700 



Fishery Bulletin 98(4) 



X 



120 



100 



80 



60 



40 



20 



-20 



NEO(«=125) ES(/7=I77) 



Species (evasion under the net) 



Figure 3 



"Box and whiskers" plot for estimated evasion index by set for northeastern offshore spotted 

 (NEOl and eastern spinner (ES) dolphins when dolphins evaded under the net (Mexican fleet. 

 PNAAPDdata 1992-95). 



herds were called "the untouchables" by fishermen (Pryor 

 and Norris, 1978) because the animals were capable (and 

 still are) of completely evading the fishing operation even 

 before the net has been set (National Research Council, 

 1992). 



Several authors have suggested that the reasons for the 

 detectable spatial patterns in evasive behavior seem to be 

 related to the learning capacity of these mammals. Stuntz 

 and Perrin^ stated that dolphins (Stenella spp.) were more 

 difficult to capture in areas where fishing effort had been 

 greatest, therefore concluding that dolphins have learned 

 to evade capture more effectively. Hewitt (1985) pointed 

 out that the dolphins' reaction distance to survey vessels 

 "may vary between geographic areas with the degree of 

 animal naivete." This possibility was also considered by 

 Hall and Boyer (1986) as an explanation for the spatial 

 heterogeneity of mortality rates. 



Although dolphin learning seems to be a logical expla- 

 nation for geographic differences in evasive behavior, ade- 

 quate measures for the dolphins' experience (i.e. their 

 exposure to the tuna fishery) have to be designed. A mea- 

 sure may be the historical fishing effort (no. of chases per 

 one-, two- or five-degree quadrant by the international 

 fleet on the northeastern offshore spotted dolphin accu- 

 mulated from 1959, when the tuna purse-seining fishery 

 started, until today) standardized with respect to herd 

 density (as described by Polachek, 1987; Reilly, 1990; or 

 Reilly and Fiedler, 1994). Standardization is necessary 

 because dolphins in areas with fewer herds will presum- 

 ably have more opportunities to practice evading capture 

 in comparison with areas where there are more herds 

 (higher density) and effort has apparently been the same 

 (number of chases per quadrant). 



Furthennore, the number of dolphins that evaded capture 

 during a set might also be affected (reduced) by the number 

 of speedboats, the fishing vessels power, and the presence of a 

 helicopter; explosives were also used to herd dolphins several 

 years ago. In addition, the dolphins' distribution ( and hence 

 possibly the fleet's effort) is influenced by changes in oceano- 

 graphic features between seasons (Au and Perryman, 1985; 

 Reilly, 1990) and yeai's (Fiedler and Reilly, 1994; Reilly and 

 Fiedler, 1994 ). Therefore, all variables recorded by the observ- 

 ers regarding the fishing operation (vessel power, number of 

 speedboats, aerial assistance, use of explosives), time, and 

 environmental features should be considered. However, this 

 thorough analysis, including the important (according to the 

 literature) variable "fishing effort" was not performed with 

 the available data because the effort of the Mexican fleet 

 should not be considered representative of the international 

 fleet (Mexican vessels tend to fish closer to Mexico than do 

 other fleets ). Our future research aims to consider these vari- 

 ables and the effort of the complete international fleet, which 

 is possible only with data from the lATTC. 



Differences between stocks 



In the mouth of the Gulf of California (area 3, Fig. 1), 

 eastern spinner dolphins ^Stenella longirostiis orientalis) 

 showed significantly higher estimated evasion indices than 

 those for northeastern offshore spotted dolphins (Stenella 

 attenuata) in our study (Fig. 2). A stronger evasion behav- 

 ior by the eastern spinner dolphin has been described by 

 other authors (Norris et al, 1978; Pryor and Kang, 1980) 

 and may relate to different evasive strategies used by the 

 two stocks to evade capture in the net, some of which were 

 analyzed in our study. 



