PASSIVE BEHAVIOR BY THE SPOTTED 



DOLPHIN, STENELLA ATTENUATA, IN 



TUNA PURSE SEINE NETS 



The purse seining method of catching yellowfin 

 tuna, Thunnus albacares, in association with 

 schools of dolphins in the eastern tropical Pacific 

 has been described by Perrin ( 1969). The primary 

 target species, in order of importance, are the spot- 

 ted dolphin, Stenella attenuata, and the spinner 

 dolphin, S. longirostris , with occasional net sets 

 made on schools of the common dolphin, De/p/imws 

 delphis. Schools of these dolphins ranging in size 

 from 50 to several thousand are herded with 

 speedboats and encircled with a purse seine net 

 that is from 900 to 1,400 m long and as much as 

 130 m deep. After the dolphins are encircled, the 

 bottom of the net is pursed, entrapping the mam- 

 mals and any tuna that are associated with the 

 school. Presently, neither the mechanisms nor the 

 function of the close association of the yellowfin 

 tuna with these schools of dolphins in the eastern 

 tropical Pacific are clearly understood. 



Studies of the behavior of dolphins while captive 

 in a purse seine net were pursued during the char- 

 tered cruise of the commercial seiner MV 

 Elizabeth C. J., in October 1976 (Norris et alM. 

 Prior to this cruise, however, beginning in 1973, an 

 intensive effort was made to develop net modifica- 

 tions and fishing techniques that would decrease 

 the incidental killing of the mammals. Much of 

 this effort was based upon general, to date unpub- 

 lished, observations of captured dolphins and tuna 

 made by National Marine Fisheries Service 

 (NMFS) observers and technologists. One of the 

 behavioral patterns of the spotted dolphin first 

 noted by NMFS divers in 1973 and then recognized 

 for its contribution to incidental mortality in 1975 

 is termed here "passive" behavior. 



In the fall of 1975, NMFS chartered the purse 

 seiner MY Bold Contender to carry on fishing gear 

 dynamics research aimed at reducing incidental 

 dolphin mortality. Part of this research included 

 the evaluation of the use of a two-man inflatable 

 raft as a dolphin rescue platform during and after 

 the release procedure known as the "backdown" 

 (Perrin 1969; Coe and Sousa 1972). A face mask 

 and snorkel were worn by the person in the rescue 



'Norris, K. S., W. E. Stuntz, and W. Rogers. 1978. The be- 

 havior of porpoises and tuna in the eastern tropical Pacific 

 yellowfin tuna fishery - preliminary studies. Available U.S. Dep. 

 Commer, Natl. Tech. Inf. Serv., Springfield, Va., as PB-283970, 

 86 p. 



nSHERY BULLETIN: VOL. 78, NO. 2, 1980. 



raft to enable him to: 1) signal when tuna were 

 approaching the release area during backdown, 2) 

 keep track of sharks, particularly the oceanic 

 whitetip shark, Carcharinus longimanus, both in- 

 side and outside of the net, 3) locate and release 

 any stray dolphins and, 4) observe the dynamics of 

 the net modification being tested during the 

 cruise. Notes on the behavior of the captured dol- 

 phins during backdown were recorded after each 

 of the 25 net sets in which the rescue raft was used. 

 The underwater passive behavior of spotted dol- 

 phins was first noted in the eighth net set of the 

 cruise and in 14 subsequent sets. 



The passive behavior manifests itself in possibly 

 two forms during the backdown release procedure. 

 The first, which has been described as "rafting" by 

 Norris et al. (see footnote 1 ), consists of groups of 

 5-50 or more spotted dolphins hanging tail-down 

 at or near the surface and showing no overt reac- 

 tions to their surroundings. This type of passive 

 behavior can be seen from the deck of the seiner 

 and is generally displayed from the time the net is 

 pursed through the backdown (about V2 h). There 

 is a steady increase in the number of rafting ani- 

 mals until backdown begins and then an apparent 

 sharp decrease as backdown proceeds. The de- 

 crease in rafting may be due to the crowding and 

 confusion during that period. Rafting behavior 

 simplifies the effective release of these animals by 

 backdown, because the net is actually pulled out 

 from under the "raft" of dolphins as it remains 

 relatively stationary in the water. Observations 

 during the Elizabeth C. J. cruise showed that in 

 every school of captured spotted dolphins some 

 portion exhibited this behavior. 



Dolphins in the captured school that are not in 

 rafts during backdown are either actively swim- 

 ming, usually in the horizontal plane directly 

 away from the advancing wall of the net, or dis- 

 play the second manifestation of passive behavior. 

 Prior to backdown, rafting dolphins are occasion- 

 ally observed to sink tail first to depths up to 5 m 

 before they swim awkwardly back to the surface, 

 breathe, and sink again. During backdown, how- 

 ever, there are occasionally relatively large num- 

 bers of animals that sink to lie on the webbing 

 (Figure 1) and many more that show signs of the 

 sinking behavior and drop out of the rafts. 



During backdowns on iheElizabeth C. J. cruise, 

 from 3 to an estimated 75 spotted dolphins and, in 

 one set, 2 bottlenose dolphins, Tursiops truncatus, 

 were observed lying on the webbing in the bottom 

 of the backdown channel. These animals, which 



535 



