FISHERY BULLETIN: VOL. 75, NO. 4 



1 ) The samples were biased with respect to repro- 

 ductive structure of the population, in one or 

 both years or differently in the 2 yr. 



2) The change was a real and normal event, 

 perhaps reflecting differential breeding rates 

 in single-year cohorts (the data suggest about a 

 3-yr cycle — see below — and the 1974 rates 

 were similar to those for 1971). 



3) An anomalous increase in pregnancy rate oc- 

 curred from 1973 to 1974, perhaps related to 

 exploitation in the tuna fishery or to natural 

 variation in the pelagic environment. 



The balance of evidence discussed above favors 

 the first alternative, suggesting that the Method 2 

 estimates of gross annual reproduction are the 

 more accurate of the two alternative sets of esti- 

 mates. 



DISCUSSION 



Comparison with the Spotted Dolphin 



The estimated gross reproductive rates (Method 

 1) for the eastern spinner dolphin are lower than 

 those estimated for the offshore spotted dolphin by 

 Perrinetal. (1976), 10 to 11%, as opposed to 14%. 

 Three major points of difference between the data 

 for the two species contribute to this disparity. 



1) A higher proportion of the spotted dolphins 

 were females (55.1% as opposed to 50.8% in the 

 present 1973-75 sample of eastern spinner 

 dolphins). 



2) The proportion of total females which were re- 

 productive was higher for the spotted dolphin 

 (55.7% as opposed to 43.7% for the eastern 

 spinner dolphin). 



3) There is apparently much less overlapping of 

 reproductive cycles in the eastern spinner dol- 

 phin than in the spotted dolphin in the eastern 

 Pacific. Only 1.4% of lactating females 

 examined were simultaneously pregnant, as 

 opposed to 9.6% in the spotted dolphin, a seven- 

 fold difference. At least part of this difference 

 may be inherent in the species; the rate in the 

 unexploited western Pacific population of spot- 

 ted dolphin is 5.1%(Kasuya et al. 1974), still 

 nearly four times greater than in the eastern 

 spinner dolphin. 



In summary, the data suggest that there is an 

 inherent difference in reproductive capability be- 



tween the spotted and spinner dolphins, but that 

 part of the total difference in present reproductive 

 rate may be related to differential exploitation. 

 Gross annual reproductive rate in the unexploited 

 western Pacific population of S. attenuata is esti- 

 mated at 0.094 (calculated from data in Kasuya et 

 al. 1974—0.57 female x 0.61 mature x 0.27 an- 

 nual pregnancy rate = 0.094/yr), as opposed to 

 0.144 in the exploited eastern Pacific population of 

 the same species, a possible example of difference 

 in rate correlated with differential exploitation. 

 Whereas the western Pacific population is thought 

 to be virtually unexploited and at its original size, 

 the eastern Pacific population is estimated to be at 

 62% of its original, preexploitation size (midpoint 

 estimate). 8 



Comparison with Other Cetaceans 



The estimates of gross annual reproductive rate 

 for the eastern spinner dolphin lie at the lower end 



8 Report of the Workshop on Stock Assessment of Porpoises 

 Involved in the Eastern Pacific Yellowfin Tuna Fishery. SWFC 

 Admin. Rep. LJ-76-29, Natl. Mar. Fish. Serv., NOAA, La Jolla, 

 Calif, 109 p. (Unpubl. rep.) 



TABLE 13. — Estimated gross annual reproductive rate of the 

 eastern spinner dolphin compared with estimated rates for other 

 cetaceans. Data for S. attenuata from Perrin et al. (1976) for 

 eastern Pacific and Kasuya et al. (1974) for western Pacific; for S. 

 coeruleoalba from Kasuya (1972), for Delphinus from 

 Danilevskiy and Tyutyunnikov (1968); for Globicephala from 

 Sergeant (1962); for Delphinapterus from Sergeant (1973); and 

 for Eschrichtius from Rice and Wolman (1971). Common and 

 scientific names follow Subcommittee on Small Cetaceans, Sci- 

 entific Committee, IWC (Anonymous 1975); alternative common 

 name in parentheses. 



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