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



581 



Gulf of Mexico," in this article. Mobile Bay, Alabama 

 (88°00'W) was considered to be the dividing line be- 

 tween the eastern and western Gulf of Mexico. 



Sightings per unit of effort (SPUE) were calculated 

 to correct for unequal effort among regions by dividing 

 the number of sightings per season by the total survey 

 effort during the same season and then multiplying the 

 resulting value by 1000. Because aerial surveys were 

 limited to summer and winter in the eastern Gulf of 

 Mexico, data from these two seasons in this region were 

 compared (by using a ^-test) to data collected during 

 surveys in the same two seasons in the western Gulf 

 of Mexico to determine if mean whale shark SPUE was 

 significantly different between the two areas. To deter- 

 mine if there was a season of peak whale shark SPUE 

 in the western Gulf of Mexico a one-way analysis of 

 variance (ANOVA) and the Student-Newman-Keuls test 

 were employed. The Mann-Whitney test was used to de- 

 termine if there was a significant relationship between 

 season and number of individuals per aggregation. This 

 test was selected because of the non-Gaussian distribu- 

 tion of these data. The location of each whale shark 

 sighting was plotted on navigational charts, produced 

 by the National Oceanic and Atmospheric Administra- 

 tion (Chart numbers 11006, 11300, 11340), to examine 

 the associations of whale sharks with bathymetric fea- 

 tures such as reefs and salt diapirs. Statistical tests 

 were preformed according to the methods of Zar (1999) 

 at a significance level of PsO.05. 



Results 



Transects totaling 89,369 km were surveyed over the 

 course of the study (Table 1). A total of 119 whale sharks 

 were counted during 81 sighting events (Fig. 2). There 

 was no statistical difference in the SPUE of whale sharks 

 between the eastern and western continental slope 

 waters of the Gulf of Mexico (/-test, df=4, f-value = 1.06. 



P=0.35). Because survey effort in the eastern Gulf of 

 Mexico was limited to summer and winter, it was not 

 possible to quantitatively analyze whale shark seasonal- 

 ity in this region. However, results indicated that whale 

 sharks are more abundant in the eastern Gulf of Mexico 

 during the summer (SPUE = 0.96) than in the winter 

 (SPUE = 0.36i. There was a statistically significant differ- 

 ence in whale shark SPUE among seasons in the western 

 Gulf of Mexico (ANOVA. df=7, F-ratio = 12.97, P=0.02). 

 The Student-Newman-Keuls test indicated that whale 

 shark SPUE was significantly greater in the summer (.f = 

 2.98, SD = 0.49) than during the fall (.r=0.30, SD = 0.18) 

 or winter (x=0.63, SD=0.32). However, there was no sig- 

 nificant difference between summer and spring (.v=1.32, 

 SD = 0.38). There was no significant difference in whale 

 shark SPUE among spring, fall, and winter. 



Of the 119 whale sharks sighted over the course of 

 the study, 45 were observed in aggregations — an ag- 

 gregation being defined as the presence of two or more 

 whale sharks in close proximity to one another. Seven 

 aggregations, ranging in size from 2 to 23 individuals, 

 were observed. Aggregations of whale sharks were seen 

 only during the winter and summer and there were sig- 

 nificantly more individuals per aggregation during the 

 summer (Mann-Whitney test, W=12.0, P=0.05). Sixty- 

 two percent (;;=74) of whale sharks were not observed 

 in association with conspecifics. 



The majority of whale sharks observed were not as- 

 sociated with discrete areas of high bathymetric relief 

 Thirty-four of the whale sharks sighted were in close 

 proximity to relatively small, high relief diapiric fea- 

 tures dominated by coralline algae (Rezak et al., 1990); 

 Ewing Bank (28°06'N, 9r02'W), Bright Bank (27°53'N, 

 93°18'W) and 28 Fathom Bank (27°55'N, 93"26'W). Of 

 the seven aggregations observed during the surveys, 

 three were associated with the aforementioned banks 

 including the aggregation consisting of 23 individu- 

 als. The area of highest whale shark abundance was 

 located in an area of approximately 16,800 km- and 



