FISHERY BULLETIN: VOL, 77. NO, 4 



time and were thus in large groups. Table 6 shows 

 a few instances of this behavior, when we were 

 certain whether or not the dolphins had been feed- 

 ing. The data confirm the impression that dolphins 

 which had been in or near feeding bouts ap- 

 proached the boat, while groups which had not 

 been surface feeding avoided or ignored it. When 

 the dolphins associated with the boat, a high level 

 of acrobatic noisy leaps was also evident. Of 

 course, it is likely that the dolphins were merely 

 associating with the boat, and that the humans on 

 board were irrelevant. 



Dolphins also associated with the southern 

 rightwhale, Eubalaena glacialis, and the sea lion, 

 Otaria flavescens, by moving around and among 

 them in a manner similar to that described for 

 bottlenose dolphins (Wiirsig and Wiirsig 1979). 

 This activity also appesu-ed related to whether 

 dolphins had or had not been surface feeding. Dif- 

 ferences in behavior between "Ted" and nonfeeding 

 dolphins were striking, and we labeled them dif- 

 ferent "activity levels," following the definition of 

 this term by Norris and Dohl (1980). 



After feeding, dolphins at times balanced kelp 

 (pieces oi Macrocystis sp.) on their pectoral flip- 

 pers. In one individual this persisted for at least 1 

 h. The activity of kelp-balancing, we suspect, may 

 be termed "play." 



Dolphins interacted with other odontocete ceta- 

 ceans as well. Groups of from one to six killer 

 whales, Orcinus orca, appeared for brief periods 

 throughout the year (Table 7 ). On the six occasions 

 when we saw them within about 1 km of dusky 



Table 6. — Number of times that dusky dolphin groups as- 

 sociated with the boat by orienting towards it and "bow riding," 

 and number of times they avoided or ignored the boat; both 

 relative to feeding activity (A significant difference from C, 

 P<O.0r, B significant difference from D, P<0.001; chi-square 

 goodness of fit test). 



Category 



Moved to boat 



Ignored boat 



Group not seen surface feeding A 10 B 18 



Group seen surface feeding 

 previously on same day C 28 D 1 



Table 7. — The number of days per month when Grampus 

 griseus and Orcinus orca were seen in the Golfo San Jose study 

 area. 



dolphins, the dolphins moved rapidly in a tightly 

 bunched group away from the killer whales. On 

 three occasions when dolphins were within 1 km of 

 land, they moved towards the shore, and then pro- 

 ceeded along shore in water less than 1 m deep, the 

 closest to shore we ever saw this species. It was 

 reported to us by reliable observers (Jen and Des 

 Bartlett^) that killer whales in our study area once 

 surfaced within a school of dusky dolphins, with 

 one whale dripping blood from its mouth, perhaps 

 indicative of having actually fed on a dolphin. 

 However, we have no conclusive evidence that 

 killer whales habitually feed on dusky dolphins. 



Risso's dolphin Grampus griseus, associated 

 with dusky dolphins from October through March 

 1974-75 and 1975-76 (Table 7). From two to six 

 individuals consistently stayed within 1-3 km of 

 feeding dusky dolphins during this time. It is pos- 

 sible that Risso's dolphins were feeding on large 

 fish found in the vicinity of southern anchovy, but 

 we have no data for this assumption. Whether or 

 not the larger cetaceans were of actual help to 

 dusky dolphins in finding food, as has been 

 suggested by Norris and Prescott (1961) for pilot 

 whales followed by bottlenose dolphins, and by 

 northern right whale dolphins, is not known. 



The bottlenose dolphin, Tursiops truncatus, was 

 found in small groupings of 8-22 animals in the 

 study area (Wiirsig 1978). They usually stayed in 

 shallower water than did dusky dolphins, and 

 were never observed moving in water >39 m. 

 There was, however, some overlap in area covered 

 by both species. On only eight occasions were both 

 species found within 0.5 km of each other. When 

 they were relatively close, each species continued 

 on its previous course, and no interactions ap- 

 peared to take place ( although they may ha ve been 

 interacting by sound). This appEU-ent lack of in- 

 teraction was especially striking because both 

 dolphin types associated with right whales and sea 

 lions. Dusky dolphins were more abundant when 

 bottlenose dolphins were not, and vice versa (Fig- 

 ure 12). 



DISCUSSION 



Dusky dolphins were present in Golfo San Jose 

 during most or all of the year, but were located in 

 the southeast portion, in the study area, mainly 

 during spring and early summer. They did not 



yen and Des Bartlett, wildlife photographers, P.O. Box 17323, 

 Tucson, AZ 85731. pers. commun. November 1974. 



882 



