Local and state health authorities have closed several North Whidbey beaches to butter clam harvesting due to high concentrations of a potentially deadly marine biotoxin associated with the occurrence of red tide.

The closure affects beaches from Ala Spit to Strawberry Point and applies only to butter clams, according to a July 29 press release. However, all shellfish harvesting, including clams, oysters, mussels and scallops, in Deception Pass and Cornet Bay was also closed for red tide earlier this month.

According to Kathleen Parvin, an environmental health specialist with Island County Public Health, North Whidbey is experiencing a full-blown bloom of the naturally forming marine algae.

An associated biotoxin, which can cause paralytic shellfish poisoning, can be deadly if ingested. Symptoms can appear within minutes or hours and usually begins with tingling lips and tongue. The sensation moves to the hands and feet and is followed by breathing difficulty, and potentially death, the release said

To date, no incidents of poisoning have been reported.

Parvin said red tide blooms are not common, but are not rare either.

“It varies from year to year,” she said. “It’s erratic.”

The last bloom occurred in 2009 and resulted in the closure of shellfish harvesting in Cornet Bay. A much more massive bloom occurred in 2006. It forced local and state officials to close beaches all along the west side of Whidbey Island. No one knows what causes them or how long they will last. They can appear any time and can last from months to years, she said.

“The data is insufficient so we just monitor,” Parvin said.

Thursday’s press release states that all beaches in San Juan and Whatcom counties been closed to shellfish harvesting for some time.

According to the state Department of Health’s website, of Whidbey Island’s total 56 public beaches, 20 are currently closed. Four are closed due to the presence of marine biotoxins, 14 are closed because of pollution, and two are closed for a combination of the two. Shellfish advisories have been posted for the remaining 36, the website says.

Red tide is a serious concern for local and state officials, Parvin said. Clamming season runs from March to October but people often ignore both the season dates and health advisories and clam year-round. Some even go out at night, she said.

“People are serious about their shellfish, but paralytic shellfish poisoning is serious too,” Parvin said.

Marine biotoxins are not destroyed by cooking or freezing, and in most cases, can only be detected through laboratory testing. Shellfish harvesters should always check the department of health’s website at www.doh.wa.gov/ehp/sf/biotoxin.htm or call the state’s Biotoxin Hotline at 1-800-562-5632 before harvesting shellfish anywhere in Washington.