582 



Fishery Bulletin 104(4) 



30°N 



28°N - 



26°N - 



24"N 



96°W 



94"W 



1)2 W <-)() W 



XS W 



Sh'W 



84 W 



Figure 2 



Locations (;!=81) of whale sharks iRhirjcodon typus) observed during aerial surveys in the northern Gulf 

 of Mexico from 1989 to 1998. Dashed line demarks the separation between the eastern and western Gulf 

 of Mexico as defined in our study. Area contained within the circle indicates region of highest observed 

 whale shark abundance. Bathynietric contours represent the 100-, 500-, 1000- and 2000-meter contours. 

 Location and season of sighting symbols: A = winter, O = spring, D = summer. = fall. 



the center of their distribution was approximately 140 

 km southwest of the Mississippi River Southwest Pass 

 (Fig. 2). Twenty-seven whale sharks were observed in 

 this region over a four-year period. 



Discussion 



We observed whale sharks throughout the year in the 

 Gulf of Mexico, and our study is the first to identify a 

 broad region where whale sharks are present during all 

 seasons and where they are perhaps resident throughout 

 the year. The highest SPUE values that we observed 

 occurred during the summer. Therefore, given the 

 reported seasonality of whale sharks in the western 

 Caribbean Sea (Heyman et al., 2001) and southeast- 

 ern Gulf of Mexico (Gudger, 1941), it is reasonable to 

 assume that a portion of the population migrates into 

 the northern Gulf of Mexico during the spring and in 

 winter moves into lower latitude waters, such as the 

 Bay of Campeche and waters off the coast of Cuba. The 

 only entrances to the Gulf of Mexico are through the 

 Yucatan Channel and the Straits of Florida; therefore 

 research with telemetry and satellite tagging in these 

 regions would help determine the timing and routes 

 of whale shark migrations and the period of residency 

 within the Gulf of Mexico. 



Whale sharks are more abundant in the western than 

 in the eastern Gulf of Mexico. However, because the 

 aerial surveys were limited to two seasons in the east- 

 ern Gulf of Mexico, little can be inferred from our data, 

 except confirmation of the presence of whale sharks in 

 the eastern Gulf of Mexico during the winter and the 

 addition of summer to the seasonality of occurrence in 



this region. Because the survey reported in our study 

 focused solely on continental slope waters, it is possible 

 that whale sharks in the eastern Gulf of Mexico use 

 continental shelf waters to a greater extent and, thus, 

 were outside of the sampled area. Despite extensive ef- 

 fort over the broad upper continental slope off the west 

 coast of Florida, no whale sharks were observed south 

 of 27'38'N. However, at higher latitudes, 13 individuals 

 were sighted in continental slope waters at depths rang- 

 ing from 70 to 180 m. Because the upper continental 

 slope is broader in the eastern than in the western Gulf 

 of Mexico, it is possible that whale shark distribution in 

 the eastern Gulf of Mexico is more diffuse than would 

 be expected, especially if this species is associated with 

 steep bathymetric relief, which promotes upwelling 

 of nutrient rich waters. During our study, two whale 

 sharks were observed at the head of the DeSoto Can- 

 yon, an upwelling area south of the Florida panhandle. 

 A large aggregation (30-100 individuals) was found in 

 this same area by Hoffmayer et al. (2005). 



In the western Gulf of Mexico, whale sharks were ob- 

 served in all sampled depth strata, and the spatial distri- 

 bution of sightings was fairly continuous along the con- 

 tinental slope from Mobile Bay (88°00'W) to the United 

 States/Mexico border (96°30'W: Fig. 2). Seasonally, whale 

 sharks were present year round in the western Gulf of 

 Mexico, except between 94°00'W and 95°35'W where they 

 were observed only during spring and fall. Bimodality in 

 seasonal occurrence in this region could be attributed to 

 seasonal fluctuations in biological productivity and, thus, 

 to prey availability resulting from ephemeral mesoscale 

 phenomena such as Loop Current eddy formation, cur- 

 rent reversals, and coastal jetting. For example, in the 

 western Gulf of Mexico the Texas Current is capable of 



