FISHERY BULLETIN: VOL. 77, NO. 4 



FIGURE 11.— Radiotrack chart of a 

 marked spinner dolphin. 31 March-2 

 April 1971. Dolphin stayed in moving 

 school of 100 animals presumably feed- 

 ing over the island's submarine slopes. 



to 2,600 m deep. By 0300 the school and the 

 radiotagged animal had moved closer to shore and 

 continued to move in ever shallower water until 

 dawn. 



Feeding schools were observed on three occa- 

 sions at dusk. Each was composed of widely scat- 

 tered groups, covering as much as 3 km in widest 

 dimension, moving together. Diving was subsyn- 

 chronous. Before a dive occurred, groups were evi- 

 dent and there was much aerial behavior across 

 the entire width of the school. Then groups of the 

 school dove individually, all following within ap- 

 proximately a minute or two. Dives were long, 

 averaging 3.5 min according to our records. Sur- 

 facing was approximately as coordinated as div- 

 ing; that is, the various groups straggled to the 

 surface over a minute or two. 



It was striking to see these very broad diffuse 

 schools reverse their course in relative synchrony 

 (within a minute or two), even at dusk, indicating 

 a communication mechanism, probably acoustic, 

 that could pass information rather quickly across 

 the school. 



Stomach contents were obtained from four spin- 

 ner dolphins caught early in the day (before noon), 

 while three animals taken in the afternoon had 

 empty stomachs. This same pattern seems to occur 

 in the oceanic spinner dolphins of the tropical 

 Pacific, and a high percentage (65.3%) of empty 

 stomachs (not segregated according to time of day) 

 from 49 spinner dolphins taken from the eastern 



tropical Pacific were empty (Perrin et al. 1973). A 

 time-stratified sample would probably show some 

 food in the stomachs in the morning before diges- 

 tion of the night's catch is complete, with empty 

 stomachs in the afternoon. If spinner dolphins 

 were diurnal feeders, one would expect few empty 

 stomachs during the day at any time. The ob- 

 served morning defecation period also fits this 

 scheme. We conclude from our own observational 

 data that the spinner dolphin in the open eastern 

 tropical Pacific and around the Hawaiian Islands 

 feeds at night. Our evidence, and that from other 

 studies, suggests that it feeds upon scattering- 

 layer organisms found at considerable depth. 

 Fitch and Brownell ( 1968) reach a similar conclu- 

 sion from otolith studies of stomach contents of 

 five spinner dolphins taken from the yellowfin 

 tuna grounds; they stated: "We feel certain that 

 three of the cetaceans we investigated (Kogia 

 simus, Stenella longirostris, and Lissodephis 

 borealis) had been feeding 800 ft (250 m) or more 

 beneath the surface . . . . " Perrin et al. (1973) 

 similarly concluded that spinner dolphins are 

 feeding mostly on mesopelagic fish and squid, with 

 a small increment of epipelagic squid species in 

 their diet. Our results (Table 3) confirm these ear- 

 lier works, but show a considerable component of 

 sergestid crustaceans in the diet of the Hawaiian 

 spinner dolphin. Epipelagic squid were absent, 

 though common in Hawaiian waters, while such 

 relatively deepwater forms as Abralia astrosticta 



836 



