Over the past two summers, a pair of remote and treeless volcanic islands in the eastern Bering Sea broadcast signals of climate change danger in the marine ecosystem that feeds Alaska residents and supports much of the state’s economy.
Tribal employees monitoring St. Paul Island’s beaches came across 10 dead but seemingly well-fed northern fur seals in August 2024, their bodies lying amid piles of dead fish and birds.
Testing revealed that the seals had been killed by an algal toxin that causes paralytic shellfish poisoning. It was the first ever conclusive case of marine mammals killed by saxitoxin, the algal toxin that causes paralytic shellfish poisoning.
The people living on St. Paul, numbering about 400, most of them Unangax, are highly dependent on the marine environment for their food. They are aware of the algal toxins that pose risks of paralytic shellfish poisoning in faraway Southeast Alaska. But seal deaths from algal toxin poisoning on their own island came as a big surprise, said Aaron Lestenkof, who is part of the tribe’s Indigenous Sentinels Network.
“It never occurred to us that it may happen to our marine mammals here,” Lestenkof said. “I guess it was just a matter of time.”
The St. Paul die-off was not a one-time incident. In August 2025, tribal residents found 21 dead fur seals on a beach at St. George Island, a sister island of St. Paul. Along with the seals were two dead fin whales, a dead sea lion and several dead seabirds.
The events show that deadly levels of algal toxins, once believed to be confined to the warmest waters in the warmest months in southernmost Alaska, are spreading north and into regions and parts of the food web that previously caused no worry for local people.
“This is the scary, ‘I-don’t-know’ moment of this event now happening in consecutive years,” said Mike Williams, one of the National Oceanic and Atmospheric Administration biologists who happened to be on scene at St. George to gather samples and document the event.
St. Paul and St. George, which has about 70 residents, are the only inhabited islands in the four-island Pribilof archipelago. Located about 750 miles west of Anchorage and 300 miles from the mainland, the islands are far from Alaska’s population centers. But the Pribilofs are at the center of a Bering Sea ecosystem so rich with marine life that they are sometimes called the “Galapagos of the North.”
The waters around the islands support some of the nation’s biggest seafood harvests, with vessels catching pollock, cod, halibut, crab and other fish. Millions of migratory seabirds of a dozen species flock each year to nest in the Pribilofs.
The Pribilofs are the breeding grounds for two-thirds of the world’s approximately 1 million northern fur seals. Each summer, they gather on the islands’ rocky beaches in noisy congregations to give birth to and nurture their young, molt their fur and rest.
The algal discoveries on the St. Paul and St. George beaches point to a bigger phenomenon in the ecosystem, but how much bigger is yet to be determined.
“The problem is they die at sea. They’re being poisoned at sea, and they can’t even make it back to land, right?” Williams said. “We don’t know how this may have population consequences because we don’t really have a true estimate of the number of animals that are dying.”
Paralytic shellfish poisoning is a long-known hazard in the southern coastal areas of Alaska and other warmer parts of the world.
Saxitoxin is colorless and odorless. It cannot be cooked out or frozen out of food. Once ingested, there is no antidote. The poison acts within minutes, interfering with signals from the nervous system that enable vital bodily functions. In mild cases, many of which may go unreported, patients feel some numbness and possibly nausea and other symptoms before recovering. In fatal cases, saxitoxin blocks the nervous system’s functions, causing paralysis that suffocates victims.
From 1992 to 2021, 132 people in Alaska were reported sickened with paralytic shellfish poisoning, according to state epidemiologists. Between 1994 and 2020, five people died after eating saxitoxin-tainted food.
Understanding the exact chemical process that leads to paralytic shellfish poisoning took decades of scientific research. Saxitoxin was first identified in 1937 in an Alaska butter clam by a team led by Hermann Sommer at the University of California, San Francisco. They named the toxin for the species of the clam in which it was found: Saxidomus gigantea.
Saxitoxin is produced by a particular algae species, Alexandrium catenella, that blooms in warm conditions. The association of warmth and algal toxin risks was well-known; an old rule of thumb for harvesters was to gather clams only in months with the letter R in their names. An even older guideline was to use the end of herring spawning — an event usually in late spring — as the signal to pause harvests of clams for the season.
There are inescapable facts about the proliferation of Alexandrium and other harmful algae in Alaska: Ocean waters are getting warmer, and staying warmer longer, meaning there are more blooms producing more toxins and creating more exposure risks for marine life and for the people who depend on food from the sea.
“Do we predict that these blooms will continue to increase and toxins will be increasing in Arctic food webs? Yes, they will,” Kathi Lefebvre, a NOAA Fisheries research biologist who specializes in algal toxins, said in her presentation.
Don Anderson, an expert on harmful algal blooms who runs a lab at the Woods Hole Oceanographic Institution in Massachusetts, said there are now two major sources of far-north blooms.
Most of that algae is believed to have been carried north by ocean currents. But climate change has created a possible bigger local source of blooms, he said: germination from massive seed beds that are, by far, the largest and most concentrated ever documented in the world.
The Bering, Chukchi and Beaufort seas used to be dead ends for Alexandrium cysts, the equivalent of seeds. They settled in the sediment after decades and centuries of being washed north, remaining dormant in the cold temperatures.
But the warm conditions that enable them to germinate have arrived, albeit sporadically. If temperatures at the seafloor warm up, the cysts can germinate within about 10 days and proliferate during the long daylight hours of Alaska summers, Anderson said.
It is unclear how the fur seals are getting exposed to saxitoxins. Unlike marine mammals considered to be at risk for algal toxins like clam-gobbling walruses and sea otters, fur seals do not eat bivalves. They do eat squid and schooling fish, including a tiny, slender, silvery fish called sand lance, which is known to absorb large concentrations of saxitoxin.
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