U.S. Department of the Interior
U.S. Geological Survey
News Release
Sept.20, 2006 Diane Noserale, USGS 703-648-4333
dnoseral@xxxxxxxx
Teri Frady, NOAA 508-495-2239
teri.frady@xxxxxxxx
Todd McLeish, URI 401-874-7892
tmcleish@xxxxxxx
Invasive Sea Squirts Persist on Georges Bank
For high resolution images and more information on Didemnum worldwide,
see the USGS website
http://woodshole.er.usgs.gov/project-pages/stellwagen/didemnum/index.htm
Woods Hole, Mass.— For the fourth consecutive year, federal and university
researchers have surveyed two areas on Georges Bank where an invasive
colonial sea squirt continues to thrive on the gravel bottom. The colonies
are denser than in 2005 over the 88 square-mile area observed. But
scientists found no colonies in nearby Canadian waters, indicating they
have not spread eastward. The Georges Bank squirts are a species of the
genus Didemnum.
“The area of seabed covered by the colonies has doubled at 75 percent of
the sites we observed in both 2005 and 2006,” said Dr. Page Valentine of
the U.S. Geological Survey, who tracks occurrences of the species off the
northeastern U.S., and elsewhere in the world. Greater density of colonies
observed during the survey is evidence that the infestation is persistent,
and not a short-lived phenomenon.
Robert Reid, a biologist with NOAA Fisheries Service and chief scientist
for the survey, agreed that the squirt appears to be proliferating in the
study area. “The fact that it is still there in high abundance over a
fairly large area certainly indicates this occurrence is not ephemeral,”
Reid said.
Scientists remain concerned that the infestation could threaten important
fisheries in the region. Sea squirt mats could prevent fish from feeding
on worms and crustaceans that live in and on the gravel bottom, reduce the
shelter required for these species to avoid predators, and limit the space
available for settlement of larvae of sea scallops and other species.
Didemnum is a nuisance to the aquaculture industry, overgrowing shellfish
in New England coastal waters.
Dr. Jeremy Collie, a biologist with the University of Rhode Island, has
been studying the benthic communities in the area since before the sea
squirts arrived, and he is monitoring the effects they are having on the
benthos. “We haven’t seen any dramatic changes yet, but as the percentage
of the area covered by sea squirts gets higher and higher, it’s going to
seal off the seafloor. That’s when we expect to see significant effects,”
he said.
As in prior years, scientists conducted the annual survey from the NOAA
Ship Delaware II. Returning researchers included Valentine and Reid, and
Collie. This year’s survey included video transects of up to 0.8 miles in
length using the USGS seabed observation and sampling system (SEABOSS).
Preliminary evaluation of the images show the gravel is 50 to 75 percent
covered at some study sites, a marked increase from last year.
Dawn Sephton, a biologist from the Department of Fisheries and Oceans
Canada, Maritimes Region, was also part of scientific team this year, since
the study included Canadian waters. Sephton currently leads a project to
detect and monitor invasive sea squirt species along the Bay of Fundy and
Nova Scotia coastlines. “While the absence of Didemnum at the Canadian
study sites is welcome news, we are concerned about its potential spread
and impact on fisheries and shellfish aquaculture in the Maritimes,”
Sephton said.
Sea squirts are also called tunicates, having a primitive spinal cord and
an outer sheath or “tunic,” from which the name derives. Tunicates spread
in several ways: by larvae that swim for only a few hours before settling;
by colonies that hitchhike onto surfaces such as boat hulls, moorings,
fishing gear, and other manmade objects and are carried to new, favorable
habitats; and by fragments of colonies that are broken up by human
activities and natural events and drift until they settle elsewhere. They
expand outward by budding new millimeter-sized individuals to form circular
mats up to a foot in diameter. The mats coalesce with neighboring colonies
to form a tough, barren layer of intergrown colonies that attach to hard
surfaces including gravel, wood, metal, and plastic. No other species is
known to eat or overgrow them.
Scientists first observed the Didemnum colonies in 2003, on the U.S. side
of the international maritime boundary separating U.S. and Canadian waters
of Georges Bank. Georges Bank is frequently fished by commercial vessels,
particularly sea scallopers and ground fishermen. The same or similar
species of Didemnum occur on the coasts of Europe, New England, California,
Washington, British Columbia, and New Zealand. So far, this is the only
occurrence reported in an offshore fishing ground.
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