Officials have issued a no-harvesting order along the North Shore for mussels and soft-shell clams after red tide spread from southern Maine to Gloucester.
The single-celled algae, which sometimes turn water a rust color, carry toxins that concentrate over time in shellfish, making them poisonous and potentially lethal. Red tide usually occurs this time of year, when algae grow explosively. Lobsters, shrimp, crabs, and other fish are not affected.
"You need a crystal ball to see whether it will go farther south," said Michael Hickey, chief biologist for the state Division of Marine Fisheries. "Right now, things are looking pretty good elsewhere in the state. But that could change overnight. It's a constant surveillance program."
Hickey added that there is no risk for residents who eat shellfish already on the market. The state suspends harvesting when they find 80 micrograms of the toxin for 100 grams of shellfish meat. It takes between 250 and 300 micrograms to make people sick.