Mullin et al.: Cetaceans of the north-central Gulf of Mexico 



775 



if any, inhabited the upper continental slope in the 

 north-central Gulf. Since studies elsewhere indicated 

 that cetaceans may concentrate in areas of high sea- 

 floor relief (Hui, 1979; Payne et al., 1986; Kenney 

 and Winn, 1986; Selzer and Payne, 1988), three sur- 

 vey blocks were initially selected on the upper conti- 

 nental slope (Fig. 1, Blocks A1-A3; Table 1): 1) the 

 Upper Mississippi Fan, 2) the Mississippi Canyon, 

 and 3) an area of submarine salt domes. The DeSoto 

 Canyon survey block (Block A4) was added in Sep- 

 tember when additional flight time was available. 



The Mississippi Canyon survey block was shifted 

 southeast to include deeper waters for the October 

 and November surveys. This was done because sperm 

 whales, Physeter macrocephalus, were sighted near 

 the 1,000-m isobath during September in the Upper 

 Mississippi Fan survey block. The sperm whale is 

 listed as an endangered species under the U.S. En- 

 dangered Species Act (USFWS, 1989), and we were 

 interested in denning its distribution. 



The results of Phase 1 indicated that a variety of 

 cetaceans were relatively abundant on the upper con- 

 tinental slope; ten species and 171 herds were 

 sighted. Therefore, Phase 2 was implemented and 

 monthly surveys, except for December, were contin- 

 ued to complete a full year period. 



Phase 2 was conducted from January through June 

 1990. The study area selected for Phase 2 consisted 

 of seven adjacent blocks 30 minutes wide (48.7 km) 

 that extended from long. 87.5'W to long. 90.5'W (Fig. 



1, Blocks B1-B7). The northern border of the study 

 area, except near the Mississippi Canyon, generally 

 followed the 200-m isobath. Each block extended 44.1 

 km south of its northern border. 



The Mississippi Canyon was not surveyed because 

 we wanted to focus on oceanic waters. Because of 

 the shape of the canyon, surveys of oceanic waters 

 would have been logistically inefficient. We also be- 

 lieved that the results of Phase 1 established the 

 canyon as important cetacean habitat; eight species 

 were identified in 54 sightings. 



Aerial surveys were conducted when the sea state 

 was Beaufort 0—4 and visibility was good and were 

 designed to sample blocks A1-A4 at least twice each 

 month during Phase 1, and blocks B1-B7 twice each 

 month during Phase 2. Line transect sampling meth- 

 ods were used (Buckland et al., 1993) although line 

 transect analyses are not presented here. 



The survey aircraft, a DeHavilland Twin Otter with 

 a large plexiglass bubble window on each side that 

 allowed observers to view an area on both side of the 

 transect line, was flown at an altitude of 750 feet 

 (229 m) and a speed of 110 knots (204 km/h). 

 Transects that were uniformly spaced from a ran- 

 dom starting point were surveyed in each block (Table 

 1). Transects ran north-south, perpendicular to the 

 bathymetry. One observer was stationed at each 

 bubble window and one at a computer station. Ob- 

 servers rotated every 30 minutes to avoid fatigue. 

 The bubble windows were divided into seven 10' 



Table 1 



Summary of area, water depth, transect length, number of transects, and effort per month for each survey block in 

 the Gulf of Mexico. 



Transect kilometers of effort per month and block 



Range of Transect Number 

 Area water length of 



Block (km 2 ) depths (m) (km) transects' 



1989 



1990 



Jul Aug Sep Oct Nov Jan Feb Mar Apr May Jun Total 



934 677 956 634 920 — — — — — — 4,121 



440 447 396 — — — — — — — — 1,283 



— — 499 167 — — — — — — 666 



394 489 412 164 — — — — — — 1,459 



— — 176 356 535 — — — — — — 1,067 



179 362 362 269 265 1,437 



178 178 223 178 352 717 1,826 



— — — — 357 176 631 360 445 449 2,418 



231 355 357 170 544 1,657 



— — — — — 128 358 360 361 361 1,568 



— — — 312 178 273 358 357 1,478 



356 180 361 354 362 1,613 



1,768 1,613 1,940 1,489 1,786 1,562 533 2,287 2,251 2,309 3,055 20,593 



Total 



' Number of planned transects each time the block was surveyed. 



