During the last 2 weeks of August, scientists from the U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), the University of Rhode Island (URI), and the Department of Fisheries and Oceans Canada spent 10 days on the Georges Bank fishing grounds documenting the occurrence and effects on gravel habitats of the invasive colonial sea squirt Didemnum sp. For the fourth consecutive year, researchers surveyed two areas on Georges Bank where the sea squirt continues to thrive. The colonies are denser than in 2005 over the 88-mi2 area observed, but scientists found no colonies in nearby Canadian waters, indicating that they have not spread eastward.
As in previous years, scientists conducted the annual survey from the NOAA ship Delaware II, out of NOAA's Northeast Fisheries Science Center in Woods Hole, Mass. On the morning of departure (August 22), Senator Barbara Mikulski (D-MD) visited the center, where she inspected the Delaware II and received a briefing on cruise objectives from scientists who showed her the sampling and camera systems to be used on the cruise. In addition, she was shown living examples of the invasive sea squirt overgrowing blue mussels collected by Dann Blackwood and Jennifer Bonin (both of USGS) from dock pilings in Woods Hole.
This year's survey included video transects as much as 0.8 mi long using the USGS Seabed Observation and Sampling System (SEABOSS; see Seabed Observation and Sampling System). Preliminary evaluation of the images indicates that the gravel is 50 to 75 percent covered at some study sites, a marked increase over last year.
"On Georges Bank, the area of seabed covered by the colonies has doubled at 75 percent of the sites we observed in both 2005 and 2006," said Page Valentine of the USGS Woods Hole Science Center, who tracks occurrences of the species off the Northeastern United States and elsewhere in the world. The 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 sea 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.
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 such surfaces 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-size individuals to form circular mats, as much as 1 ft 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. The 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.
Jeremy Collie, a URI biologist, has been studying benthic communities in the Georges Bank area since before the sea squirts arrived, and he is monitoring the effects the tunicates are having on the benthos. "We haven't seen any dramatic changes yet, but as the percentage of the area covered by the tunicate gets higher and higher, it's going to seal off the sea floor. That's when we expect to see significant effects," he said.
Dawn Sephton, a biologist from the Department of Fisheries and Oceans Canada, Maritimes Region, was also part of the scientific team this year because the study included Canadian waters. Sephton currently leads a project to detect and monitor invasive tunicate 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.
For high-resolution images and more information on Didemnum worldwide, visit Marine Nuisance Species, Didemnum, a colonial tunicate.
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