WURSIG and WURSIG: BEHAVIOR AND ECOLOGY OF THE DUSKY DOLPHIN 



\.0 

 0.8 

 0.6 

 0.4 

 0.2 



Lo atnorh y rnhut ob: 



JAS0NDJFMAMJJASOND,JFM 



1974 



1975 



1976 



FIGURE 12. — Occurrence data from Figure 2a in conjunction 

 with comparable data on bottlenose dolphins during the same 

 period. The two species occurred in the study area with approxi- 

 mately opposite frequency, i.e., when one species was abundant, 

 the other one was less often seen. 



appear to avoid low ( = 10° C) temperatures, but 

 may have been avoiding higher ( > 1 8° C ) tempera- 

 tures near shore in mid- and late summer. At 

 those times, they were found most often in cooler 

 waters near the mouth of the bay. In Wiirsig (in 

 press), it was shown that during that time they 

 moved outside of the bay as well. Yet they did not 

 show a well-defined seasonal migration pattern, 

 and marked individuals were resighted in the 

 same location during different seasons. Studies of 

 groups of Tursiops sp. have indicated that degree 

 of migration may be different for different popula- 

 tions. For example, bottlenose dolphins off Cape 

 Hatteras, N. C, migrate (Mead 1975), while those 

 of our study site did not ( Wiirsig and Wiirsig 1979). 

 Shane (1977) and Irvine et al. (see footnote 2) 

 reported localized seasonal movements of 

 bottlenose dolphins in the Gulf of Mexico, with 

 differences between their East Texas and West 

 Florida study sites. Degree of seasonally related 

 movement probably hinges on several environ- 

 mental and ecological variables, but an important 

 factor for dolphins in temperate waters may be 

 food availability (suggested by Norris 1967 , Evans 

 1971, and others). Thus it seems likely that dusky 

 dolphins moved with the food supply most of the 

 year. The main prey item appears to be southern 

 anchovy and we have some evidence that it is 

 found in deeper offshore waters in spring and 

 summer (Ciechomski 19651 and in large concen- 

 trations near the mouth of Golfo San Jose in late 

 summer (Brandhorst and Castello 1971), at the 

 same time dusky dolphins were feeding there. 



Gaskin (1968) stated that dusky dolphins are 

 present around the Hawke Bay area of New Zea- 



land generally only in winter and spring. He re- 

 lated this to the presence of the cold Canterbury 

 Current which comes close to Hawke Bay in 

 winter and spring. Clarke (1957) and Sergeant 

 (1962) described the seasonal migration by pilot 

 whales as being regulated mainly by seasonal 

 abundance of squid and certain schooling fish. 

 Wilke et al. (1953) also found seasonal movement 

 patterns for the Dall porpoise, Phocoenoides dalli; 

 the northern right whale dolphin, Lissodelphis 

 borealis; and the Pacific whitesided dolphin, 

 Lagenorhynchus obliquidens. Norris and Prescott 

 ( 196 1) and Evans ( 1 97 1 ) reported that the common 

 dolphin, Delphinus delphis, moved closer to the 

 shore of California in fall and winter, and moved 

 farther offshore in spring and summer. They 

 suggested that this movement was food related. 

 Brown and Norris (1956) stated that whitesided 

 dolphins off California were most often found near 

 shore in winter and spring, and offshore in sum- 

 mer and fall. They also reported that the move- 

 ments of the northern anchovy, Engraulis mor- 

 dax, corresponded with the seasonal dolphin 

 movements. Their observations of L. obliquidens 

 thus agree with those of L. obscurus of the present 

 study. 



Frequency of feeding in the highly visible man- 

 ner described, with birds flocking overhead, was 

 seasonal. It occurred less often in winter than at 

 other times. In winter, anchovy are found in water 

 > 100 m (which is deeper than Golfo San Jose), and 

 farther north, around lat. 36°-37° S (Brandhorst et 

 al. 1971). Thus it is probable that dolphins were 

 not feeding on southern anchovy in winter in the 

 atudy area. Yet it is not possible that mammals as 

 small and as constantly active as dusky dolphins 

 stopped feeding completely for several months. We 

 can only guess that other feeding was done on prey 

 below and not at the surface, and possibly more 

 individually instead of as a concerted group effort. 

 At any rate, in winter we observed very little aeri- 

 al behavior euid rapid movement usually atten- 

 dant on surface feeding. Instead, dolphin groups 

 consistently moved slowly and in small groups 

 near shore. Stomach content samples would be 

 helpful in solving this ambiguity. 



Dolphins exhibited a daily feeding cycle as well. 

 Morning surface feeding activity lasted for shorter 

 times than in the afternoon, so that dolphins were 

 more often seen feeding in the afternoon. They 

 moved in shallow water (5-10 m) in the morning, 

 but most afternoon surface feeding occurred in 

 35-45 m. We gained the impression that im- 



883 



