692 



Abstract— This study examines the 

 question of whether the evasive behav- 

 ior of northeastern offshore spotted dol- 

 phins (Stenella atteniwta) during fish- 

 ing for tuna (by the Mexican fleet) 

 varies in geographic areas of the east- 

 ern Pacific Ocean (EPO). It also investi- 

 gates whether evasion differs between 

 northeastern offshore spotted and east- 

 em spinner dolphins tStenella longiro- 

 stris orientalis). Observations recorded 

 in the database of the Mexican Pro- 

 grama Nacional de Aprovechamiento 

 del Atun y de Proteccion de Delfines 

 (PNAAPD) from 1992 to 1995 were ana- 

 lyzed. The calculated evasion index was 

 the estimated percentage of dolphins 

 that evaded capture in relation to the 

 herd's estimated initial size in each set. 

 Evasion index by set was averaged in 

 2x2 quadrants and then used to draw a 

 contour map. Three areas were outlined 

 with low (259^), medium (44.44'7, ), and 

 high (71.80%) median evasion indices. 

 These areas were significantly differ- 

 ent (/'<0.0001) according to the Krus- 

 kal-Wallis nonparametric multisample 

 test, thus indicating a spatial pattern 

 in evasive behavior of northeastern off- 

 shore spotted dolphins during fishing 

 operations of the Mexican fleet. Spatial 

 patterns in evasive behavior might be 

 related to the dolphins' learning capac- 

 ity, hence experience of individual dol- 

 phins or herds with tuna purse-seining 

 in the EPO should be estimated to 

 demonstrate this. To be representative, 

 future research should utilize avail- 

 able historical fishing effort data for 

 the international fleet. Furthermore, a 

 multivariate approach to this issue is 

 necessary. 



One of the investigated areas (mouth 

 of the Gulf of California) was further 

 analyzed regarding differences between 

 two stocks of dolphins. Evasion indices 

 for eastern spinners were significantly 

 different from those for northeastern 

 offshore spotted dolphins (P<0.0001, 

 Kolmogorov-Smirnov two-sample test). 

 This difference may correspond to dif- 

 ferent evasive strategies used by the 

 two stocks to evade capture in the net, 

 such as evasion under the net and 

 dispersion (division of herd into sub- 

 groups during the set). Eastern spin- 

 ners apparently evaded more frequently 

 than northeastern offshore spotted dol- 

 phins by diving under the net. During 

 the three set stages of tuna fishing 

 (before chase, during chase, and during 

 encirclement), eastern spinner dolphins 

 dispersed less often than spotted dol- 

 phins, behavior that may permit them 

 to coordinate their evasive movements 

 more effectively than northeastern off- 

 shore spotted dolphins. Evasion over 

 the net was rarely ob.served in cither 

 stock. 



Manuscript accepted 1 July 2000 

 Fish. Bull. 98: 692-70.3 (2000). 



Evasive behavior of spotted and spinner dolphins 



iStenella attenuata and S. longirostris) 



during fishing for yellowfin tuna 



iThunnus albacares) 



in the eastern Pacific Ocean 



Gisela Meckel 



Facultad de Ciencias Marinas 



Universidad Autonoma de Baia California 



Placido Mata 2309 Depto D-5 



Condominio Las Fincas 



22820 Ensenada 



Baia California, Mexico 



E-mail address: gheckeleitelnornet 



Kim E. Murphy 



Facultad de Ciencias Mannas 

 Universidad Autonoma de Baia California 

 Km 103 Carretera Ti|uana-Ensenada 

 22800 Ensenada, 

 Baia California, Mexico 



Guillermo A. Compean Jimenez 



Programa Nacional de Aprovechamiento del Atun y 



de Proteccion de Delfines 



Km 107 Carretera Ti|uana-Ensenada 



Campus CICESE 



22800 Ensenada 



Baia California, Mexico 



The tuna fishery in the eastern Pacific 

 Ocean (EPO) sets purse seines in three 

 major modes: school, log, and dolphin 

 fishing. Yellowfin tuna iTIuiiuuis alba- 

 cares} and certain species of dolphins 

 are found to associate in the EPO. The 

 spotted dolphin (Stenella attenuata) is 

 by far the most important species from 

 the point of view of its frequency of 

 association with tuna and its use by 

 fishermen for catching tuna (Perrin, 

 1969). The frequent appearance of spin- 

 ner dolphins (Stenella longirostris) in 

 sets also makes this species significant, 

 although in almost all cases it appears 

 in mixed herds with the spotted dolphin 

 (National Research Council, 1992). 



During "dolphin fishing" or "fishing 

 on dolphins," the net is set around the 

 tuna and the dolphins after a period of 

 chase. Once a dolphin herd is sighted 

 with high-power binoculars (25x), foin- 



to six speedboats are lowered, and the 

 chase begins. The speedboats herd the 

 dolphins and the accompanying tuna 

 into a tight group that can be encircled 

 by the seiner. The dolphins may try 

 to evade the boats to avoid capture. 

 Those that do not succeed are released 

 by the fishing crew during the "back- 

 down" procedure (a procedure in which 

 the vessel is run in reverse to pull the 

 corkline underwater and thus release 

 the dolphins) (Barham et al., 1977). 



Impact of the fishery on cetacean 

 populations has been assessed by esti- 

 mating cetacean abundance and distri- 

 bution involved in tuna purse-seining, 

 as well as by estimating mortality rates. 

 By 1988, the estimated abundance of 

 the northeastern offshore spotted dol- 

 phin had been reduced to between 19% 

 and 28% and that of the eastern spin- 

 ner dolphin and to between 32'7t to 58% 



