776 



Fishery Bulletin 91(4). 1993 



160° 



140" 



130' 120" 110° 



WEST LONGITUDE 



90' 



Figure 1 



Distribution of the eastern spinner dolphin (Stenella longirostris orientalis) in the eastern 

 tropical Pacific. Squares represent positions of all sightings from the 1986-90 Monitoring of 

 Porpoise Stocks (MOPS) surveys used in the abundance estimate (a total of 236 sightings). 

 The outer grey line represents the MOPS study area, and the inner solid line represents the 

 area occupied by the eastern spinner dolphin. 



For JV,., Smith (1983) used an estimated abundance 

 of 293,000 animals for the eastern spinner dolphin, 

 which was based on combined data from aerial and 

 research vessel surveys conducted in 1979 (Holt and 

 Powers, 1982). Recently, the U.S. National Marine Fish- 

 eries Service conducted large-scale research vessel sur- 

 veys annually for five years (1986-90) as part of the 

 Monitoring of Porpoise Stocks (MOPS) program, re- 

 sulting in a revised estimate of abundance of 632,700 

 (Wade and Gerrodette, 1992b 1 ). For a number of rea- 

 sons, discussed below, this estimate should be more 

 reliable (both more precise and less biased) than the 

 1979 estimate of abundance used by Smith (1983). 



This revised abundance estimate was sufficiently dif- 

 ferent from the 1979 estimate to justify re-estimation 

 of historical population size for the eastern spinner 

 dolphin. Additionally, estimates of the historical kill 

 have also been revised since Smith (1983), although 

 they did not differ greatly from the previous estimates 

 (Lo and Smith, 1986; Wahlen, 1986). Therefore, I esti- 

 mated the historical population size for the eastern 

 spinner dolphin using the same methods and the same 

 ranges for the parameters R,„ and MNPL as Smith 

 (1983), but with revised abundance and fishery mor- 



'Wade. P. R., and Gerrodette, T. 1992b. Estimates of cetacean abun- 

 dance in the eastern tropical Pacific. Paper SC/44/018 presented at 

 the annual meeting of the Int. Whal. Comm.. June 1992. 



tality estimates. This resulted in new estimates of rela- 

 tive population size for this stock. 



Confidence limits for the estimates of relative popu- 

 lation size were calculated by using Monte Carlo simu- 

 lation methods (Buckland, 1984). These confidence lim- 

 its only incorporated uncertainty due to sampling error 

 of the current population estimate and the mortality 

 estimates. They did not incorporate uncertainty in the 

 model parameters R,„ and MNPL. Therefore, confidence 

 intervals were calculated for all parameter combina- 

 tions. 



Population abundance estimate 



The MOPS cruises (1986-90) had approximately five 

 times more kilometers of survey effort in the region 

 occupied by eastern spinner dolphins than the 1979 

 survey. About 75% of the 1979 survey was concentrated 

 within 1,000 km of the coast, whereas the range of the 

 eastern spinner dolphin is up to 2,000 km from the 

 coast (Fig. 1, Perrin et al., 1985). Therefore, the 1979 

 survey provided little coverage of the western half of 

 the area occupied by eastern spinner dolphin (Holt 

 and Powers, 1982, fig. 1). Raw sample sizes show the 

 large difference in the quantity of data: a total of 285 

 schools containing eastern spinner dolphins were re- 

 corded during the MOPS surveys; a total of only 41 

 schools, during the 1979 survey. The large increase in 



