Red tide, shellfish and dogs on the beach

The Department of Public Health describes red tide as a population explosion of toxic microscopic plankton, in New England the dinoflagellate Alexandrium. Warm surface water, plenty of nutrients, lower salinity and calm seas favor a bloom, and rain followed by sunny summer weather often comes before one. The water may turn reddish brown, but it does not have to: toxic plankton can be numerous enough to poison shellfish without discoloring anything. The department adds that swimming carries no risk even during an Alexandrium red tide. The danger is eating.

Filter feeders are the problem. Hard-shell and soft-shell clams, oysters, mussels and scallops strain the plankton out of the water, and whelks and moon snails pick up the toxin by eating them. Lobster meat, crab, shrimp and most finfish do not normally build it up, though lobster tomalley is never safe. The toxin, saxitoxin, is one of the most potent known, cooking does not destroy it, and toxic shellfish look and taste exactly like safe ones. Only laboratory testing tells them apart.

Massachusetts monitors water, sediment and shellfish, and the Division of Marine Fisheries is responsible for year-round testing of shellfish and growing areas, more intensively in spring, summer and fall. When toxin passes the action level for people, 80 micrograms per 100 grams of shellfish meat, the division closes the area and notifies the city or town. When a bloom subsides and the shellfish clear the toxin, testing decides when the area reopens. The state's advice to recreational diggers is to watch for posted warnings and local announcements and to ask the local shellfish constable.

Cape Ann has had recent closures. On May 27, 2025, the division closed growing areas N8 through N12 in Gloucester and Rockport to all shellfish because of an Alexandrium bloom, and its letter says the areas would not revert to open on their own but would stay closed until examined. On June 13, 2025, it issued a precautionary closure of areas N3 through N7, from Newburyport to Gloucester and including Essex, for all shellfish and carnivorous snails. The division's North Shore field station is on Emerson Avenue in Gloucester.

Prevention on the flats is management more than training. Run the dog on firm open sand rather than across the clam beds, leash it past mussel-covered rocks, and walk the strandline yourself after a storm before the dog reaches it. A dog that digs clam holes or cracks mussels is rehearsing the habit, so interrupt the first dig. Carry treats and trade for anything it picks up rather than chasing it, because a chase turns a mouthful into a swallow. Our how-to on drop it builds that trade before you need it.

Through the summer, when Gloucester's beaches are closed to dogs and the state's monitoring is at its busiest, the flats are off the calendar anyway. On weeks when you cannot supervise a beach run at all, a walk from your door does the job: Dogtown's at-home walks are solo walks of 20, 30 or 60 minutes for $25, $35 or $55, led by Dogtown staff, with a photo and a note afterward. Ask us whether we cover your address.