The Washington Department of Health has closed much of northern Puget Sound to recreational shellfish gathering after finding dangerous levels of the toxin that causes paralytic shellfish poisoning.

Most shellfish areas on North Puget Sound, including all of Whatcom and San Juan counties and as far south as the northern end of Whidbey Island, are off-limits for recreational shellfish harvesting because of dangerous levels of marine biotoxins — “red tide” — capable of causing paralysis or even death if consumed.

Besides shellfish harvesting areas in Whatcom and San Juan counties, the state Department of Health has closed Deception Pass, Fidalgo Bay, Samish Bay, Sinclair and Cypress Islands in Skagit County to the harvesting of clams, oysters, mussels, scallops, geoduck and other mollusks, according to a health department bulletin released on Thursday.

The northern part of Whidbey Island, from Keystone Harbor on the west to Strawberry Point on the east, including Deception Pass, are on the closure list.

The red tide biotoxins that cause paralytic shellfish poisoning have been detected at levels as much as 100 times higher than the established closure level in shellfish samples collected from north Puget Sound, said Frank Cox, of the state Department of Health’s shellfish program.

Biotoxins have also been detected in Discovery Bay in Clallam and Jefferson counties, prompting a shellfish harvest closure, there, too. The health department said toxin levels in Discovery Bay weren’t as high as those found in north Puget Sound, but should also be taken seriously.

Warning signs have been posted at popular shellfish beaches, warning people not to harvest from the closed areas.

Shellfish recently harvested from closed areas should be discarded and not eaten, the health department said.

Crab is not included in the closure, but “crab butter,” the white-yellow fat inside the back of the shell of a large crab, which some consider a delicacy and is often added to dressings and sauces served with crab, should be discarded, the health department said. Only the crab meat is safe to eat.

The health department said commercially harvested shellfish currently on the market have been thoroughly tested and are safe to eat.

Marine biotoxins, produced by naturally occurring algae that tend to be more common during the warmer months of the year, are not destroyed by cooking or freezing, and can be life threatening.

Symptoms can appear within minutes or hours of eating contaminated shellfish. They usually begin with tingling lips and tongue, moving to the hands and feet, followed by difficulty breathing. Anyone with symptoms should seek medical help immediately.

The state health department has a marine biotoxin Web site — www.doh.wa.gov/ehp/sf/biotoxin.htm — for updates on recreational shellfish harvesting statewide, and a biotoxin hotline — 800-562-5632.

Charles E. Brown: 206-464-2206 or cbrown@seattletimes.com