FISHERY BULLETIN; VOL. 77. NO. 4 



Hawaiian spinner dolphins (Norris and Dohl 

 1980) and for Sousa sp. (Saayman and Tayler 

 1979). But what about the consistently shallow- 

 water movement in the morning (and all day in 

 winter) when dolphins did not appear to be surface 

 feeding much of the time? Their activity level was 

 low and they did not move rapidly. They ignored or 

 avoided boats as well as other marine mammals. 

 They moved in small, tight groups and we there- 

 fore gained the subjective impression that they 

 were schooling in an almost "fishlike" manner. 

 Because level of activity was low, objects in their 

 paths were avoided or ignored, and schooling was 

 tight, we believe that the dolphins were resting at 

 this time. 



There is some evidence that killer whales may 

 prey on dusky dolphins. On three occasions when 

 killer whales came close to dolphin groups, the 

 dolphins moved into extremely shallow water. At 

 the same time, they moved rapidly along shore, 

 perhaps to avoid nearshore predation, of which 

 killer whales Eire known to be capable on more 

 stationary prey, such as elephant seals and sea 

 lions (Norris and Prescott 1961; Tomilin 1967; 

 pers. obs.). As well, their nearshore movement 

 may serve to hide them from possible Orcinus orca 

 echolocation, which might be confused and in- 

 efficient in very shallow water. 



These observations make it likely that near- 

 shore movement while resting is a defense against 

 predation. In shallow water, killer whales (and 

 possibly deepwater sharks) cannot come from be- 

 low, nor from the flanking shoreline. When danger 

 comes from the open sea, dolphins can retreat to 

 very shallow waters in which larger predators 

 cannot maneuver as efficiently. Norris and Dohl 

 ( 1980) postulated a similar function for nearshore 

 resting of Hawaiian spinner dolphins, suggesting 

 that these animals possibly avoid large deepwater 

 sharks during morning periods of low activity. 

 Saayman and Tayler (1979) also saw Sousa sp. 

 very close to shore when killer whales were near, 

 and suggested that the dolphins might avoid pre- 

 dation in a similar manner. In the present popula- 

 tion, it is possible that nearshore movement dur- 

 ing low-activity levels may serve other functions 

 as well, but we believe that the predator- 

 avoidance hypothesis may be at least part of the 

 reason. 



CONCLUSION 



In the preceding discussion, we attempted to 

 888 



link observed behavior patterns to observed or 

 possible ecological variables. We recognize that 

 this endeavor is highly incomplete, and that many 

 more alternative explanations will be made avail- 

 able in the future. One important factor that may 

 have been somewhat obscured in the results and 

 discussion of behavior should be emphasized. Dol- 

 phin behavior in captivity as well as in the wild 

 appears highly plastic and variable. For example, 

 dusky dolphins feed on southern anchovy. Yet 

 many species are more catholic feeders (for exam- 

 ple, Gunter 1942, Leatherwood 1975, for Tursiops 

 truncatus; and Perrin et al. 1973, for Stenella 

 spp.), and it is certain that dusky dolphins engage 

 in other feeding than surface feeding described 

 here. We hope that future work will shed light on 

 other feeding modes, whether subsurface feeding 

 is done cooperatively as is surface feeding, or 

 whether it is performed more often by single dol- 

 phins on nonaggregated prey. Such an analysis 

 may help us understand the dramatic difference in 

 movement patterns and general activity levels be- 

 tween times when dolphins feed cooperatively on 

 the surface and when they feed in other ways. 



ACKNOWLEDGMENTS 



Jen and Des Bartlett, Peter Tyack, Marty 

 Hyatt, and Russ Charif helped gather data. Jan I. 

 Wolitzky wrote the computer program for analyz- 

 ing theodolite track data, and Matt Lamishaw pa- 

 tiently worked at the computer. Roger and Katy 

 Payne provided material and intellectual support. 

 George C. Williams, Kenneth Norris, Randall 

 Wells, J. L. McHugh, Douglas Smith, and an 

 anonymous reviewer for theFts/ieryBu//e<m criti- 

 cally read the manuscript. Charles Walcott sup- 

 ported and encouraged all phases of the research. 

 We are especially grateful to him. 



This study was supported by the New York 

 Zoological Society, the Committee for Research 

 and Exploration of the National Geographic Soci- 

 ety, and the Program for Neurobiology and Be- 

 havior of the State University of New York at 

 Stony Brook. 



LITERATURE CITED 



Bateson, G. 



1974 Observations ofa cetacean community, /n J. Mcln- 

 tyre (editor), Mind in the waters, p. 146-165. Charles 

 Scribner's Sons, N.Y. 

 BRANDHORST, W., AND J. P. CA.STELLO. 



1971. Evaluacion de los recursos de anchoita (Engmulis 



