Heat dome found to be deadly for some shellfish species, but not for others

June’s intense heat dome, ill-timed with extremely low tides, left shellfish dead on some beaches across Puget Sound and the Washington coast. A heat dome is a heat wave that lasts multiple days over one region.

A research paper about the event is under peer review, using data collected by scientists from tribes, government agencies, universities, non-profits and marine resources committees. The data was analyzed by the Jamestown S’Klallam, Skokomish and Swinomish tribes, University of British Columbia, Washington Department of Fish and Wildlife, Puget Sound Restoration Fund, Washington Sea Grant and the University of Washington (UW).

The impacts of the heat dome on shellfish varied across the region, said Wendel Raymond, a UW nearshore ecology research scientist who is leading the analysis.

Some beaches fared better than others. Shellfish on the coast generally had better survival than those in the Eastern Strait of Juan de Fuca and Puget Sound, Raymond said. It’s believed this had to do with the tide cycle during the heat dome, as the lowest tides happened early in the morning on the coast compared to midday in Puget Sound.

By the afternoon on the coast, the tide was high enough to cover and protect the shellfish, he said, while the beaches in Puget Sound were exposed at the hottest time of the day.

Narrowing the scope even further, scientists discovered that some species fared better in certain kinds of habitat. Pacific oysters near the Dosewallips and Duckabush rivers in Hood Canal experienced less mortality compared to Pacific oysters in other parts of Hood Canal that were far from water sources. Larger estuaries have a constant freshwater supply that may have kept shellfish cool while the tide was out, Raymond said.

Mortality estimates for oysters in Hood Canal have been as high as 30 percent, said Blair Paul, Skokomish Tribe shellfish biologist, while other beaches with some refuge ranged from 5-18 percent. Survival also depended on the species’ preferred habitat. California mussels, which live among the big waves on the coast, had a better outcome than bay mussels found in sheltered and shallow areas of interior waters.

Cockles suffered a lot of mortality, Raymond said, as they are generally found on top of the sediment or are buried just below the surface. In comparison, butter clams, which live deeper in the sediment, experienced less mortality.

“The loss of clams, particularly cockles, on our local beaches is devastating for our tribal community,” said Swinomish interim fisheries manager Tandy Wilbur. “Understanding the extent of the mortality is a first step for us in determining how to respond now and in the future to these types of events.”

“Although this event had negative effects on marine life of the Salish Sea, there is hope that can be found in this work,” said Annie Raymond, Jamestown S’Klallam shellfish biologist. “This work has suggested that not all locations and species were affected equally, offering clues to pathways to resiliency in the future.”

Overall, the data set is limited, said Wendel Raymond, as it is more qualitative than quantitative, meaning the scientists weren’t able to get exact numbers of how many animals died, but they were able to provide quality observations about what they saw on the beach. This is an essential first step in understanding the impact that the heat dome had on shellfish resources.

“The next phase of this work has begun, with a lot of discussions to determine the shellfish harvest season for 2022,” Paul said.

Extreme temperatures killed shellfish on beaches across the region. Pictured is a cockle found on the Swinomish Reservation. Story: Tiffany Royal. Photo: Swinomish Tribe.