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New Hampshire Department of Environmental Services

FAQs – Red Tide
  • What is Red Tide?
    Microscopic algae are single-celled plants that live in the sea. Most species of algae or phytoplankton are beneficial to the ecology of the ocean, serving as the base of the food chain, which supports higher forms of life such as fish, birds, marine mammals, and humans.
    Occasionally, the algae grow very fast or "bloom" and accumulate into dense, visible patches near the surface of the water. Some species of phytoplankton contain reddish pigments, and an intense bloom of these species can make the water appear to be colored red. The term "Red Tide" is a common name for such a phenomenon, although the term is really a misnomer because such events are not associated with tides. Many of the phytoplankton that can discolor the water are not harmful, and those species that are harmful may never reach the densities required to discolor the water.
    Although most phytoplankton species are not harmful, a small number of species produce potent neurotoxins. The toxins can be transferred through the food chain, where they affect and even kill the higher forms of life such as zooplankton, shellfish, fish, birds, marine mammals, and even humans that feed either directly or indirectly on them.
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  • What Causes Red Tide?
    Although there are several species of phytoplankton that can cause a variety of illnesses, the main illness of concern in NH waters is Paralytic Shellfish Poisoning, or PSP. Various species of the phytoplankton Alexandrium are the chief causative agents of PSP. Alexandrium cysts spend the winter in offshore ocean sediments in a dormant state. In response to increased sunlight and temperature in the spring, the cysts germinate into free-swimming cells, which reproduce by cell division. If water conditions are good, the cells continue to divide, and growth is exponential - a single cell can result in the reproduction of several hundred cells in just a few weeks. If other single cells are reproducing in a similar manner, this large "bloom" can result in shellfish toxicity.
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  • What are the symptoms of Red Tide poisoning?
    Paralytic Shellfish Poisoning, or PSP, is a life-threatening syndrome, and the onset of symptoms is rapid, usually within two hours of consumption. Symptoms include tingling, burning, numbness, drowsiness, incoherent speech, and respiratory paralysis. Duration of effects is a few days in non-lethal cases. The most severe cases result in respiratory arrest within 24 hours of consumption of the toxic shellfish. If the patient is not breathing or if a pulse is not detected, artificial respiration and CPR may be needed as first aid. Although there is no antidote, supportive therapy and treatment is usually adequate, and survivors recover fully. PSP is prevented by large-scale proactive monitoring programs (assessing toxin levels in filter feeding mollusks such as mussels, oysters, clams, and/or scallops) and rapid closures to harvest of suspect or demonstrated toxic areas.
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  • How is Red Tide Monitoring Conducted in NH?
    PSP monitoring in New Hampshire has been ongoing for several years. The monitoring program has typically consisted of at least weekly collection and testing of blue mussels, which tend to accumulate the PSP toxin quicker than other species of shellfish, from Hampton/Seabrook Harbor. The weekly testing is performed on samples of mussels from April through the end of October, the period when the phytoplankton may be active. Data-sharing with neighboring states of Maine and Massachusetts has been an integral part of ensuring an adequate, large-scale monitoring program.
    In the year 2000, the NH Department of Environmental Services expanded PSP monitoring in offshore NH tidal waters. A new mussel monitoring site was been established on Star Island, Isles of Shoals. This site provides critical information on PSP levels in offshore waters, which is where the phytoplankton blooms tend to originate. These data are extremely useful to not only help anticipate the occurrence of dangerous PSP levels in nearshore areas, but also to evaluate the suitability of offshore waters for activities such as aquaculture.
    To further expand our knowledge of the incidence of PSP and other Harmful Algal Blooms, volunteers from the Great Bay/Coast Watch began a phytoplankton monitoring program in 1999. Although the numbers of particular types of phytoplankton in the water are not necessarily indicative of the presence of toxin in shellfish, the data produced by the volunteers can be useful in terms of providing an early warning that high toxin levels in shellfish may occur in the near future.
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  • How often are NH waters closed due to Red Tide?
    NH shellfish waters are closed for harvesting when the PSP toxin levels in blue mussels reach 80 micrograms of poison per 100 grams of mussel tissue. However, PSP toxin levels can change very rapidly over the course of just a few days. Therefore, waters may be closed for lower amounts of toxin, especially when NH stations are showing rising levels of toxin, and nearby stations in southern Maine or northern Massachusetts are showing levels higher than the 80 microgram limit.
    High levels of toxin and resultant closures to shellfishing have occurred with greater frequency in recent years. After a period of generally low toxicity and few closures from 1994 through 2003, high toxin levels have been observed each year at Star Island since 2004, with the highest toxin levels recorded during the historic red tide closures in the summer of 2005. Offshore closures have typically begun in May and have remained in place until mid-to-late summer. High toxin levels have also been more frequent in nearshore areas along the coast, although toxicity levels have been lower, with shorter harvesting closures required.
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NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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