Heckel et al.: Evasive behavior of Stenella attenuata and 5 longirost/is 



701 



Northeastern offshore spotted dolphin 

 n = 308 



100% - 

 i 80% - 

 t 60% - 

 I 40% - 

 ? 20% - 

 0% — 



BC 



DC 



Set stage 



DE 



Grouping codes: 



1 - one group 



2 - two or three groups 



3 - more than three groups 



Eastern spinner dolphin 

 n =235 



100% , 

 80% ] 

 60% 4 

 40% ! 

 20% r 



0% i 



Set stages: 



BC - before chase 



DC - during chase 



DE - during encirclement 



n 



Grouping 

 code 



02 



03 



DC 



Set stage 



DE 



Figure 4 



Dispersion (the ability of herds to "explode" (separate suddenly) into subgroups in relation to their configiaration before 

 chase) of northeastern offshore spotted and eastern spinner herds in evasion area 3 (Gulf of California mouth, Mexican 

 fleet, PNAAPD data 1992-951. 



Analysis of evasive strategies indicated that eastern 

 spinner dolphins seemed to evade capture more effectively 

 under the net than northeastern offshore spotted dolphins 

 (Fig. 3). According to stomach-content analyses of spinner 

 and spotted dolphins, the spinner dolphin is thought to 

 forage deeper (approx. 250 m) than the spotted dolphin 

 lapprox. 30 m) (Fitch and Brownell, 1968), Therefore, the 

 spinner dolphin may be more habituated to dive deeply 

 enough to escape under the bottom of the net approxi- 

 mately 200 m from the surface, before the net is pursed. 



In addition, even though herds of both stocks tended to 

 disperse from one set stage to the next, there seemed to 

 be a significant difference in dispersion between the stocks 

 (according to logistic regression analysis results), i.e. east- 

 ern spinner dolphins apparently dispersed less often than 

 the northeastern offshore spotted (Fig. 4). This evasive 

 strategy (previously described as "school exploding" by 

 Allen et al., 1980) also might have contributed to the east- 

 ern spinner dolphin's higher estimated evasion index in 

 our study iFig. 2). Because they disperse less and also tend 

 to form larger subgroups than spotted dolphins during the 

 fishing operation (Pryor and Kang, 1980), eastern spinner 

 dolphin herds could be more cohesive and this behavior 

 may coordinate their evasive movements more effectively 

 than northeastern offshore spotted dolphins. 



Moreover, the apparent higher evasive ability of east- 

 ern spinner dolphins might also be associated with dif- 

 ferent activity levels in these dolphins. Spinner dolphin 

 activity level during the set tends to be higher than that of 

 the spotted dolphin (Schramm, 1997) and could probably 

 enhance evasion by spinners. 



The frequency of evasion over the net was negligible 

 in both stocks (spotted dolphins leaped over the net in 

 only O.GV'r of the sets analyzed; spinners, in 1.8*^^. Fish- 

 ermen and scientists (Pryor and Kang, 1980) have also 



obsei^ved that dolphins encountered in fishing for tuna 

 seldom attempt to leap out of the net to escape, even 

 though individuals of both dolphin species are capable 

 of doing so. The reasons why this event seems to be so 

 rare are still unknown. Norris et al. (1978) mentioned that 

 oceanic dolphins (like spinners and spotted) in captivity 

 may take more time than coastal dolphins (like bottlenose 

 dolphins, Tursiops truncatus) to cross a white line or a 

 sunken rope on the tank bottom; this behavior might be 

 explained by the fact that the animals may come from 

 open waters where no barriers are present at all and there 

 is no confinement. Thus, confinement in a tuna set may be 

 foreign to these animals, as are the corkline and other fea- 

 tures of a set (Norris et al., 1978). 



Acknowledgments 



Data were provided by the PNAAPD. M. Hall and M. 

 Scott (lATTC) made thorough revisions of early drafts 

 of the manuscript. The final version was considerably 

 improved thanks to three anonymous reviewers. I. Mendez 

 (PNAAPD) and A. Trujillo (Universidad Autonoma de 

 Baja California) gave statistical advise. J. A. Delgado 

 (Centro de Investigacion Cientifica y Educacion Superior 

 de Ensenada) shared his mapping program. G. Heckel was 

 financially supported by Consejo Nacional de Ciencia y 

 Tecnologi'a, Mexico, the Baitenmann family, and C. Jordan. 



Literature cited 



Allen, R. L., D. A. Bratten, J. L. Laake. J. F. Lambert, 

 W. L. Ferryman, and M. D. Scott. 



1980. Report on estimating the size of dolphin schools, 

 based on data obtained during a charter cruise of the M/V 



