Site Selection. Prior to the initiation of this study, scientists at USGS had compiled side scan sonar images from much of Georges Bank. Since marks produced by commercial trawlers were easily observed in these images, it was possible to use the side scan sonar data to classify areas as either “disturbed” or “undisturbed” by bottom fishing. Sediment and bathymetry maps of Georges Bank were also consulted in order to choose study sites that were located at similar depths (i.e., 45-90 m) and possessed a similar sediment composition. Based on these factors, six sites were initially selected in 1994. In later years, additional sites were added to the sampling regime.

 

Naturalist Dredge Samples. The Naturalist dredge is primarily used to obtain samples of megafaunal species. During annual research cruises, a 1-m wide dredge is used to collect replicate samples from haphazardly selected locations within each study site. The Naturalist dredge is typically towed along the seafloor for 30-45 s (i.e., approximately 10-60 m distance) in order avoid overfilling it and losing part of the sample. All megafauna from the dredge samples are picked out by hand and immediately placed in a container of seawater. The samples are then preserved in a mixture of 10% buffered formalin and seawater. A subsample is sieved through a 5-mm screen in order to check for any animals missed during the initial sorting process. Upon returning to the laboratory, the organisms in the samples are sorted, identified to the species level, counted, and weighed. The abundance and biomass of organisms in each sample are standardized by the volume of sediment collected during the tow.

 

Bottom Photographs and Videos. Bottom photographs and videos are used to estimate the percentage of the seafloor on Georges Bank that is covered by benthic megafauna. Within each study site, three 15-minute long video transects are conducted annually at stations that differ slightly from where the Naturalist dredge samples are collected. While ROVs and submersibles have also been used to film these videos, the Seabed Observation and Sampling System (SEABOSS) has proven to be the most effective device for recording underwater videos. The SEABOSS consists of a tethered grab sampler that has been equipped with one video recorder and two photographic cameras. While the SEABOSS is filming video transects, it concurrently takes photographs of the seafloor at 30-60 s intervals. Downward pointing twin lasers have also been mounted on the SEABOSS. The distance between these laser points in the bottom photographs is indicative of the camera's distance from the seafloor. Generally, photographs are taken when the camera was approximately 1 m above the sea bottom.

 

Beam Trawl Samples. A 2-m beam trawl was deployed during the 2003 sampling season in order to target juvenile fish, which are infrequently caught by the Naturalist dredge. The beam trawl was towed for a 10-minute duration at a speed of 3 knots (i.e., approximately 900 m distance). Larger organisms caught by the beam trawl were identified, counted, and weighed in the field, while smaller organisms were preserved in formalin and subsequently examined in the laboratory.