AU and PERRYMAN: MOVEMENT AND SPEED OF DOLPHIN SCHOOLS 



highest initial speeds, 10.0 and 8.8 kn, respec- 

 tively, and had average speeds of 8.6 and 7.2 kn, 

 respectively. Both were moving with the waves 

 in a Beaufort 4 seastate(l 1-16 kn wind) and were 

 probably utilizing the forward momentum of the 

 swell as described by Lang (1975). The speed of 

 school 5 was also high ( >8 kn) after the first 5 

 min that it was observed. Its average speed while 

 actively avoiding the ship was 8.8 kn. This higher 

 sustained speed may have been related to its level 

 of excitement that was evident in its apparently 

 confused state, when it turned toward and then 

 away from the ship (Table 1, Fig. 4). Schools 3 

 through 7 showed some tendency for increased 

 speeds as the ship drew nearer. 



Swimming Behavior and School Structure 



Field descriptions of each school, and later 

 study of the aerial movie and still photographs, 

 revealed no obvious indication of dominant, or 

 leading, individuals or subgroups. The schools 

 were seen to progress in an almost amoeboid 

 fashion with subgroups of two to five individuals 

 striking off in different directions or accelerat- 

 ing to higher speeds, then drifting back to the 

 main body of the school if not followed by others 

 in the school. Although individuals and sub- 

 groups within a school were constantly changing 

 course, sometimes abruptly, the heading of the 

 main body of the school remained nearly 

 constant or changed slowly. The schools ap- 

 peared as loose aggregations of individuals and 

 small subgroups, most proceeding along similar 

 headings. Individualistic rather than coordi- 

 nated movements were the general feature of 

 these schools. The schools appeared to be one- 

 layered, i.e., groups of animals were not 

 swimming beneath others. 



As the vessel closed to within 2 nmi of the 

 schools, the subgroups within the schools were 

 seen to be increasingly oriented in lines abreast. 

 Animals in the rear third of a school could be 

 seen swimming faster than those ahead. The 

 result was that the width of a school in the 

 direction of its swimming axis narrowed as the 

 distance between ship and school decreased. 



DISCUSSION 



Our first impression from the observed school 

 behavior and structure was that the dolphins 

 were not noticeably disturbed by the vessel's 



presence. Only at a distance of less than a mile 

 did bunching or compaction of the relatively dis- 

 persed individuals and small subgroups become 

 common and did the schools obviously appear to 

 be running, i.e., in flight (Table 1). Radical, 

 evasive maneuvers were not regularly seen until 

 the last 200 m of distance between ship and 

 school. Examination of the relative motion plots 

 and the consecutive vectors of swimming speed 

 and course made it clear, however, that the 

 dolphins were actually avoiding the ship much 

 earlier, sometimes beginning at distances 

 approaching the horizon for a shipboard ob- 

 server. Though ship-avoidance behavior should 

 not be surprising, considering the extent of 

 "porpoise fishing," in the study area, it is a 

 behavior not easily studied from a surface 

 platform. These observations have important 

 implications relative to population studies of 

 dolphins, especially those conducted from ships. 



Because a shipboard observer sees a dolphin 

 school increasingly in profile view as distance 

 increases, an understanding of its structure and 

 behavior is helpful for proper interpretation of 

 its characteristics. A travelling school appears to 

 be a loose aggregation of relatively widely sep- 

 arated individuals or subgroups of 2-5 animals. 

 Rather than being made up of relatively few, 

 tight subgroups of various sizes, as observed 

 for spotted dolphins in a purse seine (Norris et al. 

 1978), most of the animals in these schools 

 appeared to be swimming independently, as 

 individuals or in pairs. This school configuration 

 appeared typical all during vessel avoidance, 

 except at radial distances of less than a mile from 

 the ship. 



The schools we observed remained incon- 

 spicuous to the shipboard observers because they 

 swam smoothly, without much splashing, at 

 speeds that averaged 6.8 kn. Even at swimming 

 speeds of 7-9 kn, the animals often broke the 

 water surface with little commotion and swam 

 most of the distance between breaths just under 

 the surface. Bursts of higher speed, with 

 attendant long leaps (2-3 body lengths) and large 

 splashes, occurred only temporarily. 



The swimming speeds presented in Table 3 

 pertain to these pelagic dolphins when swim- 

 ming in the cruising mode, i.e., moving smoothly 

 with little splashing for sustained periods. The 

 higher observed speeds of 7-9 kn are still in the 

 upper range for prolonged cruising speeds of 

 smaller dolphins (Webb 1975). That this must be 

 so is indicated by the fact that research ships 



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