Red tide has spread from central Maine to Gloucester, making it unsafe to harvest soft-shell clams or mussels from those coastal waters and potentially signaling the onset of a business-damaging season, state officials say.
The single-celled algae carries toxins that concentrate over time in shellfish, making them poisonous, even lethal. Red tide often occurs in late spring and summer, when the algae grow rapidly. Crabs, lobsters, fish, and shrimp are not affected.
A combination of abundant beds of the algae seeds and excess winter precipitation could translate into the worst red tide season since 2005, oceanographers at the Woods Hole Oceanographic Institution say.
The 2005 outbreak, which also began in the middle of May, extended from northern Maine to Nantucket with algae counts 40 to 100 times higher than normal. It halted business for nearly 2,000 clammers, oyster farmers, and mussel harvesters for much of the summer. They lost tens of millions of dollars.
"I'm not in a position to make predictions, but we're always worried," said Michael Hickey, chief biologist for the state Division of Marine Fisheries.
He said the no-harvesting order along the North Shore took effect on Thursday for mussels, carnivorous marine snails, and soft-shell clams. But he said it was not unusual to see red tide this time of year.
"We are always watchful of this from the beginning of May," he said.
Hickey added there's no risk for residents who eat clams or mussels already on the market. The state suspends harvesting when officials find 80 micrograms of the toxin for 100 grams of shell fish meat. It takes 250 to 300 micrograms to make people ill.
Don Anderson, a senior scientist at the Woods Hole Oceanographic Institution who specializes in red tide issues, said his oceanographic models show this year's bloom could approach the damage that occurred in 2005.
He said the amount of winter snow and spring rainfall, the number of seeds found on the bottom of the Gulf of Maine, and shellfish toxicity have followed the same pattern as 2005.
"It is strikingly similar," Anderson said. "It seems likely we'll see further closures in the coming week further south."
He said those who work in the shellfish industry should hope for sustained southwesterly winds, which could dissipate the effect of the algae bloom by blowing the cells off to sea. Gloucester Shellfish Constable Dave Sargent said about 500 commercial shellfish harvesters and about 2,000 recreational harvesters could be devastated by a large red tide outbreak.
"So far, it looks like it could be a bad year, but you really never know," Sargent said.
The state has sampling stations that test shellfish every two days. Massachusetts has been monitoring red tide since 1972, when there was also a large outbreak.