NORMS and DOHL; BEHAVIOR OF THE HAWAIIAN SPINNER DOLPHIN 



Their excursions were entirely confined to shallow 

 water (about 3-50 m). Only when disturbed or 

 when rest was over did the schools begin to edge 

 into deeper water. For instance, resting schools 

 seldom ventured into the deep channel that en- 

 tered the bay at Kaawaloa, but instead moved 

 mostly back and forth between our observation 

 station #1 and Napoopoo (Figure 1), an area of 

 shallow sandy bottom dotted here and there with 

 isolated coral heads. 



Resting schools often changed direction under- 

 water, which made it difficult to predict where 

 they would surface. Interindividual and inter- 

 group distances gradually decreased as rest 

 deepened, until many animals were very close to 

 or actually touching one another, while the arriv- 

 ing school showed evident group structure and 

 independence of both movement and diving time 

 within the school; the resting school swam in 

 much closer synchrony. Arrival of synchronous 

 diving by an entire school provided a good, if arbi- 

 trary, indicator of the onset of rest. 



On a few occasions we were able to watch syn- 

 chronous dives from the MOC. The underwater 

 vantage point allowed the observer to see sub- 

 groups of animals in the school as a whole. Tight, 

 uniformly oriented groups dove slowly, with mea- 

 sured tail beats, toward the sand bottom below; 

 leveled out a few body lengths above the bottom 

 and moved slowly along, schooled tightly; and 

 swam largely without individual exploratory 

 movements. Occasionally an animal descended to 

 the bottom and beat boils of sand up into the water 

 with its flukes. At the end of a dive the animals 

 rose rather steeply to the surface, not as a single 

 tightly integrated group, but more or less 

 seriatum, as a column of subgroups. Often after 

 rising, the animals spread outward from this ris- 

 ing column a short distance before turning to 

 define the compact confines of the surface school, 

 like the petals of a flower opening. Once on the 

 surface, group structure could be seen, but the 

 animals seemed much more regularly spaced than 

 is the case in active schools. Diving, too, was 

 steeper and slower than in travelling groups. 

 While individuals in resting schools seemed less 

 alert than animals in feeding or travelling 

 schools, the resting school itself was very wary of 

 strange features of its environment. Any strange 

 object placed in a rest cove, such as a buoy, boat, 

 or line was avoided for a matter of days before a 

 school seemed to habituate to its presence. It was 

 striking that they reacted to foreign objects in 



much the same way as we have come to expect 

 from fish schools, and not with typical dolphin 

 individuality. For instance, when a resting school 

 cruised inshore of us near the cliffs, we waited in 

 a quietly rocking skiff some 75 m offshore, and 

 the school approached slowly as a discoidal group, 

 thinned as it reached a point directly inshore of 

 us, streamed betweeen the skiff and the cliff as a 

 long line of quietly moving animals, and re- 

 formed its discoidal group once past us. We found 

 that our skiff or our anchored workboat could de- 

 form such discoidal groups from some distance, 

 causing the side nearest the skiff to become dent- 

 ed or malformed as the entire school reacted to 

 our presence. When a four-hydrophone array 

 capable of sound triangulation was placed near 

 the path of such resting schools, it was assidu- 

 ously avoided and no animals were known to pass 

 through it for 6 days after its placement (Watkins 

 and Schevill 1974). A line stretched across the 

 surface of the water was capable of deflecting such 

 schools. In such cases, even though the animals 

 moved slowly and other evidence of alertness, such 

 as complex phonation or aerial behavior, was 

 nearly absent, the school as a whole remained 

 alert. We suppose this is due to sensory integra- 

 tion by the closely packed school, that is, by the 

 reception of environmental information by some 

 members of a school and transmission of its oc- 

 currence to all or most of them. It was usually 

 possible to cruise among alert schools, and many 

 individuals might station at the bow within a few 

 feet of an observer, but resting animals very sel- 

 dom came to a vessel. 



A graphic demonstration of spinner dolphin's 

 fear of strange objects was given by our attempts 

 to encircle quiescent spinner dolphin schools with 

 a modified Hawaiian "hukilau''." 



Our hope was that this fear might be utilized to 

 assist in their release from tuna seines, since at 

 the time large numbers were being killed per year 

 in the yellowfin tuna fishery. We conceived that 

 light weight gear of this sort could be deployed in a 

 tuna net to crowd the captive animals, and thus 

 assist in their release. Our tests, run in August 

 1973 in Kealakekua Bay, used a hukilau com- 

 posed of 450 m polyporpylene cork line I '/•> in; 1 .27 

 cm) from which were hung every 2 m, thin poly- 



'A hukilau is a Hawaiian "net" made of a cork line with palm 

 fronds woven through it at intervals, which is towed across 

 coral areas, chasing fish in front of it. Because a mesh net is not 

 involved, it does not entangle on the rough bottom but still 

 serves to concentrate the fish, which are then netted from inside 

 the hukilau over sandy bottom. 



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