Cooking does not eliminate the toxins, and people should be extremely careful when harvesting shellfish on public or private tidelands, officials said.
The large bloom of toxic algae also was worrisome for the state's shellfish farmers, who battled a recall of raw oysters last month after a separate outbreak of vibriosis, an bacterial illness.
Some of the affected harvest areas have since reopened, but the industry is leery of damage to its market from more bad news about shellfish in Washington.
Shelton-based Taylor Shellfish, the West Coast's largest farmed shellfish producer, lost about $150,000 a week in live oyster sales because of the bacterial outbreak, spokesman Bill Dewey said.
Industry officials were watching the newer toxic algae blooms closely, hoping they would stay away from the major farming areas in the northern and southern ends of Puget Sound.
"It'd be a shame if we just got relief from the vibrio to get shut down for red tide," Dewey said.
