580 



Fishery Bulletin 104(4) 



SO-N - 



28"N - 



26-'N 



24"N 



46'VV 



44 W 



92°W 



mi W 



ssw 



Sd'W 



84 W 



Figure 1 



Transect lines surveyed by aircraft during three aerial studies conducted in the northern Gulf of Mexico 

 from 1989 to 1998. Note that some lines overlap others, particularly in the eastern Gulf of Mexico. 

 Bathymetric contours represent the 100-. 500-, 1000- and 2000-meter contours. 



tive seasonal abundance, distribution, and aggregations 

 of whale sharks observed in the northern Gulf of Mexico 

 during three spatially and temporally intensive aerial 

 surveys conducted by the National Marine Fisheries 

 Service, Mississippi Laboratories. 



Materials and methods 



Between 1989 and 1998, three aerial surveys were con- 

 ducted in the northern Gulf of Mexico with a DeHavil- 

 land Twin Otter turbine engine aircraft. The primary 

 objective of one of these surveys, referred to as the Upper 

 Continental Slope (UCS) Survey, was to examine species 

 composition, distribution, and seasonality of cetaceans 

 in the north-central Gulf of Mexico (Mullin et al., 1994). 

 The purpose of the other two surveys, referred to as 

 the Gulf of Mexico Cetacean Studies I (GulfCet I) and 

 Gulf of Mexico Cetacean Studies II (GulfCet II) (Mullin 

 and Hoggard-) surveys, was to assess possible impacts 

 of petroleum industry activities on cetaceans and sea 

 turtles in the Gulf of Mexico. Standard line-transect 

 sampling methods were used for each survey (Buckland 

 et al., 1993). Surveys were flown at a constant altitude 

 and air speed of 229 m and 200 km/h, respectively. 

 The aircraft was modified with a Plexiglas observation 

 bubble on the port and starboard sides of the fuselage to 

 permit an unobstructed view of the area along transect 

 lines and a lateral view to each horizon. When fauna 



Mullin, K. D., and W. Hoggard. 2000. Visual surveys of 

 cetaceans and sea turtles from aircraft and ships. In OCS 

 Study MMS 96-0027, vol. II, Cetaceans, sea turtles and sea- 

 birds in the northern Gulf of Mexico: distribution, abundance 

 and habitat associations (R. W. Davis, W. E. Evans, and D. 

 Wursig, eds.), p. 111-172. Minerals Management Service, 

 Gulf of Mexico OCS Region, New Orleans, LA 70123. 



of interest were sighted, the pilot circled the area until 

 all observed fauna were identified to the lowest pos- 

 sible taxa and the latitude and longitude of the sighting 

 and numbers of conspecifics sighted were recorded. All 

 survey effort was limited to waters associated with the 

 continental slope in Beaufort sea states less than 4 and 

 restricted to areas within the United States Exclusive 

 Economic Zone (EEZ; Fig. 1). For all analyses and dis- 

 cussion, seasons were defined as spring (April-June), 

 summer (July-September), fall (October-December), 

 and winter (January-March). 



The UCS survey was conducted from the summer 

 of 1989 through the spring of 1990 and was carried 

 out during all four seasons. The study area was lo- 

 cated in the central northern Gulf of Mexico along the 

 continental shelf break (~200-m isobath) south of the 

 Mississippi River Delta and extended from the DeSoto 

 Canyon (87°00'W) to the western edge of the Missis- 

 sippi Trough (90°05'W). GulfCet I surveys occurred 

 from 1992 to 1994 over continental slope waters in the 

 western Gulf of Mexico between the United States and 

 Mexico border (25°57'N) and the Mississippi-Alabama 

 border (88°25'W) and were conducted during all four 

 seasons (Mullin et al., 2004). GulfCet I transects began 

 at the 100-m isobath and extended to the 1000-m iso- 

 bath west of 90°00'W, and to the 2000-m isobath east of 

 90°00'W. GulfCet II surveys were conducted only during 

 the winter and summer between 1996 and 1998 in the 

 northeastern Gulf of Mexico and covered continental 

 slope waters (100-2000 m deep) and a portion of the 

 continental shelf (Mullin and Hoggard-). The entire 

 spatial range of the three areas was surveyed within 

 each season of operations. Because of the spatial overlap 

 of the UCS and GulfCet II surveys, the central northern 

 Gulf of Mexico and the western Gulf of Mexico were 

 treated as a single region, referred to as "the western 



