Skokomish Tribe investigates impacts on shellfish from blue-hued Hood Canal

The Skokomish Tribe is concerned that the rich turquoise water showing up annually in Hood Canal is affecting the shellfish resource.

The colorful water is caused by blooming phytoplankton called coccolithophores, which are single-cell organisms covered in calcium carbonate coccoliths (similar to scales) that reflect light, creating the Caribbean blue color. The bloom also clouds the water, preventing light from reaching the algae that need it to photosynthesize, which reduces the amount of food available to shellfish.

NASA satellite images have captured the brightly colored water on and off for more than a decade, but annually since 2016, said Blair Paul, the tribe’s lead shellfish biologist. It typically starts in Dabob Bay in the summer and makes its way toward southern Hood Canal.

“It’s not toxic to humans but it could be affecting the food resource for the shellfish, and affecting the tribe’s treaty resource,” said Seth Book, the tribe’s Environmental Protection Agency coordinator.

The tribe started to question the correlation between the water conditions and reduced shellfish survival after observing oyster mortalities in 2017. Also that year, Paul conducted a geoduck dive survey in the middle of the bloom. He noticed geoduck weren’t eating and the light levels within the bloom were darker than usual.

Paul started talking with shellfish farmers along the canal, discovering that their shellfish beds were having extremely high mortality rates at the same time as the bloom. In 2018, many farms took heavy losses in central Hood Canal.

“Now we want to know two things,” Paul said. “If there is a correlation between low crab and shrimp abundance when there is a coccolithophore bloom, and if there is reduction in food production in the water column for all shellfish nutrition.”

The tribe is looking at “light attenuation” – how much of the light is reflected out of the water by coccolithophores. The tribe is using a spectroradiometer to take light readings inside and outside the blooms, as well as water quality and plankton samples.

It’s difficult to determine whether the shellfish eat the coccolithophores, Paul said. While this species of coccolithophores isn’t believed to be toxic, it may not be edible to shellfish because of the abrasive nature of the coccoliths, he said.

The tribe received a Bureau of Indian Affairs grant to investigate coccolithophores. A report is expected in the fall, and the tribe also will write a mitigation plan for shellfish impacts.

“The tribes have been here thousands of years and will continue to be here,” Book said. “It could be a natural cycle, but what we’re seeing is having implications to shellfish and treaty resources. It could possibly spread to other parts of Puget Sound as well.”

Skokomish Tribe EPA coordinator Seth Book uses a fishing line to slowly lower a spectroradiometer into Hood Canal, which takes light readings every foot, in the middle of a coccolithophore bloom. Photo: Tiffany Royal

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