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Fishery Bulletin 101(4) 



i 28- 



Figure 1 



Survey effort in Beaufort sea state of <3 (dark lines), east (1342 km) and west (2202 km) of Mobile Bay, 

 Alabama (bgld vertical line), in the northern U.S. Gulf of Mexico outer continental shelf (20-200 m) during 

 fall 1998-2001. The 20- and 200-m isobaths (thin lines) are shown. 



Methods 



Study area 



The study area (245,800 km^) included continental shelf 

 waters of the U.S. GOM between the U.S-Mexico border and 

 Key West, Florida, between the 20- and 200-m isobaths ( Fig. 

 1). However, survey effort did not extend south of 26.0°N in 

 the southeastern GOM and therefore abundance estimates 

 were extrapolated for this region. The shelf is wide (up to 

 200 km) off the Florida peninsula and off northern Texas 

 and Louisiana, and narrower off the Florida Panhandle 

 near DeSoto Canyon, the Mississippi River Delta, and 

 southern Texas. The continental slope is a steep escarp- 

 ment from 1000 to 2000 m in the eastern GOM. 



Survey design and data collection 



Surveys were conducted from the 68-m NOAA Ship Gordon 

 Gunter (1998, 1999, and 2001) and the 52-m NOAA Ship 

 Oregon II (2000). The four surveys ranged from 28 to 32 

 days between 28 August and 2 October and were divided 

 into two legs of 12 to 19 days. Standard ship-based, line- 

 transect survey methods for cetaceans, similar to those 

 used in the Pacific Ocean and U.S. GOM, were used (e.g. 

 Barlow, 1995; Hansen et al.'*). Surveys were conducted 

 24 hours a day along a predetermined trackline between 

 plankton stations uniformly spaced 30 nmi apart. The 

 trackline uniformly covered the shelf waters roughly 

 10-200 m deep in 1998-2001 (Fig. 1). 



■• Hansen, L. J., K. D. MuUin, T. A. Jefferson, and G. P. Scott. 

 1996. Visual surveys aboard ships and aircraft. In Distribu- 

 tion and abundance of marine mammals in the north-central 

 and western Gulf of Mexico: Final report. Volume II: Techni- 

 cal report (R.W. Davis and G.S. Fargion, eds.), p. 55-132. OCS 

 Study MMS 96-0027. Minerals Management Service, Gulf of 

 Mexico OCS Region, New Orleans, LA. 70123. 



Data were collected by two teams of three observer — one 

 team positioned on the flying bridge 14.5 m above the wa- 

 terline (Gunter) and the other team positioned 9.2 m above 

 the waterline (Oregon II) during daylight hours while the 

 vessels moved between plankton stations, weather permit- 

 ting (i.e. no rain, Beaufort sea state <6). Each team had at 

 least two members experienced in ship-based, line-transect 

 methods and in identification of tropical cetaceans. The 

 left- and right-side observers searched to the horizon in the 

 arc from 10° right and left of the ship's bow to the left and 

 right beams (90°), respectively, using 25x binoculars. The 

 third observer searched, using unaided eye or 7x hand-held 

 binoculars, and recorded data. Observers changed position 

 every 30-40 minutes, and the two teams alternated 2-h 

 watches throughout daylight hours. Survey speed was usu- 

 ally 18 km/h (-10 knots) but varied with sea conditions. 



Data were recorded on a computer interfaced with a glob- 

 al positioning system (GPS) by an in-house BASIC data 

 acquisition program (Southeast Fisheries Science Center, 

 NMFS, Pascagoula, MS). For each cetacean sighting, 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) were recorded. The bearing and radial distance 

 for groups sighted without 25x binoculars and close to the 

 ship were estimated. Survey effort data were automatically 

 recorded every 2 minutes and included the ship's position 

 and direction, effort status, observer positions, and envi- 

 ronmental conditions that could affect the observers' abil- 

 ity to sight animals (e.g. Beaufort sea state, sun position). 

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

 side of the ship, the ship was diverted from the trackline 

 to approach the group to allow the observers to identify 

 species and estimate group-size by consensus. 



Cetaceans were identified to the lowest taxonomic 

 level possible. Observers' ability to make identifications 

 depended on weather and animal behavior. Differences 

 between T. truncatus and S. frontalis could not always be 



