Harmful Algal Blooms
Overview
Harmful Algal Blooms (HABs) occur when certain phytoplankton species bloom and produce toxic compounds that accumulate in marine organisms when consumed. These toxins can cause diseases like Paralytic Shellfish Poisoning (PSP). In Alaska, the main source of these toxins is Alexandrium catenella, which releases Paralytic Shellfish Toxins (PSTs) that build up in shellfish, fish, birds, and marine mammals. Since 1973, at least 15 Alaskans have died from PSP—all during non-commercial harvests, indicating a disproportionate impact from subsistence and recreational harvests. Monitoring HABs requires funding for sampling and lab testing. This infrastructure is more established in urban-based regions with long-standing PSP activity like Southeast Alaska and Kodiak. However, warming oceans are creating new HABs risks in Western Alaska, where laboratory and testing infrastructure is limited.
HAB formation: what is a bloom?
Alexandrium cells begin as cysts (Stage 1 in the figure below) in the seafloor, and can remain dormant for multiple decades. The cells germinate (Stage 2) into a swimming stage when conditions such as water temperature, oxygen, nutrients, and light are right for growth. Then, they rapidly divide (Stage 3), increasing their cell concentration, sometimes producing and releasing toxins into the water column. Once conditions become unfavorable for growth, the cells stop dividing, reproduce, and re-enter the cyst stage, ready to bloom again (Stages: 4, 5, 6). A “bloom” occurs when the cell concentration at Stage 3 surpasses 1,000 cells/liter (advisory limit). This is separate from the regulatory limit, which is a legally determined threshold of PSTs found in a marine organism’s tissue (above which consuming the organism may cause PSP). Both advisory and regulatory limits are used to inform communities of HAB-related risks.
HAB lifecycle adapted from U.S. National Office for Harmful Algal Blooms at Woods Hole Oceanographic Institution by Anjali Shah (ACCAP)
HABs and PSP through history* (by region)
* These timelines are non-exhaustive. They only include events pertaining to the Alexandrium genus and non-commercial harvests. Many PSP cases go unreported.
North Slope to Aleutian Islands
July 1980: 1 PSP case at King Cove from butter clams, patient medevaced to Anchorage
Aug. 1982: 5 PSP cases in Perryville from mussels tested at 62 times the regulatory limit
June 1990: 5 PSP cases, 1 PSP death, butter clams near King Cove
April 1995: 7 PSP cases in Perryville from razor clams testing at 12 times the regulatory limit
May & June 2012: Pacific Walrus** off of St. Lawrence Island tests positive for saxitoxin
2014: Suspect PSP case recorded in Nome Census Area
Aug. & Sept.2017: Pacific Walrus die-off, 37 walruses found dead near Diomede and Shishmaref, 1 of 37 had detectable saxitoxin concentrations above the regulatory limit
Aug. 2018: HAB detected in northern Bering Sea by icebreaker USCGC Healy, 1,500-1,800 cells/liter detected (1,000 cells/liter is the advisory limit)
July 2019: 1 PSP case in Perryville from butter clams
Aug. 2019: HAB detected in northern Bering Sea by icebreaker Healy, clams near St. Lawrence Island and Cape Lisburne tested above the regulatory limit
July 2020: PSP Death in Unalaska from blue mussels testing at 100 times the regulatory limit
Dec. 2021: Multiple beached herring in Kotzebue test positive for saxitoxin
June & July 2022: King Cove shellfish advisory, Razor clams tested 6-7 times the regulatory limit, mussels test at the regulatory limit
Aug. 2022: Largest, most toxic Alexandrium bloom ever in the US detected in the Bering Strait by the Norseman II. Clams and gastropods test above regulatory limit.
Aug. 2023: Alexandrium bloom in Bering Strait linked to clams and worms testing above regulatory limit.
Summer 2024: Alexandrium bloom detected in Bering Sea, northern fur seal mortality event on St. Paul Island
July 2025: Alexandrium bloom detected in Bering Sea, seabird and northern fur seal die offs on St. George Island
**The Pacific Walrus is an important nutritional and cultural food source for people throughout Northern and Western Alaska
Kodiak
May 1976: 1 confirmed PSP case from razor clams
July 1987: 2 PSP cases in Monashka Bay from mussels testing at 70 times the regulatory limit
July 1990: 1 confirmed PSP case from mussels, shellfish found at 25 times the regulatory limit
May 1994: 16 confirmed PSP cases around Kodiak, 1 PSP death from mussels, 3 patients required hospitalization in Kalsin Bay and 1 death in Old Harbor, mussels tested at ~230 times the regulatory limit at Chiniak Bay and Old Harbor
February 1995: 1 confirmed PSP case from butter clams testing 2-3 times the regulatory limit
June 1997: 3 confirmed PSP cases, 1 PSP death near Sturgeon River Lagoon and Larsen Bay from butter and littleneck clams testing at 100 times the regulatory limit.
