Scientists investigate paralytic shellfish poisoning as possible factor in large marine die offs 

Gloved hands holding a bunch of mussels
Blue mussels at Nahku Bay. (Claire Stremple/KHNS)

Paralytic shellfish poisoning, or PSP, is a growing concern for Alaska’s entire marine food web, according to a scientist who presented testing data at Alaska Forum on the Environment on Monday.

Bruce Wright is senior scientist at the Knik Tribe and leads PSP monitoring efforts across the state. He said while PSP has been a concern for subsistence shellfish harvesters for centuries, he and other scientists have begun noticing deadly marine toxins targeting animals at every level of the food chain, from plankton to whales.

“We’re finding PSP in just about everything we test, all the animals we test, at different levels, different times of the year, different locations,” Wright said.

Paralytic shellfish poisoning can occur when a type of algae called Alexandrium blooms in the ocean, releasing lethal neurotoxins that end up lodged in many species of clams, mussels, scallops and other shellfish. In high concentrations, a single clam could kill a person. 

“One of the pathways is shellfish,” Wright said. “But the other pathway is Alexandrium to zooplankton, to macrozooplankton and forage fish, to predatory fish like salmon and cod, and then the top predators like whales.”

A graphic depicting how paralytic shellfish toxins enter the food web.
Graphic depicting trophic transfer of PSP toxins produced by Alexandrium cells to shellfish, zooplankton, shellfish consumers, forage fish, predatory fish, and eventually to top predators such as seabirds and marine mammals. (Credit: Steve Kibler, NOAA.)

Wright said this research could potentially help explain recent population declines and die-offs in Alaska’s marine ecosystem — like the Pacific cod crash in the Gulf of Alaska or the ongoing Yukon River king salmon collapse.

“I know there’s other issues out there with the Yukon River king salmon, but this might be one of them,” he said. 

Wright presented recent data showing detectable levels of paralytic shellfish toxin in many species of predatory fish, including cod and salmon. A Yukon king salmon liver harvested in 2023 nearly broke the Food & Drug Administration’s maximum limit of toxin for safe consumption. 

Wright said the data his team has collected is anecdotal so far, but he’s continuing to test samples from animals all over the state. He said he suspects toxins will show up in more places as climate change warms ocean waters and pushes harmful algal blooms further north.

Steve Kibler, a NOAA research biologist who studies harmful algal blooms all over the country says there are still a lot of unanswered questions, like how paralytic shellfish toxins move through the food web.

He said without more data it’s hard to know how much toxin is lethal for different animals, or what the biological effects are.

“It does make me worry that the toxins could be having an impact,” Kibler said. “It definitely hurts their fitness. Maybe they can’t feed as well, can’t reproduce as well, can’t migrate. Those are all major issues.”

Kibler and Wright both have more data collection planned this summer to begin piecing together how paralytic shellfish toxins may be impacting the health of Alaska’s ocean food web.

Kavitha George is Alaska Public Media’s climate change reporter. Reach her at Read more about Kavitha here.

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