Toxin that causes paralytic shellfish poisoning blamed for Alaska seal deaths
- Yereth Rosen
- Jul 14
- 7 min read
A die-off last year at St. Paul Island, the first compelling case of fatal saxitoxin poisoning in marine mammals, comes as more harmful algae is found farther north
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.
It was the clearest evidence ever found for this type of toxin-caused death, scientists say.
“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 local 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.
Dead fur seals and fish
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 called Benson Beach. 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. “So we were pretty adamant about really trying to do the best job that we could to collect as much information as we could about the event, and also with quite little capacity that we have out in the remote community.”
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.
Anderson, who noted that his team has already found a huge Alexandrium cyst bed near St. Paul, the seal discovery is significant for two reasons.
“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,” he said by email.
Additionally, because scientists know that ocean currents run north from that region, “this might be yet another source of cells that can affect the Alaskan Arctic,” he said.
Lefebvre said all the evidence about the seals’ bodies, the fish samples and the environmental conditions were needed to show that the algal toxins killed the seals.
Up to now, it has not been possible to prove such a strong link between saxitoxin and marine mammal deaths, she said.
Mammals poisoned by saxitoxin are likely to disappear while at sea, she said. “They become paralyzed. They’re going to basically suffocate, not even drown, not even taking a breath of air, of water,” she said. “They’re going to just stop breathing.”
That contrasts with effects of a different type of algal toxin that, since the 1990s, has caused mass strandings and deaths since 1990 of marine mammals in California. That toxin, domoic acid, is produced by the Pseudo-nitzschia algae. Rather than causing paralysis, it overstimulates the nervous system, causing seizures that can result in death.
Lefebvre and others have documented thousands of cases of domoic acid poisoning among marine mammals in California, including sea lions, dolphins, seals, and whales, with animals easily seen by people on the beach. Mass die-offs of seabirds have also been documented.
So far, there have been no domoic acid poisoning cases documented in Alaska, though Lefebvre and her colleagues have consistently found low levels of it in various marine mammal species.
But a new study suggests that continued warming may make domoic acid a future problem in Alaska. The study, also led by Lefebvre, found that bowhead whales hunted over a two-decade period carried higher levels of both saxitoxin and domoic acid in years when waters were warm and sea ice was low.
Saxitoxin risks in various wild foods?
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.
Information about saxitoxin poisoning in wildlife has been more difficult to pinpoint.
In past years, it was suspected in some marine mammal deaths in Alaska, but it was not proven.
In the fall of 2017, four dead walruses found in the Bering Strait region had saxitoxin in their stomachs or intestines. They were among 39 walruses that, though otherwise in good body condition, were found dead in the region that August and September.
Two years later, another piece of the puzzle came when scientists retrieved clams from the Bering Strait area and the Chukchi Sea that had levels of saxitoxin above the thresholds for safe consumption by humans. That added another piece to the puzzle. Clams are an important part of the Pacific walrus diet, though the saxitoxin threshold for walruses has yet to be determined.
There are other suspected cases. For example, saxitoxin poisoning was a suggested cause of a 1987 sea otter die-off in the Kodiak Archipelago, though test results were inconclusive.
Beyond mammals, Alaska seabirds are known to have been killed by saxitoxin. That toxin caused a 2019 die-off of Arctic terns in the Juneau area; the birds had been feeding on sand lance, a type of fish known to accumulate saxitoxin.
For people in St. Paul and elsewhere in Western Alaska, who live far away from the two Alaska labs that currently test shellfish for toxins, the new information is concerning, Divine said.
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, she 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.
A grant from a NOAA program called ECOHAB funded much of the work that supported the new study. The grant for that work, which is led by Lefebvre and Anderson, runs through this year.
Anderson said the team just learned that it has been awarded ECOHAB grant funding for another year of work, part of what had been planned as a five-year follow-up program to better understand the saxitoxin risk in wild foods gathered in Indigenous subsistence harvests.
News of the one-year award is encouraging, but the scientists are worried about future years, Anderson said
The Trump administration has targeted NOAA for deep cuts, he noted. The administration is proposing to entirely terminate NOAA’s National Centers for Coastal Ocean Sciences, the agency arm that supports research into algal toxins.
Termination of the National Centers for Coastal Ocean Sciences would end funding for NOAA’s entire Integrated Ocean Observing System. Within the IOOS is the Alaska Ocean Observing System, which monitors algal blooms, among other work.
Divine said the same people who have been drawn together by their concerns about toxins in the food web are also worried about the potential loss of scientific research.
“We have just an incredible amount of interest in this, and the funding that has been secured to really tackle this in a coordinated way is all on hold in this administration. And I do think that that’s worth noting,” she said.
• Yereth Rosen came to Alaska in 1987 to work for the Anchorage Times. She has been reporting on Alaska news ever since, covering stories ranging from oil spills to sled-dog races. She has reported for Reuters, for the Alaska Dispatch News, for Arctic Today and for other organizations. She covers environmental issues, energy, climate change, natural resources, economic and business news, health, science and Arctic concerns -- subjects with a lot of overlap. Alaska Beacon is part of States Newsroom, the nation’s largest state-focused nonprofit news organization.
