‘Red tide’ makes shellfish poisonous in Massachusetts waters


Algal blooms can make clams, oysters and mussels dead.

Bivalve shellfish like clams can become loaded with neurotoxins from algal blooms known as red tides. Maleo Photography/Adobe Stock

Shellfish harvesting is temporarily prohibited in large areas of Massachusetts coastal waters, particularly off the North Shore. These restrictions are the result of harmful algal blooms that can cause paralytic shellfish poisoning (PSP), also known as red tide.

The ban was originally announced by the Massachusetts Division of Marine Fisheries (DMF) last month, but remains in effect until toxicity levels drop. The collection of all bivalve shellfish and carnivorous snails has been banned in fourteen growing areas, stretching from Salisbury Beach to the waters off Gloucester.

A ban on picking blue mussels was announced at the end of June affecting many growing areas on the South Shore.

On July 7, it was determined that soft clams were safe to pick up and eat in the Essex Bay area, but all other bivalve molluscs and carnivorous snails in the area are still considered dangerous.

In an update released July 8, DMF officials said that while toxicity levels have decreased, several areas will still be closed until mid-July.

The red tide in New England waters is caused by a sudden population boom in Alexandrium fundyense, a naturally occurring microscopic plankton. Blooms of this plankton are caused by environmental conditions conducive to explosive growth, such as warm surface temperatures, high nutrient content, low salinity and calm seas, according to the DMF. When periods of rain are followed by sunny weather in summer, these blooms are particularly common.

PSP is a disease caused by eating contaminated shellfish. Some marine species, like seashells, feed on microscopic algae harvested by filtering seawater. During a harmful algal bloom, these creatures can collect enough algae containing neurotoxins to be dangerous and potentially deadly. if they are consumed by humans. The toxin does not affect the shellfish themselves, and the animals eventually get rid of the toxin.

These poisonous flowers can sometimes turn water reddish brown, hence the name Red Tide. This discoloration does not always occur, although a particular algal bloom has rendered the shellfish toxic. Conversely, many non-toxic species of reddish-brown plankton can cause similar discolorations.

There is no risk of swimming in waters affected by the red tide, according to the DMF.

Hard clams, soft clams, oysters, mussels, whelks, and moon snails are most commonly affected, but scallops and lobster meat are generally safe to eat.

The poison immediately affects the nervous system once ingested and symptoms can develop within 30 minutes, according to DMF. A common initial reaction is tingling of the lips and tongue, which can quickly spread to the face, neck, fingertips and toes. Headaches, dizziness and nausea come next. In severe cases, muscle paralysis and difficulty breathing may occur. Deaths from respiratory paralysis caused by PSP have been documented.

With these restrictions in place, restaurants have been forced to purchase shellfish from out of state, WBUR reported. This usually increases the price for restaurants and customers. Typically, restaurateurs turn to Maine clams when local clams are no longer on the table.

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