GROWTH AND REPRODUCTION OF THE EASTERN SPINNER DOLPHIN, 



A GEOGRAPHICAL FORM OF STENELLA LONGIROSTRIS 



IN THE EASTERN TROPICAL PACIFIC 



William F. Perrin, David B. Holts, and Ruth B. Miller 1 



ABSTRACT 



Estimates of life history parameters for use in assessing status of the population of the eastern spinner 

 dolphin and assessing impact of incidental mortality in the yellowfin tuna fishery are developed from 

 data on 2,675 specimens collected from 1968 to 1975. Average length at birth is 77.0 cm, gestation is 

 10.6 mo, average length at 1 yr is approximately 134 cm. Three alternative hypotheses of rate of 

 deposition of dentinal growth layers are: I) 1.5 layers/yr throughout life; II) 1.5 layers in the first year, 

 1/yr thereafter ( most favored hypothesis); and III) 1.5 layers until puberty (at 5.5 layers in females and 

 9 layers in males), and 1/yr thereafter. Males attain sexual maturity on the average at about 170 cm 

 (range 160-195) and 9-12 layers (6.0-11.5 yr), depending on the criterion of testis-epididymis weight 

 chosen. Average length at attainment of social maturity is unknown. Average length of adult males is 

 174-176 cm. Females on the average attain sexual maturity at 165 cm (5.5 layers or 3.7, 5.0, or 3.7 yr 

 under Hypotheses I, II, and III). Average length of sexually adult females is 171 cm (range 152-187). 

 Ovulation rate is 1/growth layer (1/0.67-1.00 yr) until about 10 ovarian corpora have been accumu- 

 lated, after which the rate declines. Approximately 1% of adult females are postreproductive. Best 

 estimates of annual pregnancy rate range from 0.450 (based on 1973 data) to 0.474 (based on 1974 

 data). The pooled estimate for all years' data is 0.461. The corresponding estimates of calving interval 

 (reciprocal of pregnancy rate) are 2.22 yr, 2.11 yr, and 2.17 yr, respectively. Pregnancy rate decreases 

 after age of about 12 layers (8.0, 11.5, or 10.2 yr) concomitant with increase in lactation rate. Overall 

 sex ratio is near parity, but there are about 6% more females than males in adults. Best estimates of 

 gross annual reproductive rate based on the 1973, 1974, and 1975 data are 0.099, 0.103, and 0.105, 

 respectively. The estimate based on pooled data for the 3 yr is 0.102. The estimates are compared with 

 estimates for the spotted dolphin, Stenella attenuata, and for other cetaceans. 



This paper presents the results of a study of the life 

 history of the eastern spinner dolphin, a geograph- 

 ical form 2 of Stenella longirostris (Gray 1828), in 

 the eastern tropical Pacific. The eastern spinner 

 dolphin accounted for the second-highest level of 

 incidental mortality in the purse seine fishery for 

 yellowfin tuna, Thunnus albacares, in the eastern 

 Pacific through 1975, after the offshore spotted 

 dolphin, a form of S. attenuata (Gray 1846) (Perrin 

 1969, 1975a; Perrin et al. 3 ) The purpose of the 



study was to develop estimates of life history 

 parameters for use in assessing the status of the 

 eastern spinner dolphin stock and the impact on 

 the stock of incidental mortality in the yellowfin 

 tuna fishery. Preliminary results of partially com- 

 pleted analyses reported here in full appeared in 

 Perrin et al. 4 



Very little previously published information is 

 available on growth and reproduction of the pan- 

 tropically distributed S. longirostris. Cadenat and 

 Doutre (1959) listed body weights and gonadal 



'Southwest Fisheries Center, National Marine Fisheries Ser- 

 vice, NOAA, P.O. Box 271, La Jolla, CA 92038. 



2 Perrin (1975b) gave the eastern spinner dolphin subspecific 

 rank but stated that the nomenclature is not yet resolved, be- 

 cause the holotype of the species is from an unknown locality. 

 The term "geographical form" is used here as a substitute for the 

 more unwieldy "[Stenella longirostris] subspecies (unnamed)." 

 The term "dolphin" is used in conformance with the "list of 

 smaller cetaceans recognized" adopted by the Subcommittee on 

 Smaller Cetaceans, Scientific Committee, International Whal- 

 ing Commission (Anonymous 1975). 



3 Perrin, W. F., T. D. Smith, and G. T. Sakagawa. 1974. Status 

 of populations of spotted dolphin, Stenella attenuata, and spinner 

 dolphin, Stenella longirostris, in the eastern tropical Pacific. 



Manuscript accepted February 1977. 

 FISHERY BULLETIN: VOL. 75, NO. 4, 1977. 



Working Document for Meeting of Ad Hoc Consultants Group on 

 Small Cetaceans and Sirenians (Ad Hoc Group 2), Working 

 Party on Marine Mammals, Advisory Committee of Experts on 

 Marine Resources Research ( ACMRR) of the Food and Agricul- 

 ture Organization of the United Nations, La Jolla, Calif.. De- 

 cember 16-19, 1974. SWFC Admin. Rep. LJ-74-42, Natl. Mar. 

 Fish. Serv., NOAA, La Jolla, Calif., 22 p. (Unpubl. real 



"Perrin, W. F., D. B. Holts, and R. B. Miller. 1975. Preliminary 

 estimates of some parameters of growth and reproduction of the 

 eastern spinner porpoise, Stenella longirostris subspecies. 

 SWFC Admin. Rep. LJ-75-66, Natl. Mar. Fish. Serv., NOAA, La 

 Jolla, Calif., 33 p. (Unpubl. rep.) 



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