The state Department of Health closed Penn Cove and other areas of Whidbey Island’s east side to shellfish harvest after tests of mussel and clam showed the highest level of domoic acid ever detected in the inland waters.

Domoic acid is a naturally-occurring marine toxin that can be dangerous if ingested in high levels, causing amnesic shellfish poisoning. Similar to so-called red tide, domoic acid is produced by microscopic algae.

Ian Jefferds, owner and general manager of Penn Cove Shellfish, said none of the mussels or clams with elevated levels of the neurotoxin were distributed. Shellfish currently on the market should be safe to eat.

Jefferds said this was the first time Penn Cove has ever been closed due to domoic acid. Saratoga Passage and Holmes Harbor are also closed as a precaution.

“Hopefully, the cool weather will make it go away,” he said. “It came quickly, hopefully it will leave quickly.”

Jefferds said it’s unclear what environmental conditions might cause high levels of the toxin to show up in a particular area, though he stresses it’s a natural part of the ecosystem. He said he doesn’t know whether a large sewage leak in Oak Harbor last month could have contributed to the problem.

The Health Department reports that domoic acid was first detected in razor clams off the coast of Washington in the fall of 1991. Ever since, clamming on the coast has been dependent on domoic acid levels.

The symptoms of amnesic shellfish poisoning, or ASP, include vomiting, diarrhea and abdominal cramps. In more severe cases, it can cause headaches, dizziness, confusion, disorientation, short-term memory loss, seizures and even death.

The Island County and the state Departments of Health will continue monitoring and will open the areas when the levels drop.