SCIENCE: Invasive sea squirts persist on Georges Bank
EcoPolis
Invasive sea squirts persist on Georges Bank
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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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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
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(Source: United States Geological Survey)