Summer 2012: Kittlitz’s Murrelet nestling die off tied to high saxitoxin levels in prey sand lance
July 2014: Butter clam levels reported as highly toxic at Old Harbor (2-2.5 times more toxic than the rest of the island)
July 2016: 1 confirmed PSP case at Roslyn Beach, butter clams testing at 8 times the regulatory limit in Old Harbor
April, Aug., Dec. 2018: Butter clam levels reported as highly toxic at Ouzinkie (2-2.5 times more toxic than the rest of the island)
May & June 2019: State shellfish advisory, butter clam toxin levels high (some 50 times the regulatory limit)
June 2021: Region-wide shellfish advisory, all species tested high in the City of Kodiak and south Trident Basin (including mussels, butter clams, oysters, and moon snails)
Southeast
Aug. 1973: 2 families report PSP from butter clams in Tenakee Springs
June 1980: 2 PSP cases reported from butter clams which tested at 22 times the regulatory limit at Sunny Bay near Wrangell
July 1980: 2 PSP cases reported from mussels near Young Bay and Eagle River. Both people medevaced to Juneau
May 1982: 14 PSP cases reported in Ketchikan and Craig. Butter clams, cockles, and mussels all tested at dangerous PSP levels, one clam found at 151 times the regulatory limit
June 1997: 2 PSP cases
Nov. 1997: 4 PSP cases reported from butter clams at Auke Bay and Amalga Harbor. Both families “taste tested” clams
June 2010: 3 PSP cases, 1 death in Haines from dungeness crab viscera, 1 death in Auke Bay from cockles tested at 25 times the regulatory limit
May 2011: 5 confirmed, 8 probable PSP cases in Metlakatla from cockles and 3 confirmed, 5 probable cases in Ketchikan from mussels, littleneck, and butter clams. Cockles tested ~6.25 times the regulatory limit, mussels tested at 62 times the regulatory limit
Dec. 2014: PSP case reported in the Southeast region from 4 butter clams
May & June 2019: State shellfish advisory. Butter clam toxin levels high (some 50 times the regulatory limit)
June 2021: Region-wide shellfish advisory. All species tested high in Hoonah and Ketchikan (including mussels, butter clams, oysters, and moon snails)
Kenai Peninsula
June 1977: 17 PSP cases reported in Homer from sea snails
Sept. 2025: Saxitoxin event at Kachemak Bay. At Yukon Island, Homer Harbor, Gull Island, and Peterson Bay, butter clams tested at 1.5 times regulatory limit and blue mussels as high as 2 times regulatory limit
Event Spotlight: 2022 Bering Strait
In 2022, researchers aboard the Norseman II in the Bering Strait region detected the largest, most toxic HAB event ever recorded in the United States, an area with historically low PSP rates. Alexandrium cell concentrations were up to 173,000 cells per liter, 173 times the advisory limit of 1,000 cells/liter. Though the shellfish toxin levels were pending lab analysis, regionally-based Alaska Sea Grant and Norton Sound Health Corporation in Nome worked to notify Bering Strait regional community leadership and healthcare facilities by providing awareness, education, and updates. While there were no PSP reports in people, results for harvested seafood sent for testing showed saxitoxin at 5 times the regulatory limit, marking a close call.
HABs and ocean temperatures
Alexandrium germinate in response to certain environmental conditions, particularly ocean temperatures above 8°C, which have been more common in all Alaskan waters over the last few decades. Recent research from Woods Hole Oceanographic Institute designates a metric called heating degree-days (DD), a measure of accumulated heat.
Germination by region
Figure from Anderson et al. 2021
Alexandrium cells from the Chukchi Sea require 85 DD (red line) to germinate, which encompasses conditions from over 60 consecutive days at 0°C (left) to 5 days at 14°C (right). Warmer conditions occur more frequently now than in the past due to ocean warming (see below), increasing HAB risks in the Bering and Chukchi seas off of Western Alaska.
HAB fast facts
53% of reported PSP cases from 1993-2021 were Alaska Native, reflecting a higher reliance on non-commercial harvests
35% of PSP cases in Alaskan waters from 1993-2021 were from consuming butter clams, 18% from mussels, 18% from cockles, and 18% from shellfish combinations.
Alaska Department of Environmental Conservation (DEC) is required to test only commercial shellfish for PSTs, whereas there is no requirement to test subsistence and recreational harvests.
DEC recommends NO non-commercial harvest of shellfish in Alaskan waters
A special thanks to Gay Sheffield and Julie Matweyou from Alaska Sea Grant and Thomas Farrugia and Rosie Masui from the Alaska Harmful Algal Bloom Network for help with this product.