Wade: Population size of Stenella longirostns onentalis 



785 



estimate, respectively. These recent kill estimates were 

 the highest since 1976 (Table 2), and may have been 

 high enough to prevent recent recovery of the popula- 

 tion. The most recent estimates of the abundance in- 

 dex from tuna vessel sighting data indicated the popu- 

 lation was declining from 1986 to 1991 (Anganuzzi et 

 al., 1992 fi ). However, the most recent kill information 

 indicated a substantial reduction in kill to less than 

 m of the population in both 1990 (5.378, Hall and 

 Boyer, 1992) and 1991 (5,879, Hall and Lennert, in 

 press), which resulted in an average kill per year for 

 1988-91 of 11,324, or 1.8% of the population estimate 

 of 632,700. Therefore, the current status of the popu- 

 lation is unlikely to be substantially different from 

 what it was in 1988. Managing kill levels so that they 

 do not exceed some fraction of the expected maximum 

 net recruitment rate may be the most reasonable man- 

 agement strategy for promoting recovery of the popu- 

 lation (DeMaster et al, 1992). The U.S. National Ma- 

 rine Fisheries Service has recently proposed listing 

 the eastern spinner dolphin as depleted under the U.S. 

 MMPA (Federal Register, 17 June 1992, 57FR27010). 

 A separate proposal to list the eastern spinner dolphin 

 as "threatened" under the Endangered Species Act was 

 not warranted at this time, as the population is in no 

 immediate danger of extinction (Federal Register, 19 

 October 1992, 57FR47620). Proposed international quo- 

 tas on fisheries kill for each dolphin stock in the east- 

 ern tropical Pacific (IATTC, in press; MMC, 1993), if 

 implemented, would ensure that mortality levels stayed 

 low enough to allow recovery of the population to the 

 OSP level. 



dolphin population was well below historical abundance 

 levels in 1988. Most uncertainties appear to lead to 

 over-estimates of relative population size, indicating 

 the population may be at a lower level than indicated 

 here. Calculation of confidence limits for relative popu- 

 lation size showed that the precision of the estimates 

 was sufficient to make a status determination except 

 for higher values of R m . However, higher growth rates 

 (R m > 0.04) were not supported by independent evi- 

 dence available about the population trend since 1975. 

 The results indicated that, as of 1988, the stock of 

 eastern spinner dolphins was depleted as defined by 

 the U.S. MMPA. The substantial fisheries kill that oc- 

 curred after 1988 makes it unlikely that the popula- 

 tion has experienced any significant recovery since 

 then. 



Acknowledgments 



I would like to thank Douglas P. DeMaster, Tim 

 Gerrodette, and James T Enright for the many edito- 

 rial comments that improved this manuscript consid- 

 erably. I would also like to thank an anonymous re- 

 viewer who pointed out the method of exactly 

 calculating the z values. Finally, I would like to thank 

 the many people who contributed to the collection and 

 analysis of the data used in this paper, particularly 

 the observers on the research and tuna vessels. 



Literature cited 



Conclusions 



Based on the best data available on abundance, kill, 

 and population dynamics, the population size in 1988 

 of the eastern spinner dolphin was estimated to be 

 below MNPL, within the range of 32% to 58% of pre- 

 exploitation population size. Based on available life 

 history data, the population size was estimated at 44% 

 of pre-exploitation population size. Relative population 

 size was estimated to be higher than Smith's (1983) 

 estimate for 1979, but this difference was due mostly 

 to the use of a new, better estimate of abundance, 

 rather than to a recovery of the population between 

 1979 and 1988. Although there are uncertainties asso- 

 ciated with this analysis, especially with the early kill 

 data, the results indicated that the eastern spinner 



"Anganuzzi. A. A., S. T. Buckland, and K. L. Cattanach. 1992. Rela- 

 tive abundance of dolphins associated with tuna in the eastern Pa- 

 cific Ocean: analysis of 1991 data. Paper SC/44/SM23 presented at 

 the annual meeting of the Int. Whal. Comm. 



Au, D., and W. Perryman. 



1982. Movement and speed of dolphin schools respond- 

 ing to an approaching ship. Fish. Bull. 80( 2 ):37 1-379. 

 Barlow, J., and A. Hohn. 



1984. Interpreting spotted dolphin age distribu- 

 tions. NOAA Tech. Memo. NMFS-SWFC-48, 22 p. 

 Breiwick, J. M., and H. W. Braham. 



1990. Historical population estimates of Bowhead 

 whales: sensitivity to current population size. Rep. 

 Int. Whal. Comm. 40:423^126. 

 Breiwick, J. M., E. D. Mitchell, and D. G. Chapman. 

 1980. Estimated initial population size of the Bering 

 Sea stock of bowhead whale. Balaena mysticetus: an 

 iterative method. Fish. Bull. 78( 4 1:843-853. 

 Breiwick, J. M., L. L. Eberhardt, and H. W. Braham. 

 1984. Population dynamics of western Arctic bow-head 

 whales (Balaena mysticetus). Can. J. Fish. Aquat. 

 Sci. 41:484-496. 

 Buckland, S. T. 



1984. Monte Carlo confidence intervals. Biometrics 

 40:811-817. 



1985. Perpendicular distance models for line transect 

 sampling. Biometrics 41:177-195. 



