SHANE; OCCURRENCE AND DISTRIBUTION OF BOTTLENOSE DOLPHIN 



T. truncatus in northeastern Florida on a seasonal 

 basis. Wiirsig (1978) found no evidence for a sea- 

 sonal migration of T. truncatus in Argentina. 



The belief that the seasonal variation in abun- 

 dance of bottlenose dolphins in the study area was 

 an annual occurrence and not a result of the un- 

 usually cold winter of 1976-77 was supported by 

 my 1977 fall and winter observations. Later obser- 

 vations on the seasonal presence of Thick Fin and 

 Bent Fin in the study area by Poff (footnote 5) 

 further substantiated the first year's data. 



Bottlenose dolphins in the Port Aransas area 

 responded to seasonal changes by emigrating out 

 of the study area for the winter, immigrating into 

 the study area for the winter or by remaining 

 year-round residents. Hogan (footnote 2) and True 

 (1890) reported the first and third patterns for T. 

 truncatus in the Atlantic. 



The three responses to seasonal changes exhib- 

 ited by dolphins in the study area prohibit a simple 

 explanation of seasonal occurrence patterns. Two 

 factors cited as affecting cetacean distribution are 

 water temperature (Gaskin 1968) and food avail- 

 ability (Mercer 1975). Water temperature in the 

 study area changed from a winter mean of 11.4° C 

 to a summer mean of 28.4° C, and this change 

 could have had a direct or indirect effect upon 

 dolphin movements. Food is available to dolphins 

 both in the study area and in the Gulf of Mexico 

 during the winter: most fish species emigrate from 

 the bays to the Gulf for the winter, but a few 

 species spend the winter in the bays ( Gunter 1945). 

 Three of the latter species — striped mullet, Mugil 

 cephalus; sand trout, Cynoscion arenarius; and 

 black drum, Pogonias cromis — have been found in 

 the stomachs of bottlenose dolphins ( Gunter 1942). 

 Other fish species wintering in the bays near Port 

 Aransas were killed by cold spells during the win- 

 ters of 1940 and 1951 (Gunter 1941; Gunter and 

 Hildebrand 1951). Of these, spot, Leiostomus 

 xanthurus; croaker, Micropogon undulatus; 

 sheepshead, Archosargus probatocephalus; spot- 

 ted trout, C. nebulosus; pinfish, Lagodon rhom- 

 boides; and ribbonfish, Trichiurus lepturus, are 

 eaten by T. truncatus (Gunter 1942; Kemp'). If 

 water temperature and food availability influence 

 the seasonal occurrence of dolphins in the study 

 area, individual dolphins or social groups may 

 have different temperature or food preferences. 



Daily Movements 



Dolphins generally move against the tide in the 

 lower sections of the study area, although they 

 moved against the ebb tide more consistently than 

 they moved against the flood tide (Table 2). In all 

 other studies where cetacean movements and tides 

 were correlated, cetaceans moved with tidal cur- 

 rents (Irvine and Wells 1972; Norris et al. 1977; 

 Irvine et al. footnote 3; Wiirsig^). Tide was sig- 

 nificantly related to dolphin movements in only 

 one case in the upper section of the study area, and 

 the strength of the relationship there was weak ( V 

 = 0.139). In two other cases where tide and direc- 

 tion were significantly related in the upper section 

 of the study area, insufficient data made the chi- 

 square results potentially invalid. This lack of 

 tidal effect for the upper bays agrees with the 

 observation that gray whales moved with tidal 

 currents in channels but ignored tidal flow in bays 

 (Norris et al. 1977). 



Dolphins consistently moved against the ebb 

 tide in Aransas Pass where the strongest ebb tides 

 were slightly stronger (2.5 km/h) than the 

 strongest flood tides (2.4 km/h) (Smith 1979) and 

 where there was a net outflow averaging 0.3 km/h 

 (Smith 1978). Dolphins showed little or no consis- 

 tency in their tide-related movements in Morris 

 and Cummings Cut where inflow and outflow 

 should be approximately equal in strength and net 

 effect (Smith 1978). The tidal data indicate that 

 dolphins respond to the dominant tidal currents in 

 this area by moving against them, and that their 

 movements are less affected by tide in areas where 

 tidal effects are diluted such as in the upper bays. 



Two explanations for the movement of dolphins 

 against tidal flow seem possible. First, the coun- 

 tercurrent movement may represent a method of 

 feeding. Dolphins may catch fish more easily when 

 the fish are swimming with or being carried by the 

 current. The possible increase in feeding efficiency 

 might outweigh the energy expenditure required 

 to move against a strong current. There is evi- 

 dence that more fish move through Aransas Pass 

 during ebb tides than during fiood tides: "The dif- 

 ference in catch [of fishes in Aransas Pass] be- 

 tween flood tide collections and ebb tide collections 

 was tremendous. Few specimens were collected 

 during flood tide, although the tide trap was low- 



'Kemp, R. J. 1949. Report on stomach analysis- 

 Delphininae. Annual Report of the Marine Laboratory of the 

 Texas Game, Fish and Oyster Commission for the fiscal year 

 1948-1949. Unpubl. manuscr., p. 111-112, 126-127. 



^Wiirsig, B. 1976. Radio tracking of dusky porpoises 

 (Lagenorhynchus obscurus) in the south Atlantic, a preliminary 

 analysis. In FAO-ACMRR Scientific Consultation on Marine 

 Mammals, Bergen, Norway, p. 1-21. 



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