NORRIS and DOHL: BEHAVIOR OF THE HAWAIIAN SPINNER DOLPHIN 



Perhaps the dolphins can make such identifica- 

 tions underwater by sound. 



What can the use be of such sound signals? The 

 following possibilities seem apparent: 1) If we can 

 gage the activity state of a school by its aerial 

 behavior, it is likely that the dolphins can do so 

 too, and probably in a more refined way than we 

 can; 2) such sound signals may be used where 

 vision is useless; 3) school cohesiveness in the 

 dark, or when animals are spread beyond the 

 limits of vision, may be promoted by repeated 

 short-range omnidirectional sound signals from 

 all parts of a school. The incidence of aerial be- 

 havior is correlated with times when such signals 

 would be most useful. This seems to us to be the 

 most likely function. 



We considered, and rejected the idea, that the 

 spin might be a pattern relating to dominance or 

 courtship in the school. This seems refuted be- 

 cause animals of all age classes and both sexes 

 spin and because captive observations have shown 

 that even animals that have been rejected from 

 the social structure of a school spin. 



Another possibility is that spinning is related to 

 removal of ectoparasites such as remoras and 

 copepods. While it might be useful occasionally in 

 detaching such creatures, we have never seen a 

 case in which this seemed to be occurring, and 

 essentially every animal observed to spin was ap- 

 parently without parasites. Captives spin regu- 

 larly even though free of parasites. 



THE DAILY ACTIVITY CYCLE 



Obgervations of spinner dolphin schools along 

 the Kona coast of the island of Hawaii, and to a 



minor extent elsewhere in the Hawaiian chain, 

 show a regular sequence of activities during each 

 24-h period. Broadly, this consists of nighttime 

 feeding, about which we know little, morning 

 coastward movement that brings the animals into 

 coves and sheltered coastline areas, rest, awaken- 

 ing, zigzag swimming, and then departure to the 

 feeding grounds. Each of these activities will be 

 discussed in turn. 



Feeding 



Feeding is upon scattering-layer organisms 

 (Table 3) and seems to be performed during syn- 

 chronous or subsynchronous dives of large and 

 dispersed schools. What we take to be feeding 

 dives start as early as dusk, before most of the 

 scattering laj'er approaches the surface, and such 

 evidence as we have (mainly from a single radio- 

 tracking, from chance encounters with schools at 

 night, and from schools that we have followed to 

 the feeding grounds) suggests that the schools pa- 

 trol along the breaks in the submarine island slope 

 and toward morning gradually make their way 

 into shallow water over the shelf A radiotracked 

 animal, caught over approximately 140 m of water 

 at Keahole Point just before dusk (at 1650 h) on 

 1 March 1971, moved back and forth along the 

 shore between a point near Kailua-Kona and 

 Kiholo Point. A detailed radiotrack was made dur- 

 ing the night of 1-2 April (Figure 11). The animal 

 stayed with a large school that moved slowly 

 offshore and by 2000 was over the island slope. The 

 group then patrolled back and forth over the slope 

 within a stretch of coast approximately 20 km in 

 length and over water that varied from about 360 



Table 3. — Squid and shrimp in the diet of Hawaiian spinner dolphins. 



Sample and dale Capture locality 



Squid 



Shrimp 



0170-42 



Sepl 24, 1970 



0170-35 



Mar 25. 1970 



0171-1 

 Jar 8, 



0171-2 

 Jan 8. 



1 km oH Ala Wai, Oahu 28 mantles (mantle lengtti 25-52 mm, mean 38 9 mm) 

 5 Abralia astrosticta 

 1 4 Abralia tngonura 

 67 squid beaks, probably of the same species 



Oft Waikiki, Oahu 



200 m off Kailua-Kona 

 Harbor, Hawaii 



200 m off Kailua-Kona 

 Harbor. Hawaii 



2 Abralia astrosticta 



7 Abralia tngonura 



1 52 squid beaks of the above species 

 49 macerated squid 

 2 Abralia astrostica 

 6 Abralia trigonura 

 204 squid beaks, probably of the above species 



1 Histioteuthis sp 



2 Abralia astrosticta 



8 Abralia trigonura 



310 squid beaks of the above species 



11 pasiphaeids (to 17 8 mm carapace length) 



1 small 



4 abdominal portions 



Icandean cephalothorax 

 Probable euphausid fragments 

 No identifiable remains 



20Sergia fulgens (12.5-15.5 mm carapace length, mean 

 14 6 mm) 



1 Acanthephyra sp 



1 Pasiphaea sp 



2 Pasiptiaea sp 



1 5 Sergia fulgens (12-14 5 mm mean 1 3 6 mm 

 Some of the above may be of undetermined species) 

 1 Opiophorus grimaldii (identiftcation probable) 



3 Acanthephyra sp (identification probable) 



835 



