Scientists from the U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), and the University of Rhode Island estimate that mats made of thousands of individual squirts infest a 40 square mile area of seabed that is highly productive for fish and sea scallops. In large parts of the affected area, the sea squirts cover 50 percent or more of the seabed. The Georges Bank infestation is unique, the only known occurrence of this magnitude in a major offshore fishing ground.

Sea squirts are tunicates, a type of sea life with a primitive spinal cord and a firm, flexible outer covering called a "tunic," from which the name derives. A filter-feeding species of the genus Didemnum, they form dense mats, made of thousands of individuals, by attaching to firm substrates such as gravel, sea scallops, mussels, docks and other structures, and even seaweed. Tunicates can overgrow sea scallops and mussels, and they may affect other species of clams and worms that live in the seabed below the tunicate colony. An "invasive species" is one that is not native to an ecosystem, and that may harm that ecosystem if introduced.

In the fall of 2003, the same research team first spotted this infestation over roughly 6 square miles of ocean bottom during a scientific cruise to study habitats on Georges Bank. This year, the team surveyed a larger area with tunicate coverage.

The scientists observed mats over at least 40 square miles of the gravel substrate on the northern edge of Georges Bank. Video and photo transects using the USGS seabed observation and sampling system (SEABOSS) documented the distribution of the tunicate colonies in water depths of
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Contact: Ellen Mecray
emecray@usgs.gov
508-457-2213
United States Geological Survey
19-Nov-2004