Culprit Behind Mass Shellfish Mortality Identified

Researchers with the Washington Sea Grant program say they have identified yessotoxins, produced by blooms of certain phytoplankton, as the culprit behind summer mass shellfish mortality events in Washington state.

The study was released by aquaculture and maritime water quality specialist Teri King of Washington Sea Grant and partners from NOAA National Centers for Coastal Ocean Science, NOAA Northwest Fisheries Science Center, Northwest Indian Collage and Aquatechnics Inc.
Their findings have significant implications for shellfish growers in the region, they said.

Since yessotoxins are not a threat to human health, their presence in Washington had not been closely monitored.

Researchers used data collected by the NOAA Northwest Fisheries Science Center and NOAA National Centers for Coastal Ocean Science for other purposes, plus observations from the SoundToxins phytoplankton monitoring program. They found that two algae species, Protoceratium reticulatum and Akashiwa sanguinea, are correlated with shellfish mortality events as far back as the 1930s. Researchers also used data from SoundToxins partners in 2018 and 2019, along with reports of dying shellfish from the Washington Department of Fish and Wildlife and the shellfish industry to collect shellfish and water samples for analysis.

King said the research team is now working toward being able to help growers count the cells of yessotoxin-producing organisms in the water and create that information to take action, “SoundToxins has been conducting similar work for the Washington Department of Health for three “human health marine biotoxins since 2006,” King said.

Adding the shellfish killing plankton species to the real-time mapping capability of the SoundToxins partnership would allow for shellfish producers and natural resource managers to make informed decisions, such as harvesting their product early or otherwise strategizing to save as much crop as possible, she said.

The research also demonstrates the value of partnerships between shellfish producers, plankton monitors, Native tribes, agencies and researchers, as a team of oceanographers, biologists and chemists working together, King said.

The paper can be seen online at