Meckel et al.: Evasive behavior of Stenella attenuata and 5. longirostris 



693 



in relation to estimated pre-exploitation levels (Wade, 

 1993; Wade". Therefore, these stocks have been desig- 

 nated as depleted under the U.S. Marine Mammal Protec- 

 tion Act. However, when compared to abundance estimates 

 from 1986-90 research sui-veys (Wade and Gerrodette, 

 1993), preliminary estimates from the most recent surveys 

 (Gerrodette-) show a noticeable increase in the abundance 

 of the northeastern offshore spotted and the eastern spin- 

 ner dolphin compared to previous estimates (Wade and 

 Gerrodette, 1993). 



In addition to abundance estimation, the total inciden- 

 tal mortality (all species and stocks involved in the fishery 

 and for the international fleet) was estimated at 3274 for 

 1995, which represents 0.03'^i of the total population (Hall 

 and Lennert, 1997) of 9.6 million for all dolphin species 

 involved in the tuna fishery (Wade and Gerodette, 1993). 

 The Mexican Programa Nacional para el Aprovechamiento 

 del Atiin y de Proteccion de Delfines (PNAAPD) estimated 

 the incidental mortality (of all species and stocks) of the 

 Mexican fleet to be 1819 dolphins in 1994 (Compean et 

 al.3). 



Important efforts have been aimed at reducing mortal- 

 ity rates, such as improving fishing gear (Barham et al., 

 1977; Coe et al., 1984), placing quotas on the number of 

 dolphins killed for each stock (Wade^), placing a quota on 

 the number of dolphins killed by fishing vessel and making 

 the backdown procedure mandatory (Colson, 1992). 



Other efforts in reducing mortality have been directed 

 towards research on dolphin behavior during tuna purse- 

 seining (Norris et al., 1978; Pryor and Kang^). Mortality 

 rates may also be reduced by understanding more about 

 the dolphins' behavior, so that they are less likely to injure 

 themselves or die during fishing practices (Norris et al., 

 1978). 



Some research indicates that some dolphin stocks may 

 have reduced mortality through behavioral adaptations. 

 The first ethological research on Stenella attenuata and 

 Stenella longirostris during fishing operations was accom- 

 plished by Norris et al. (1978). They focused on overall 

 herd movements and interanimal distances and devel- 

 oped the first ethogram for these animals during a net 

 set for tuna. Pryor and Kang (1980) also made obser- 

 vations during seining operations but focused more on 

 individual and subgroup behavior Although Norris et al. 

 (1978) described high stress levels in dolphins that were 

 involved in sets, Pryor and Kang interpreted their own 

 data as indicating much learned adaptive behavior and 

 low stress levels. By analyzing records from obsei-vers on 

 tuna boats, Stuntz and Perrin'' noted and discussed the 

 fact that dolphins (Stenella spp. ) were more difficult to 

 capture in areas where fishing effort had been higher and 

 the authors concluded that dolphins had been able to learn 

 to evade capture from tuna boats. 



Data collected by observers on board tuna purse-sein- 

 ers from 1992 to 1995 (PNAAPD database) were used to 

 investigate whether the evasive behavior' of northeastern 

 offshore spotted dolphins {Stenella attenuata ) during fish- 

 ing for yellowfin tuna varied geographically in the EPO. 

 Differences in evasive behavior between the northeastern 

 offshore spotted and the eastern spinner dolphin (Stenella 

 longii'ostris orientalis) were also assessed. Furthermore, 

 differences in evasive strategies'* between these two stocks 

 were described. 



Methods 



Data collection 



^ Wade, P. R. 1993. Estimation of historical population size 

 of the northeastern stock of offshore spotted dolphin iStenelta 

 attenuata). Southwest Fisheries Science Center Administra- 

 tive Report LJ-9.3-18, 18 p. Southwest Fisheries Science Center, 

 Natl. Mar Fish. Serv., NOAA, 8604 La Jolla Shores Drive, La 

 JoUa.CA 92038-0271. 



^ Gerrodette, T. 1999. Preliminary estimates of 1998 abundance 

 of four dolphin stocks in the Eastern Tropical Pacific, http:// 

 swfsc.ucsd.edu/lDCPA/Abund98.html. Southwest Fisheries Sci- 

 ence Center. Natl. Mar. Fish. Serv., NOAA, P.O. Box 271. La 

 Jolla, CA 92038 



^ Compean J., G. A., I. Mendez G.-H. and I. Mendez R. In 

 preparation. Annual estimates of incidental mortality for dol- 

 phin species associated with the Mexican tuna fishery during 

 1992-1995. Programa Nacional para el Aprovechamiento del 

 Atiin V de Proteccion de Delfines (PNAAPD). ICjn 107 Carretera 

 Tijuana-Ensenada, Campus CICESE, 22800 Ensenada, B.C.. 

 Mexico. 



■" Wade, P. R. 1993. Revised estimates of fisheries kill of dolphin 

 stocks in the eastern tropical Pacific, 19.59-1972. Southwest 

 Fisheries Science Center Administrative Report LJ-93-17. 19 

 p. Southwest Fisheries Science Center, Natl. Mar Fish. Sei-v.. 

 NOAA, 8604 La Jolla Shores Dr. La Jolla, CA 92038-0271. 



= Pryor, K., and I. Kang. 1980. Social behavior and school struc- 

 ture in pelagic porpoises tStenel/a attenuata and S. longirostris:) 

 during purse seining for tuna. Southwest Fisheries Center 

 Admin. Rep. LJ-80-11C. Southwest Fisheries Science Center, 

 8604 La Jolla, Shores Drive, La Jolla, CA 92038-0271. 



Data from 1992 to 1995 from the PNAAPD database (Mex- 

 ican fleet) were used to study evasive behavior of two 

 stocks of different species in the EPO: the northeastern 

 offshore spotted dolphin, a stock of the pantropical spot- 

 ted dolphin (Stenella attenuata), and the eastern spinner 

 dolphin (Stenella longirostris orientalis). a subspecies and 

 stock of the spinner dolphin (Stenella longirostris) (Dizon 

 et al., 1994; Perrin and Gilpatrick, 1994; Perrin et al., 

 1994). 



Observers (also referred to as scientific technicians) from 

 the Inter-American Tropical Tuna Commission (lATTC) 



Stuntz, W. E., and W. F Perrin. 1979. Learned evasive behav- 

 ior by dolphins involved in the eastern tropical Pacific tuna 

 purse seine fishery, Unpubl. abstract. Third Biennial Confer- 

 ence on the Biology of Marine Mammals, October 7-11. 1979, 

 Seattle, WA. 



Evasive behavior: In this study, calculated as a percentage of 

 the estimated number of dolphins that avoided capture in the 

 net relative to the best estimate of the initial herd size (before 

 the set started) by the obsei'ver or scientific technician. See 

 "Methods" section. 

 ' Evasive strategies: movements of the herd or subgroups of dol- 

 phins in relation to seiner, speedboats, and net (when it has 

 been set and before rings up) by which dolphins attempt to 

 avoid capture in the net. 



