Red Tide Resource Center

Information and Frequently Asked Questions About Red Tide

This web resource center was created to offer information to the public and the media about "red tide" or paralytic shellfish poisoning (PSP).


It's OK to eat shellfish

  • It's OK to eat shellfish from a reputable dealer or restaurant - these shellfish have been sourced from shellfish beds that recently tested negative for red tide and other contaminants.
  • It's OK to eat shellfish if you harvested it yourself from an "open" shellfishing area.
  • To avoid red tide, don't harvest or eat shellfish from closed areas.
  • You cannot "get" red tide from swimming or inadvertently drinking sea water while swimming.

Information on this page was compiled from the NH Department of Environmental Services and the Maine Department of Marine Resources.

Red Tide - Frequently Asked Questions

What is red tide?

"Red tide" refers to a condition in which filter-feeding shellfish such as clams, oysters, and mussels accumulate a potent toxin produced by naturally occurring marine algae. The toxin affects the human central nervous system, and eating contaminated shellfish, whether raw or cooked, can be fatal. The term "red tide" is used to describe this condition because the intensity of some algal blooms can make the seawater appear red; however, dangerous blooms do not always discolor the water, nor do they actually have anything to do with the tides. In New England, red tide episodes can occur throughout the spring, summer and fall.

What causes red tide?

Although there are several species of marine algae that can cause a variety of illnesses, the main illness of concern in New England coastal waters is paralytic shellfish poisoning, or PSP. Various species of the Alexandrium alga cause PSP in the Gulf of Maine. Off the coastlines of Maine, New Hampshire and Massachusetts, Alexandrium blooms typically occur in the spring and early summer, in response to increased sunlight and nutrients. Other water conditions such as low salinity and warmer temperatures, which often develop following springtime rains, snowmelt, and warmer air temperatures, are also associated with blooms. If water conditions are ideal, the cells of the toxic alga reproduce, and growth is exponential – a single cell can result in the reproduction of several hundred cells in just a few weeks. If weather patterns move these offshore blooms closer to the shore, filter-feeding shellfish in recreational and commercial harvesting areas can accumulate dangerous levels of PSP toxin, making harvest closures necessary. By the end of autumn, the Alexandrium cells settle in offshore ocean sediments in the form of cysts, where they lie dormant for the winter. The cycle repeats itself the following spring, when the cysts germinate into free-swimming, reproducing cells.

Where does red tide occur?

Red tide occurs in the Gulf of Maine, starting offshore and - depending on weather - it can move to inshore areas where commercial and recreational shellfish harvesting takes place.

When does red tide usually occur?

Blooms can occur any time from early spring to late fall, but usually blooms begin in mid-April to early May. Often, a bloom peaks in June and begins to subside. Sometimes, secondary blooms occur in late summer/early fall.

What does red tide do to shellfish?

If populations of the red-tide-causing alga are high, shellfish can accumulate dangerous levels of PSP toxin in their tissues. That's because shellfish are "filter feeders," meaning that they eat by straining food particles from water, including toxic algae. Eating these shellfish, whether cooked or raw, can cause a life-threatening illness.

Is it OK to eat shellfish during a red tide event?

If you buy your shellfish from a reputable dealer, or harvest your own shellfish from an area that has not been closed to harvest, then you can enjoy all the shellfish you can eat! The NH Department of Environmental Services, the Maine Department of Marine Resources and the Massachusetts Division of Marine Fisheries have extensive monitoring programs to ensure that contaminated shellfish do not enter the market.

Can I get rid of red tide if I cook the shellfish?

No. The toxin in red tide is heat-stable. Cooking will not destroy it.

What are the symptoms of red tide?

Red tide causes Paralytic Shellfish Poisoning (PSP). Symptoms include tingling of the mouth and extremities, headache and nausea, and difficulty breathing. The onset of symptoms is rapid, usually within two hours of consumption. Symptoms include tingling, burning, numbness, drowsiness, incoherent speech, and respiratory paralysis, and can last a few days in non-lethal cases. Severe cases can result in death by respiratory arrest within 24 hours of consumption. There is no antidote for PSP, but supportive therapy and treatment is usually adequate, and survivors typically make a full recovery.

What should I do if I think I might have red tide (PSP) poisoning?

Seek medical help immediately. There are no antidotes or treatments for red tide (PSP) poisoning, and a serious case would require being placed on a ventilator in the hospital until the poison has cleared your system.

