GROWTH AND REPRODUCTION OF THE SPOTTED PORPOISE, 

 STENELLA ATTENUATA, IN THE OFFSHORE EASTERN 



TROPICAL PACIFIC 



William F. Perrin, James M. Coe, and James R. Zweifel^ 



ABSTRACT 



This study is based on data from several thousand specimens of spotted porpoise, Stenella attenuata, 

 incidentally killed in the purse seine fishery for yellowfin tuna, Thunnus albacares. Average length 

 at birth is 82.5 cm. Gestation is 11.5 mo. Average length at 1 yr is 138 cm. Length-weight equations 

 are given for fetuses and postnatal males and females. Age was estimated from dentinal layers in 

 thin sections of teeth. A two-phase Laird-Gompertz growth model was fitted to the layer-length data. 

 Direct calibration of the dentinal layers beyond the first year (two layers) was not possible, and three 

 alternative hypotheses were considered: 1) two layers per year, until pulp cavity occluded, 2) two 

 layers per year in first year, and one per year thereafter, and 3 1 two layers per year until puberty, and 

 one per year thereafter The second alternative is most probably the correct one, but reproductive 

 parameters were estimated in terms of layers. Breeding is diffusely seasonal, with prolonged calving 

 seasons in spring and fall and a pronounced low in winter A third calving season may exist in the 

 summer. Average age at attainment of sexual maturity of males is approximately 12 layers (average 

 length about 195 cm and averjige weight about 75 kg). Females attain sexual maturity on the 

 average at about 9 layers and 181 cm. Ovarian changes in adult females are described. Apparently 

 postreproductive females were encountered in the samples. It is concluded that corpora albicantia of 

 ovulation and pregnancy persist indefinitely in the ovaries. It was not possible to distinguish between 

 the two types of corpora. Ovulation rate changes with age, from about four per layer in very young 

 adult females, to about one per layer in older females. The average calving interval is 26 mo long and 

 consists of 11.5 mo of pregnancy, 11.2 mo of lactation, and 3.3 mo of resting and/or estrus. About 9.6% 

 of lactating females are also pregnant. Pregnancy rate decreases with age, from about 0.6 per year at 

 8 to 10 layers, to about 0.3 at 16 layers. The overall sample contained 44.9% males and 55.1% females. 

 Sex ratio changes with age, from near parity at birth, indicating higher mortality rates for males. 

 Gross annual production of calves, based on age and sex structures of the sample and the estimated 

 pregnancy rate, is 14.4% of the papulation per year. No evidence was found of age or sex segregation 

 in schooling. The estimated parameters differ in a consistent way from those estimated for a 

 population of Stenella attenuata in the western Pacific, possibly reflecting the exploitation in the 

 eastern Pacific. 



Porpoises of the genera Stenella and Delphinus 

 are killed incidentally in the tuna seine fishery in 

 the eastern tropical Pacific (Perrin 1969, 1970a; 

 National Oceanic and Atmospheric Admin- 

 istration^). Since 1968, the National Ma- 

 rine Fisheries Service (NMFS) has conducted 

 a program of research into the population biology 

 of the major porpoise species to assess the impact 

 of this fishing mortality on the porpoise stocks. 

 The purpose of this paper is to describe the life 

 history of the spotted porpoise, Stenella attenuata 



^Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 

 92039. 



^National Oceanic and Atmospheric Administration. 1972. 

 Report of the NOAA Tuna-Porpoise Review Committee, Sep- 

 tember 8, 1972. Unpubl. rep. U.S. Dep. Commer, Wash., 

 D.C., 63 p. 



(Gray),^ the animal most frequently killed in the 

 fishery. 



Little information on life history of the spotted 

 porpoise has been available until very recently. 

 Harrison et al. (1972) examined the gonads of 6 

 specimens from Japan (5 males and 1 female) and 

 45 specimens of S. attenuata fi-om the eastern 

 tropical Pacific (19 males and 26 females), but did 

 not separate their results and conclusions fi-om 



Manuscript accepted December 1975. 

 FISHERY BULLETIN: VOL. 74, NO. 2, 1976. 



^The taxonomy of the spotted porpoise has long been con- 

 fused. Recent morphological studies (Perrin in press) have 

 shown that the spotted porpoise in the tuna fishery is 

 conspecific with the spotted porpoise occurring around 

 Hawaii. The name S. attenuata (Gray 1846, holotype from un- 

 known locality) applied by True (1903) to the Hawaiian form is 

 used here for the eastern Pacific form, taking priority over S. 

 graffmani (Lonnberg 1934). This usage is strictly provisional, 

 pending the completion of current taxonomic studies, when a 

 different name, such as S. dubla (G. Cuvier 1812) or S. frontalis 

 (G. Cuvier 1829) may take priority. 



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