Burks et a\ : Abundance and distribution of Rhincodon typus in the northern Gulf of Mexico 



583 



generating localized upwelling on the continental shelf 

 break, thus increasing primary productivity and the 

 availability of prey species (Sahl et al., 1993). However, 

 the Texas Current occasionally flows in a direction oppo- 

 site to its normal eastward course because of wind forc- 

 ing (Sahl et al., 1997). Such a reversal of direction in the 

 Texas Current could affect productivity in this area and 

 thus influence the seasonal abundance of whale sharks 

 in this region. Given the stochastic nature of the physical 

 oceanography of this region, additional research will be 

 needed to determine if whale sharks make limited use 

 of this area during discrete times of the year or if they 

 are in fact present year round. 



From observations made by commercial mariners 

 in transit between the southern tip of Florida and 

 unidentified ports in Texas, Gudger (1941) reported 

 sightings of 68 whale sharks from a circular area in 

 the north-central Gulf of Mexico with a diameter of ap- 

 proximately 280 km; however no specific locations were 

 given. He concluded that the density of whale sharks 

 in this area must be related to high prey densities. 

 The area of highest regional abundance during the 

 present study occurred in a circular area, bounded by 

 89°30'W and 91°00'W longitude, and having a diameter 

 of approximately 165 km (Fig. 2). Three aggregations 

 of between 30 and 100 whale sharks in this same area 

 were reported by Hoffmayer et al. (2005). The safety 

 fairway used by shipping traffic transiting between 

 the eastern and western Gulf of Mexico crosses this 

 area. It is probable, therefore, that the observations 

 of whale sharks by commercial mariners reported by 

 Gudger (1941) were in the same area. Because of nu- 

 trient loading from the Mississippi and Atchafalaya 

 rivers, primary productivity is higher in this region of 

 the Gulf of Mexico than in any other region (Lohrenz 

 et al., 1999) and may explain the high localized abun- 

 dance of whale sharks in this region. Other highly 

 productive areas where whale sharks are known be 

 present in relatively large numbers include Ningaloo 

 Reef, Australia, and Gladden Spit, Belize (Taylor, 1996; 

 Heyman et al., 2001). 



The sighting of 23 whale sharks on 9 August 1993 

 was the largest aggregation observed during our study. 

 The sharks, which were observed for about 35 minutes, 

 were distributed over an area of approximately 2.6 km'-. 

 Coral spawning occurred on 9 August 1993 at the East 

 Flower Gardens (27°55'N, 93°36'W; Gittings^) which is 

 approximately 33 km due west of the location of the ag- 

 gregation sighting. A similar relationship between coral 

 spawning events and increased localized abundance 

 of whale sharks has also been noted to occur off the 

 coast of Western Australia (Taylor, 1996). Future efforts 

 should examine the relationship between annual coral 

 spawning events and whale shark occurrence near the 

 East Flower Gardens to determine if there is an annual 

 migration of whale sharks to this area. 



^ Gittings, S. R. 1993. Personal commun. Science Program 

 Manager, NOAA Office of National Marine Sanctuaries. 

 1305 East West Hwy, Silver Springs, MD 20910. 



There are inherent difficulties associated with exam- 

 ining the seasonality of marine organisms using aerial 

 surveys. This is particularly true when the species of 

 interest is not obligated to surface waters where visual 

 observations can be made. Because the whale shark is 

 not a surface obligate species, factors such as changes 

 in feeding behavior resulting from seasonal variabil- 

 ity in the vertical distribution of prey species could 

 have introduced significant bias into our sighting data, 

 subsequent analyses, and data interpretation. We as- 

 sumed that no differences exist among seasons in the 

 amount of time whale sharks spend at or in close prox- 

 imity to the surface. Using telemetry and archival tags, 

 Gunn et al. (1999) determined that whale sharks spend 

 17-53% of their time at the surface during daylight 

 hours off the coast of Australia. Over periods ranging 

 from 28-1144 days, Eckert and Stewart (2001) tracked 

 vertical movements of whale sharks through the water 

 column in the Sea of Cortez using satellite tags. They 

 concluded that during all seasons whale sharks spend 

 greater than 80% of their time at depths of 10 m or 

 less during all seasons — depth well within the photic 

 zone of continental slope waters of the Gulf of Mexico. 

 The findings of both Gunn et al. (1999) and Eckert and 

 Stewart (2001) support our assumption. 



Due to the economic value of their meat, fins, and 

 liver oil, whale sharks have recently been designated 

 as vulnerable to harvesting-induced extinction by the 

 lUCN (Stewart and Wilson, 2005). Furthermore, the 

 Convention on International Trade in Endangered Spe- 

 cies of Wild Fauna and Flora (CITES) has placed the 

 whale shark under Appendix II, which limits the trade 

 of whale shark products among cooperating nations 

 (Stewart and Wilson, 2005). Within the United States 

 EEZ, the retention of whale sharks caught commercially 

 or recreationally is prohibited (NMFS, 1993). However, 

 because whale sharks are highly migratory and their 

 movements cross numerous boundaries, detailed infor- 

 mation on their abundance and seasonal distribution is 

 needed to ensure their well being. 



Acknowledgments 



We thank W. Hoggard, S. O'Sullivan, and C. Roden 

 for participating in the surveys and for their valuable 

 suggestions. We also thank R. Avent, C. Gledhill, D. 

 Hanisko, A. Jahncke, K. Mitchell, and K. Rademacher 

 who assisted us in varying capacities. Comments by E. 

 Clarke, M. Grace, T. Henwood, C. Hubard, C. Jones, 

 K. Maze-Foley, S. Nichols, C. Roden, and the three 

 anonymous reviewers significantly improved the man- 

 uscript. The research was authorized under Marine 

 Mammal Research permits 738 and 779-1339 issued to 

 the SEFSC, and was supported by interagency agree- 

 ments between the NMFS-SEFSC and U.S. Geological 

 Survey, Biological Resources Division (no. 1445-IA09- 

 96-0009); and the NMFS-SEFSC and Minerals Manage- 

 ment Service, Gulf of Mexico Region (nos. 15958 and 

 14-12-0001-30398). 



