The scallop survey uses a random stratified design, in which the ocean is divided into zones, or strata, of similar depth and habitat, and then dredge and camera samples are taken randomly within these zones. This is similar to the sampling design used in political polls to achieve a statistically accurate assessment of political views for all U.S.citizens.
Samples are collected using a modified commercial eight-foot sea scallop dredge with a mesh liner to retain the very small seed scallops. At each sampling station, the gear is deployed on the bottom for 15 minutes at an average tow speed of 3.8 knots (just over four miles per hour). Each station covers an area about 4,500 square meters, or about one acre. After each haul, the catch is sorted, counted and measured on deck. Some additional stations are included in the survey to monitor annual recruitment and growth rates. HabCam was deployed at many of the same locations, taking high resolution digital still images of the bottom at a rate of more than one per second. The images can then be formed into a mosaic.
In 2007 and 2008, the HabCam survey images also revealed the occurrence and distribution of the invasive tunicate species Didemnum, first reported on Georges Bank in 2003. Didemnum smothers many bottom-dwelling organisms, and its acidic "skin" can prevent baby scallops from settling on the ocean floor.
"We saw it in patches in only one area we surveyed, but it is a real concern," Hart said. "Physical samples came up on one of the dredges, so biologists will have some specimens to study."
Atlantic sea scallops (Placopecten magellanicus) are distributed in the Northwest Atlantic from Cape Hatteras to Newfoundland and also occur in the Gulf of St. Lawrence. They live on the oc
|Contact: Shelley Dawicki|
NOAA National Marine Fisheries Service