Can I get sick if I swim in the water where there might be red tide?

No. Red tide will make you sick only if you eat shellfish that are contaminated.

I’ve heard that they close the beaches in Florida during red tide, and people can have breathing problems. Does this ever happen here?

No. The toxic "red tide" blooms in Florida are caused by a different phytoplankton, one that does not grow in New England waters.

Can the color of the water determine if the water is toxic to shellfish?

No. There are totally harmless blooms that may color the water a deep red, AND the toxic phytoplankton that causes red tide may bloom in large numbers in an area, but not change the color of the water at all. This is why scientists prefer the term “harmful algal blooms” (or HABs) instead of red tide, as a more appropriate term for toxic blooms.

Is red tide caused by pollution?

No. See "What causes Red Tide," above.

How do I know if there is red tide in an area?

New Hampshire: NH Fish and Game sends a news release to statewide media, and updates the Clam Flat Hotline at 1-800-43-CLAMS

Maine: The Department of Marine Resources (DMR) sends out a notice to every town office in an area that has red tide. You can also call the Shellfish Sanitation Hotline – 1-800-232-4733, or you can check the DMR website for all current closures.

Have people gotten sick from red tide?

In 2007 and 2008, a total of seven people in Maine were sickened and hospitalized when they brought home mussels that were growing in an area that had been closed due to red tide. The people either did not check to see that there was a red tide closure in that area, or they ignored the warnings. Everyone recovered, but several of them spent nearly a week in the hospital.

Should I be worried about buying shellfish during a red tide event?

No. The NH Department of Environmental Services, the Maine Department of Marine Resources and the Massachusetts Division of Marine Fisheries have extensive monitoring programs. If you buy your shellfish from a reputable dealer, or harvest your own shellfish from an area that has not been closed to harvest, then you can enjoy all the shellfish you can eat!

What kinds of shellfish are affected by PSP?

Bivalve shellfish such as clams, oysters and mussels can accumulate the toxin in their digestive system, because as "filter feeders" they filter microscopic food from the water. Whole scallops can also be toxic, but because the adductor muscle is typically the only part consumed, they are not considered a public health threat. Whelks and moon snails feeding on contaminated shellfish can be toxic.

What about lobsters, crabs and shrimp - are they affected by PSP?

Lobsters, crabs and shrimp are not filter-feeders - they're crustaceans, and therefore do not accumulate the toxin in their meat, so as a general rule they are not affected by red tide. The one exception is that lobster tomalley can contain the PSP toxin, as well as other pollutants. The tomalley is the "green stuff" found in the lobster's body cavity; it acts something like a liver and filters toxins out from the animal's system. So, lobster meat is fine to eat, but lobster tomalley should NOT be eaten.

What does the NH Department of Environmental Services do to protect shellfish consumers?

The most effective way to prevent PSP illnesses is by large-scale monitoring programs designed to assess toxin levels in mussels, oysters, and/or clams, and rapid closures of harvesting areas known or suspected to be toxic. PSP monitoring in New Hampshire has been ongoing for several years, typically consisting of weekly testing of blue mussels from Hampton/Seabrook Harbor and at the Isles of Shoals. Secondary sites and additional shellfish species are sampled as necessary. The testing occurs from April through the end of October, the period when the algae may be active. Data-sharing with Maine and Massachusetts has been an integral part of ensuring an adequate, large-scale monitoring program. Because they accumulate PSP toxin so rapidly, blue mussels are the primary species used in DES's monitoring program.

How often are New Hampshire waters closed due to red tide?

New Hampshire shellfish waters are closed for harvesting when the PSP toxin levels in blue mussels reach the regulatory threshold of 80 micrograms of toxin per 100 grams of mussel tissue. Because PSP toxin levels can change very rapidly over the course of just a few days, waters may be closed for lower, but rising, amounts of toxin. High levels of toxin and harvesting closures do not occur every year, but they do occur. One of the worst outbreaks of PSP in New England occurred in the spring/summer of 2005, causing widespread closures of recreational and commercial shellfish areas in Maine, New Hampshire and Massachusetts. Record levels of toxicity were measured at several New Hampshire monitoring stations, and many areas were closed for harvesting for most of May, June and July. A seven-month closure on the harvest of Atlantic surf clams was necessary because this species tends to retain the PSP toxin for a long period of time, often months after the algae bloom has subsided.