604 



Fishery Bulletin 101(3) 



40- 



35- 



30- 



North 

 Atlantic 

 Ocean 



-70 



Figure 1 



Survey effort (4163 km; thin lines) in Beaufort sea state s4 

 in the southern U.S. Atlantic study area (outlined by thick 

 line) during summer 1998. Blank areas indicate Beaufort sea 

 states >4 that were not included in the survey effort. The 200-, 

 500-, 1000-, 2000-, and 3000-m isobaths are shown. 



Methods 



Study area and survey design 



The study area (573,000 km^) was North Atlantic Ocean 

 waters between central Florida (28.00°N) and Maryland 

 (38.00°N) from the 10-m isobath to the boundary of the 

 U.S. EEZ, generally 371 km (200 nmi) from the nearest 

 U.S. point of land (Fig. 1). The study area has a diverse 

 bottom topography and includes a very narrow continental 

 shelf (<200 m) at Cape Hatteras which broadens to form 

 the mid-Atlantic Bight to the north and the Florida-Hat- 

 teras Shelf to the south. Beyond the shelf, south of Cape 

 Hatteras are found the following features: the Florida-Hat- 

 teras Slope, the Blake Plateau (700-1000 m deep), and the 

 Blake Escarpment. North of the Blake Plateau, the conti- 

 nental slope from 200-2000 m deep is steep and most of 

 the study area has water depths >2000 m. The Gulf Stream 

 is the dominant oceanographic feature in the study area. 

 From the south, the Gulf Stream Front generally follows 

 the upper continental slope northward to Cape Hatteras, 

 where it flows to the northeast. Seaward of the Gulf Stream 

 are Sargasso Sea waters. North ol' Cape Hatteras and the 

 Gulf Stream Front, cooler waters, which largely originate 

 in the Labrador Sea, drift into the study area from the 

 north and northeast. 



Transects covered the study area uniformly in a saw- 

 tooth pattern from a random start at the southernmost 

 inshore point and were surveyed from the 68-m NOAA ship 

 Relentless (renamed Gordon Gunter in 1999) between 8 Ju- 

 ly and 17 August 1998 from south to north, and from north 

 to south. Transects were placed to cross the bathymetry 

 gradient. The narrow band of U.S. waters between central 

 Florida and Key West, Florida, were partially surveyed but 

 were not included in the present report. 



Data collection 



Data were collected by two teams of three observers from 

 the ship's flying bridge, located 14.5 m above the surface of 

 the water, during daylight hours, weather permitting (i.e. 

 no rain, Beaufort sea state <6). Observers used standard 

 line-transect survey methods for cetaceans that were simi- 

 lar to those used from ships in the Pacific Ocean and Gulf 

 of Mexico (e.g.. Barlow, 1995; Hansen et al.'*). Each team 

 had at least two members experienced in shipboard line- 

 transect methods and in the identification of tropical and 

 temperate cetaceans. Two observers searched for cetaceans 

 using 25x binoculars and another observer searched using 

 unaided eye or 7x hand-held binoculars and recorded data. 

 These three observers constituted the "primary team." From 

 18 July to 17 August, a fourth observer was added to one 

 team to act as a conditionally independent observer (CIO, 

 see below). The area from 90° left and right of the ship's 

 bow to the horizon was searched by the primary team. 

 Observers changed position (including the CIO position) 

 every 30^0 minutes, and each team alternated two-hour 

 watches throughout daylight hours. The survey speed was 

 usually 18 km/li but varied with sea conditions. 



Data were recorded on a computer interfaced with a glob- 

 al positioning system (GPS) by a data acquisition program. 

 Data collected for each cetacean sighting included time, 

 position, bearing, and reticle (a measure of radial distance) 

 of the sighting, species, group-size, behavior, bottom depth, 

 sea surface temperature, and associated animals (e.g. sea- 

 birds, fish). The bearing and radial distance for sightings 

 that were close to the ship were estimated. Survey effort 

 data were automatically recorded every two minutes and 

 included position, heading, effort status, observer position, 

 and environmental conditions that could affect the observ- 

 ers' ability to sight animals (e.g. Beaufort sea state, position 

 of the sun). 



Typically, if a sighting was within a 5.5-km strip on ei- 

 ther side of the ship, the ship was diverted from the tran- 

 sect line and approached the group so that observers could 

 identify species and obtain group-size estimates. For each 

 sighting, the final group-size was estimated by a consensus 



■* Hansen, L. J., K. D. Mullin, T. A. Jefferson, and G. P. 

 Scott. 1996. Visual surveys aboard ships and aircraft. In 

 Distribution and abundance of marine mammals in the north- 

 central and western (lulf of Mexico: final report; vol, II: technical 

 report (R.W. Davis and G. S. Fargion. eds.), p. 5.5-132. Outer 

 Contnu-ntal Shelf (OCS) Study MMS 96-0027. U.S. Dep. Inte- 

 rior, Minerals Mgnit. Service, Gulf of Mexico OCS Region, New 

 Orleans, LA. 



