A Towed Optical Habitat Mapping System Monitors the Invasive Tunicate Species Didemnum vexillum along the U.S. Northeast Continental Shelf

The invasive tunicate Didemnum vexillum is a colonial ascidian of unknown origin that has spread down the Atlantic Coast since its appearance in Maine during the 1980s.  Optical image surveys conducted with HabCam (Habitat Mapping Camera System) average about 100 nautical miles a day of continuous bottom imagery and have to date revealed that three regions of Georges Bank contain areas of D. vexillum growth: the Northern Edge (Closed Area II), south of Nantucket Shoals (Nantucket Lightship Closed Area) and Great South Channel (Closed Area I), the latter two regions being previously not known to contain D. vexillum.  The population along the Northern Edge spans from outside to inside of an area closed to ground fish and scallop fishing thereby providing a site to study the differences in D. vexillum growth under disturbed and undisturbed conditions.  Areas of D. vexillum growth within Closed Area II were denser (>95% cover) than areas outside of Closed Area II based on measurements of percent cover of D. vexillum on the bottom.  Increasing density of D. vexillum was correlated with a decrease in number per m² of sea scallops (Placopecten magellanicus), lacy tube worms (Filograna implexa), northern seastar (Asterias vulgaris), mussels, crabs, barnacles, and siphons of infaunal organisms.  D. vexillum was also found to be limited to substrates containing gravel or more complex structure.  Mats of this tunicate were found to overgrow live and dead sea scallops, anemones, sponges, dead shells, other ascidians, barnacles, rock crabs, and skate egg cases.  Massive die-backs of D. vexillum occurred in Closed Area II between cruises in August of 2007 and early May of 2008, and again between cruises conducted in June 2009 and May 2010. Little or no die-back occurred between cruises conducted in November 2008 and April 2009 suggesting that although a seasonal cycle is evident in some years this is not necessarily consistent. Correlation with environmental variables has yet to produce a strong relationship with seasonality of D. vexillum abundance. Continued year-round measurements of hydrography and near-bottom currents are needed to characterize offshore seasonality in detail.