The state has closed New Hampshire's tidal waters to shellfish harvesting after finding unsafe levels of the toxin that causes paralytic shellfish poisoning, better known as red tide.

Last year's red tide season, which lasted from early May to late July, was the worst in more than 30 years.

Experts say there's no way to predict how long the algae that produces the toxin will bloom this year or whether the wind will carry it inshore. All they can do is monitor its presence and let commercial and recreational harvesters know when the shellfish are again safe to eat.

"These are really not predictable," said Chris Nash, shellfish program manager for the Department of Environmental Services. "What we do is just keep track of what's out there."

Blue mussels tested on Star Island on Monday contained levels of the toxin higher than the closure threshold of 80 micrograms per 100 grams of shellfish tissue.

Samples taken inshore at the Hampton/Seabrook estuary were below that limit but contained more of the toxin than last week's samples did, Nash said.

Inshore waters, or those immediately along the coast, have been closed to shellfish harvesting since the beginning of the month for concerns about bacteria levels due to recent rains. Offshore waters now have a red tide ban, which affects three commercial aquaculture farms.

Through the winter, cysts of the algae lie dormant in the Gulf of Maine. As the water warms in the spring, algae become active and reproduce. Mussels, clams and oysters, which are filter feeders, consume the poison.

When those shellfish are eaten by humans, the toxin can cause temporary impairment of the nervous system, fatal in only the most extreme cases.

Shellfish such as lobsters, shrimp and crabs, which aren't filter feeders, are generally safe to eat, although Nash said it's a good idea to stay away from lobster tamale.

The toxic blooms got their name because they sometimes color the water red.

But the blooms, which have nothing to do with the tide, can be dangerous without having that effect.

In 2004, there were two blooms of red tide, one in the spring and one in the fall.

Going into 2005, scientists found that there were 10 times the number of cysts buried in the gulf mud than there were in 1997, more than 500 per cubic centimeter. Samples taken in May and June showed very high toxicity levels, Nash said.

This winter, there were about half the number of cysts in the gulf, but Nash couldn't say if that will mean a shorter or less intense season. "Only time, and continued monitoring, will tell,"he said.

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