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PSP (red tide) monitoring

The Division of Marine Fisheries (DMF) monitors coastal waters for harmful algal blooms. These blooms produce biotoxins that can accumulate in shellfish, making it harmful to eat.

What is paralytic shellfish poisoning?

Paralytic Shellfish Poisoning (PSP) is a serious illness caused by eating shellfish contaminated with harmful neurotoxins. These neurotoxins are produced by microscopic algae that can bloom in certain environmental conditions. 

Certain species of marine organisms, such as shellfish, feed on a variety of microscopic algae they collect by filtering large quantities of seawater. During harmful algal blooms, the shellfish can accumulate the red tide algae containing neurotoxins to dangerous, and even lethal, levels if they are eaten. However, the toxin does not affect the shellfish themselves, and when the bloom diminishes, they will eventually rid themselves of the toxin and will once again be safe to eat.

Current monitoring

Our shellfish biologist monitors for marine biotoxins produced by microscopic algae, which can cause PSP. Consumption of shellfish with PSP toxins can cause illness or even death.

DMF collects shellfish from 13 primary stations from March through October. We analyze samples at our labs to test the toxin levels in shellfish. If a sample contains toxin, testing increases for that site. Shellfish areas are closed if toxin level exceeds safe limits.

View PSP notices

9/16/22: Biotoxin Narrative by DMF Staff

Additional Resources

Response to elevated toxin levels

  • As biotoxin levels rise above 50µg/100g shellfish meat, sampling increases at affected sites. At this threshold, DMF sampling extends into selected secondary sites.
  • When levels exceed 80µg, affected areas close and sampling continues. Once 3 consecutive samples result in levels below 80µg, the area reopens. Some species can reduce their toxin levels faster than others in the same area. Sampling and closures reflect these changing levels.
  • DMF will maintain specific closures until each restricted species is toxin free. Thus, one species may be open to harvest in an area while another species is not.

Harvesting controls during toxic events

Once an area reaches the 80ug threshold, DMF:

  • Changes the area status to "CLOSED TO ALL SHELLFISHING" and mails closure notices.
  • Calls shellfish constables or other officials of affected towns.
  • Sends an electronic notice to state personnel responsible for monitoring PSP events.
  • Sends a notice to the Environmental Police.
  • Mails written notices containing exact area descriptions and the species affected.

DMF sends an e-mail notice to select state agencies every week following the first event. The notice informs them of the ongoing status of the toxin events.

When toxin levels fall below 80µg for three consecutive samples, DMF changes the status of affected areas to "OPEN". We notify state and local personnel to ensure a rapid re-opening of shellfish growing areas.

For further information on the Division's monitoring efforts, contact the Shellfish program at our New Bedford office.

Additional Resources

Contact   for PSP (red tide) monitoring


DMF Staff Directory View 


(508) 990-0449


836 S Rodney French Blvd, New Bedford, MA 02744

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