BEHAVIOR OF THE HAWAIIAN SPINNER DOLPHIN, 

 STENELLA LONGIROSTRIS 



Kenneth S. Norris and Thomas P. Dohl' 



ABSTRACT 



The Hawaiian spinner dolphin, Stenella longirostris, was recorded from Kure Atoll to the island of 

 Hawaii. It enters atoll lagoons or specific coves or swims over shallow sandy areas, usually near deep 

 water, to rest. Access to nighttime feeding grounds may regulate the location of these rest areas. Rest 

 areas are generally 50 meters or less in depth. 



Natural scars and marks allowed study of movements and school structure. Schools are fluid 

 assemblages of variable size and composition. Only small subgroups within schools may have long- 

 term integrity. 



Spinner dolphins exhibit several aerial patterns including spinning which is mostly associated with 

 sound production upon reentry, and each is typical of a specific school activity level. Sounds may serve 

 as omnidirectional sound sources maintaining school cohesion beyond the limits of vision. 



The daily cycle of spinner dolphins consists of nighttime feeding, morning approach to shore, 

 morning-midday rest, and travel to feeding grounds near dusk. Feeding is upon scattering layer fishes, 

 squid, and shrimp. 



Dolphins very commonly show scars from large sharks and from the small squaloid shark, Isistius 

 brasiliensis, which scoops disc-shaped pieces of blubber from them. These wounds heal to form dollar- 

 shaped scars. 



Most that is known in any depth about the be- 

 havior of dolphins has come from observations of 

 captive animals. Yet the environment of captivity, 

 which is at best a pool a few dozen meters in 

 longest dimension and 5 or 10 m deep, can allow 

 only certain aspects of normal behavior to occur. 

 Intragroup relationships may persist, but are 

 usually distorted because relationships seldom 

 remain intact. At best only hints of normal move- 

 ment and activity patterns can persist where feed- 

 ing schedules are determined by the workdays of 

 trainers. In nature spinner dolphins, at least, 

 travel constantly, even during rest. Dolphins of 

 many species dive and feed in very deep water. 



Thus, however difficult it might be, the 

 naturalist who would study dolphin behavior feels 

 the need to study them in nature. It is usually no 

 simple task. They are wary and travel many 

 kilometers in a day. The presence of the observer 

 almost inevitably causes bias. Dolphins hear ex- 

 ceedingly well, and dolphin schools may be aware 

 of an approaching ship a kilometer or more away. 



Our first 2-yr effort ( 1968-69), with a spotted, or 

 "kiko," dolphin, Stenella attenuata, school which 

 we knew to frequent the area within a few 



'Center for Coastal Marine Studies, University of California, 

 Santa Cruz, CA 95064. 



kilometers of Kaena Point, Oahu, Hawaii, was 

 abandoned for the reasons mentioned above. It 

 simply proved too expensive in time, money, and 

 effort to work with the animals. Our work was 

 never free from observer bias. Reports of a school 

 of the spinner dolphin, Stenella longirostris, 

 (Schlegel 1841), living permanently in 

 Kealakekua Bay, on the Kona or lee coast of the 

 island of Hawaii, caused us to visit the area to see 

 if work was feasible. We found an unusual situa- 

 tion in which several vexing observational prob- 

 lems were ameliorated. 



Spinner dolphins do occur in Kealakekua Bay 

 frequently (our figures indicate occupancy about 

 74% of the time). The bay itself is remarkably good 

 for observation. The Kona coast is normally quite 

 calm, especially in morning hours. Lateral visabil- 

 ity is usually 20 m or more. The local people sel- 

 dom disturb the dolphins. Only cruise boats, which 

 seek out the schools and run through them, are a 

 predictable disturbance. An abrupt 150 m lava 

 clifT backs the bay. Schools sometimes came close 

 to the cliff base at places where our visibility was 

 blocked and could not be seen from the clifftop. 

 But, most of the time we could watch wholly undis- 

 turbed schools at reasonably close range, although 

 the distance proved too long for individual iden- 

 tification, and the lack of contrast between animal 



Manuscript accepted June 1979 



F1.SHERY BULLETIN VOL. 77. NO, 4. 1980, 



821-2''f'7 



