consumption safety, Divine said. But there is not much known yet about saxitoxin levels in the full breadth of subsistence foods, she said. “We just don’t have robust information on how much is accumulating and how things are impacted across the food web,” she said.

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