Using a towed optical habitat mapping system to monitor the invasive tunicate species Didemnum sp. along the northeast continental shelf

The invasive tunicate Didemnum sp. is a colonial ascidian of unknown origin that has spread down the Atlantic Coast since its appearance in Maine during the 1980s. Mats of this tunicate were found to overgrow gravel, pebble, cobble, boulder, live and dead sea scallops, anemones, sponges, dead shells, other ascidians, barnacles, and rock crabs, and skate egg cases. 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 Didemnum sp. 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 Didemnum sp. 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 Didemnum sp. growth under disturbed and undisturbed conditions. Areas of Didemnum sp. growth within Closed Area II were denser than areas of Didemnum sp. growth outside of Closed Area II based on measurements of percent cover of Didemnum sp. on the bottom. Increasing density of Didemnum sp. 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. Didemnum sp. was also found to be limited to substrates containing gravel or more complex structure. A massive die-back of the sites in Closed Area II occurred between cruises in August of 2007 and early May of 2008, a period during which bottom ranged between 2.8 to 18.5degC. This suggests that Didemnum sp. mats on Georges Bank exhibit the same seasonal fluctuations in density regulated by temperature as seen in coastal Didemnum sp. populations. However, continued year-round sampling is needed to characterize offshore seasonality in detail.