MOVEMENT AND SPEED OF DOLPHIN SCHOOLS RESPONDING TO 



AN APPROACHING SHIP 



D. Au AND W. Perryman 1 



ABSTRACT 



Eight dolphin schools of the species Stenella attenuata, S. longirostris, and S. coeruleoalba were 

 approached by ship and observed from a helicopter in the eastern Pacific to study their response to 

 the vessel. All schools swam away from the projected track of the aproaching ship. Their 

 movement, relative to the ship, followed paths that curved around the ship. Average swimming 

 speeds while avoiding the ship varied from 5.1 to 8.8 knots. In some cases avoidance apparently 

 began at 6 or more miles away from the ship. The effect of this behavior on shipboard censusingof 

 dolphins is discussed. 



In the eastern tropical Pacific, tuna fishermen 

 encircle with purse seine nets schools of certain 

 small cetaceans, mainly spotted and spinner 

 dolphins, Stenella attenuata and S. longirostris, 

 to capture the yellowfin tuna, Thunnus albacares, 

 with which they are associated (Perrin 1969, 

 1970). The resulting incidental kill of dolphins 

 has led the National Marine Fisheries Service to 

 study the status of these cetacean populations, as 

 required by the Marine Mammal Protection Act 

 of 1972. Data collected from commercial fishing 

 boats and research vessels are important in 

 determining the distribution and abundance of 

 the dolphins. 



In the areas of intensive "porpoise fishing," 

 dolphins are apparently learning from their 

 experience with nets and fishing vessels. The 

 animals are recaptured with purse seines fre- 

 quently enough to have possibly learned to posi- 

 tion themselves within the net to better facilitate 

 their own release (Pryor and Kang 2 ). More im- 

 portantly, they may also have developed various 

 behaviors to avoid detection by a fishing vessel 

 and to reduce their chances of capture (Pryor 

 and Kang footnote 2; Stuntz and Perrin 3 ). 

 Dolphin schools, especially of the spotted and 

 spinner dolphin species, commonly swim rapidly 

 away from approaching ships. This behavior is 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, La Jolla, CA 92038. 



2 Pryor, K., and I. Kang. 1980. Social behavior and school 

 structure in pelagic porpoises (Stenella attenuata and S. 

 longirostris) during purse seining for tuna. Southwest Fish. 

 Cent. Admin. Rep. LJ-80-11C. 



3 Stuntz, W. E., and W. F. Perrin. Learned evasive behavior 

 by dolphins involved in the eastern tropical Pacific purse seine 

 fishery. (Abstr.) Third Conference on the Biology of Marine 

 Mammals, Seattle, Wash., October 7-11, 1979. _ 



3-)l~^ 



Manuscript accepted October 1981. 

 FISHERY BULLETIN: Vol. 80, NO. 2, 1982. 



our usual observation when studying dolphins 

 from research ships. 



In November 1976 we conducted a study to 

 describe ship-avoidance behavior of dolphins. 

 The purpose was to quantitatively describe 

 school trajectories around an approaching ship 

 and to evaluate the effect on shipboard censusing 

 of dolphins. This study also allowed us to 

 measure the swimming speeds of the schools and 

 to make observations on school structure and 

 behavior. 



METHODS AND MATERIALS 



We conducted this study from the NOAA Ship 

 Surveyor, a 300-ft (91.4 m) steam-powered 

 research vessel, and its Bell 4 204 helicopter. We 

 worked in the study area, the vicinity of 

 Clipperton Island (lat. 10°15'N, long. 109°10'W) 

 in the eastern Pacific, for 9 d (26 November to 4 

 December 1976). During six of these days, we 

 made observations from the helicopter, flying 

 twice daily in a crossing pattern ahead of the 

 ship's track (Fig. 1). This enabled us to detect 

 dolphin schools ahead of the ship and to follow 

 the sequence of events leading to avoidance or the 

 detection of the school by the shipboard 

 observers. The 2.5-h flights began in mid- 

 morning (ca. 0900 h) and early afternoon (ca. 

 1330 h) to take advantage of the best lighting 

 conditions for aerial observations and photo- 

 graphy. Air speed was about 80 kn (1 kn = 1.85 

 km/h) at altitudes between 1200 and 1800 ft (366- 



4 Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



371 



