SHANE: OCCURRENCE AND DISTRIBUTION OF BOTTLENOSE DOLPHIN 



RESULTS 



Seasonal Occurrence 



Seasonal occurrence patterns were derived from 

 seasonal variation in dolphin numbers and from 

 observations of individual dolphins. The monthly 

 mean number of dolphins in the study area varied 

 from a low of 48 in October (range = 8-104) (the 

 September low is inaccurate because no zigzag 

 censuses were conducted in some sections) to a 

 high of 164 in January (range = 65-281) (Figure 

 2). Dolphin abundance declined from summer to 

 fall, rose in the winter, and declined again in the 

 spring. Boat and land observations from August 

 through December 1977 showed the same pattern 

 as the previous fall and early winter: dolphin 

 numbers declined noticeably in early fall and then 

 increased to higher than summer numbers in 

 November and December. 



Sightings of identifiable dolphins confirmed a 

 seasonal occurrence in the study area. Sixteen of 

 the 19 recognizable dolphins in the study area 

 were seen on 5 or more days and were identified by 

 the fifth month of the study. The other three dol- 



280- 



240 



200 



160 



o 

 a 



° 120 



o 



Z 



80 



40 



1 



■J- 



J J A S O N D 

 Month 



F M A M 



Figure 2. — Mean number of Atlantic bottlenose dolphins 

 counted in the study area near Port Aransas, Texas, for each 

 month from June 1976 through May 1977. Ranges are indicated 

 by the vertical lines. The September count is unreliable because 

 an insufficient number of censuses were conducted that month. 



phins were seen on <5 d or were not identified 

 until the eighth month of the study. Three patterns 

 of seasonal occurrence in the study area were 

 demonstrated by the 16 dolphins (Table 1). Four 

 individuals (Thick Fin, Jagger, Notched Fin, and 

 Short Triangle) were predominantly spring and 

 summer residents: over 6(¥7c of the days each was 

 seen in the study area occurred during spring and 

 summer and <409c occurred during fall and 

 winter. Five individuals (Lumpy, Cloud, Bent Fin, 

 Twin, and Tiki) were predominantly fall and 

 winter residents: over 60% of the days each was 

 seen in the study area occurred during fall and 

 winter and <40'7f occurred during spring and 

 summer. The remaining seven dolphins spent ap- 

 proximately the same amount of time in the study 

 area during spring and summer as during fall and 

 winter. 



My observations from June through December 

 1977 confirmed the seasonal occurrence pattern of 

 some dolpins established during the 1-yr study. 

 Lumpy, a fall and winter resident who had been 

 seen only between 20 November 1976 and 2 Feb- 

 ruary 1977, was seen again on 1 and 3 December 

 1977. Bent Fin, a fall and winter resident who was 

 seen regularly from October 1976 through Feb- 

 ruary 1977 and then only on 3 d through November 

 1977, was again sighted in the study area on 5 and 

 8 December 1977. I saw Thick Fin (spring and 

 summer resident) in the study area frequently 

 from June 1977 to 8 September 1977 but never 

 after that through December 1977, when I left the 



Table l. — Seasonal occurrence of 16 bottlenose dolphins in the 

 Port Aransas, Texas, area. Percentage of days each dolphin was 

 seen during each season and each spring and summer (Sp/S) 

 period and fall and winter (F/W) period are given. Individual 

 dolphin occurrence patterns suggested the spring-summer and 

 fall-winter links. Dashes indicate that the dolphin was not iden- 

 tified until after that season. 



595 



