AbStfelCt.— An assessment of the 

 population status of the eastern 

 spinner dolphin (Stenella longirostris 

 orientalis) in the eastern tropical Pa- 

 cific is required by the U.S. Marine 

 Mammal Protection Act (MMPAl, be- 

 cause dolphin are killed in the tuna 

 purse-seine fishery. A pooled esti- 

 mate of abundance from recent 

 (1986-90) research vessel surveys, 

 in combination with estimates of 

 fisheries kills from tuna vessel ob- 

 server data, was used to estimate 

 the historical (pre-exploitation) 

 population size with a population 

 dynamics model. Estimates of rela- 

 tive population size (current popu- 

 lation size divided by historical popu- 

 lation size) were calculated by using 

 a range of values for the maximum 

 net recruitment rate and the maxi- 

 mum net productivity level (MNPL). 

 The resulting estimates of relative 

 population size ranged from 0.32 to 

 0.58, with a best estimate of 0.44 

 based on available life history data. 

 Estimates of relative population size 

 were all below the value of MNPL 

 used to calculate each estimate. Cal- 

 culation of confidence limits for rela- 

 tive population size by Monte Carlo 

 simulation showed that the precision 

 of the estimates was sufficient to 

 make a status determination. The 

 results indicated that, as of 1988, 

 the stock of eastern spinner dolphin 

 was depleted as defined by the U.S. 

 MMPA. 



Estimation of historical population 

 size of the eastern spinner dolphin 

 (Stenella longirostris orientalis) 



Paul R. Wade 



Scripps Institution of Oceanography 

 University of California. San Diego 

 La Jolla. CA 92093 



Southwest Fisheries Sciences Center. National Marine Fisheries Service. NOAA 

 PO Box 271, La Jolla, CA 92038 



Manuscript accepted 15 June 1993. 

 Fishery Bulletin 91:775-787 (1993). 



The range of the eastern spinner dol- 

 phin, Stenella longirostris orientalis 

 (Perrin, 1990), is entirely contained 

 within the eastern tropical Pacific 

 ( Fig. 1 ). An assessment of population 

 condition or status of this stock is 

 required under the U.S. Marine 

 Mammal Protection Act (MMPA), be- 

 cause eastern spinner dolphins are 

 killed in the tuna purse-seine fishery, 

 which includes some U.S. vessels, 

 that occurs in this region. The MMPA 

 requires that each marine mammal 

 population be maintained at an "op- 

 timum sustainable population" (OSP) 

 level, which has been defined by the 

 U.S. National Marine Fisheries Ser- 

 vice as a population size between the 

 maximum net productivity level 

 (MNPL) and carrying capacity (Fed- 

 eral Register, 21 December 1976, 

 41FR55536). Therefore, assessing the 

 status of a marine mammal stock in- 

 volves, if possible, determining if it 

 is above its MNPL. Populations 

 shown to be below MNPL are consid- 

 ered depleted under the MMPA. 



One method for determining a 

 population's status relative to MNPL 

 is to estimate historical abundance, 

 meaning abundance prior to significant 

 fisheries mortality, which is assumed 

 equivalent to the equilibrium popula- 

 tion size (i.e., carrying capacity). The 

 current population size is then com- 

 pared with what is thought to be the 

 MNPL for the population, given the 

 estimate of equilibrium population size 

 (Gerrodette and DeMaster, 1990). The 



historical abundance of several ceta- 

 cean populations has been estimated 

 by back-calculating from a current 

 abundance estimate, with a population 

 model and annual records of the num- 

 ber of animals harvested (Reilly, 1981; 

 Breiwick et al, 1980, 1984; Lankester 

 and Beddington. 1986). Smith (1983) 

 described a method for back-calculat- 

 ing historical population size (N h ) for 

 spinner and spotted dolphins (Stenella 

 spp.) from estimates of the current 

 population size (A 7 ,), the historical kill 

 in the tuna fishery, the maximum net 

 recruitment rate (i?,„), and the maxi- 

 mum net productivity level. He used 

 this technique to estimate historical 

 abundance for the eastern spinner dol- 

 phin, resulting in estimates of relative 

 population size ( NJN h ) for 1979 rang- 

 ing from 0.17 to 0.25. 



An estimate of N h for a population 

 of spinner or spotted dolphins, which 

 have a relatively low /?,„, can be very 

 sensitive to the estimate of A 7 ,, as long 

 as the time period between N c and 

 N h is not too great (Smith and Pola- 

 check, 1979). Over a long time pe- 

 riod (138 years), the estimate of A 7 ,, 

 has been shown to be insensitive to 

 the estimate of A 7 for a baleen whale 

 iBalaena mysticetus) population with 

 a similarly low R,„ (Breiwick and 

 Braham, 1990). However, for a popu- 

 lation that has experienced a rela- 

 tively recent decline from known 

 losses, the estimate of N h should still 

 be sensitive to the estimate of N, 

 (Gerrodette and DeMaster, 1990). 



775 



