FISHEKY BULLETIN VOL 77, NO 2 



m) water, and moved more rapidly at that time. 

 These two distinct movements were marked as 

 well by a difference in group formation. While 

 slowly traveling near shore, the dolphin subgroup 

 was 87'^r of the time (226.2 h of 260 h) in a tight 

 formation about 10-15 m wide and 50-75 m long. 

 Because of this narrow formation, no individual 

 was far from shore, and all were in similar depth. 

 On the other hand, when dolphins moved into 

 deeper water they advanced as a wider than long 

 rank, with each animal separated from the next by 

 as much as 25 m on its flank, yet the entire group 

 presented one wide front. In this manner, the sub- 

 gi'oup was able to cover a large swath of sea (up to 

 about 300 m I as it rapidly moved ahead. During 13 

 of 134 times (9.7'^f ) that rapid movement was ob- 

 served, we noted 1-12 terns and at times gulls 

 flying in front of this advancing dolphin line and 

 diving into the water to pick up 10-15 cm long fish. 

 We suspect that they may have been near parts of 

 schools of southern anchovy, Engraulis anvhoita. 

 because of the abundance of this fish in the area. 

 The wide front movement in deeper water may 

 thus be a searching pattern by bottlenose dolphins 

 for such schooling fish. When the subgroup slowed 

 at the end of an individual-abreast run, the ani- 

 mals milled in different locations (122 of 134 

 times, 91% ), giving us the impression that they 

 were feeding in that location. However, such mil- 

 ling after rapid movement was usually short (60 

 s±30 s), and never lasted more than 5 min. At the 

 end of milling, the subgroup usually (115 of 122 

 times, 94^/ I continued to move slowly in shallow 

 water, but less often (7 of 122 times, &7c ) began a 

 new period of 16-min-long rapid movement in deep 

 water. 



Aerial Behavior 



Aerial behavior was not as frequent in the 

 bottlenose dolphin population we studied as in 

 many other species of cetaceans ( e.g., see Saayman 

 and Tayler in press; Norris and Dohl see footnote 

 3; Wiirsig and Wiirsig see footnote 4). Individuals 

 engaged in any form of aerial display <5'^f of the 

 observed time. These displays were the 1 1 leap, 2) 

 headslap, 3) noseout, 4) tailslap, and 5) kelp toss. 

 For the sake of conformity, the aerial displays 

 discussed below, except for 5l, follow the names 

 and descriptions of aerial behavior given for 

 Hawaiian spinner dolphins by Norris and Dohl 

 (see footnote 3): 



1. Leaping either produced a loud sound when 



the animal fell back into the water onto its belly or 

 side ("noisy leap"), or was relatively silent when 

 the animal arched its body during the leap and 

 reentered nose first ( "clean leap" ). Noisy and clean 

 leaps occurred at any time of day when the ani- 

 mals were moving slowly close to shore. However, 

 noisy leaps were most often performed by calves 

 and subadults (calves leaped approximately 3 

 times as often as adults), and occurred more often 

 in the afternoon ( morning, 24 leaps in 1 17 h, mean 

 = 0.21/h; afternoon, 70 leaps in 143 h, mean = 

 0.49/h; significant difference at P<0.001, testing 

 equality of percentages, Sokal and Rohlf 1969). 

 Clean leaps were performed only by adults and 

 usually occurred when the gi'oup was relatively 

 stationary in medium-deep (10 m±5 m) water. At 

 times (about 25'f , not adequately quantified for 

 exact numbers) such leaps were attended by terns 

 diving in the vicinity and feeding on small fish, 

 leading to the inference that such clean leaps, 

 which allow animals to rapidly descend headfirst, 

 may be food-related. No daily or seasonal pattern 

 was evident for clean leaps. 



2. The headslap was seen only twice in 260 h of 

 observation, each time performed by a subadult 

 while the subgroup slowly moved along shore. 



3. Noseouts, when dolphins poked their heads 

 out of the water to beyond at least one eye, were 

 about as frequent as noisy leaps and usually oc- 

 curred at the same time (i.e., more often in the 

 afternoon). 



4. Tailslaps were the most frequent form of aer- 

 ial behavior. They also occurred at any time dur- 

 ing the day, but were frequent only when some 

 disturbance occurred. Thus tailslaps were noted a) 

 when our outboard engine was started 300-500 m 

 from the dolphins, b) 14 of 95 times ( 15'y ) our boat 

 initially approached a subgroup of dolphins, c) 

 when the subgroup had been split into two adja- 

 cent groupings for several hours and then rejoined 

 (this happened five times), and d) once when a 

 light plane flew overhead at low altitude. In these 

 cases, the tailslapper was always an adult, and 

 most of the time was a large, recognizable indi- 

 vidual who was part of a stable subunit of five 

 individuals which consistently stayed together 

 (dolphin no. 1 in Wiirsig 1978). Unlike any of the 

 other forms of aerial behavior, which usually were 

 performed only once by one animal at a particular 

 time, tailslapping occurred from 10 to 20 times 

 during one bout. 



5. "Kelp tossing" usually was accompanied by J 

 high incidences of noisy leaps and noseouts. Dur- ' 



406 



