REPRODUCTIVE BIOLOGY OF 



FEMALE SPOTTED DOLPHINS, STENELLA ATTENUATA, 



FROM THE EASTERN TROPICAL PACIFIC 



A. C. Myrick, Jr., A. A. Hohn, J. Barlow, and 

 P. A. Sloan 1 



ABSTRACT 



Reproductive parameters were estimated from about 4,700 female spotted dolphins collected in the eastern 

 tropical Pacific from 1973 to 1981. From this sample, specimens for which ages were estimated were 

 divided into two subsets and were used to estimate age-specific rates for the northern offshore stock 

 of this species. The youngest sexually mature individual was 10 years old; the oldest immature was 17 

 years; the youngest and oldest pregnant individuals were 10 and 35 years, respectively. There was high 

 individual variability in the accumulation of corpora with age; the ovulation rate appears to slow abruptly 

 after the eighth ovulation. Average age at attainment of sexual maturity (ASM) for all years ranged 

 from 10.7 to 12.2 years (x = 11.4 years) for two sets of age estimates; no significant temporal change 

 in ASM was detected. Correlation between color phase and state of sexual maturity suggests that color 

 phase may be a good indicator of maturity for this stock. The average annual pregnancy rate was about 

 0.33; this rate did not change significantly with age. The calving interval was 3.03 years (SE = 0.205). 

 The lactation period was 1.66 years, but there was a significant increase noted in the percent lactating 

 from 1973 to 1981. A low percentage of postreproductive females was found in the sample (0.4%) in- 

 dicating that reproductive senescence is of little importance in reproductive rates of this stock. 



Purse seine operations of the yellowfin tuna fishery 

 in the eastern tropical Pacific Ocean (ETP) have 

 caused high mortality of the spotted dolphin, 

 Stenella attenuata (Perrin 1969a, 1970). Estimated 

 incidental kills for the northern offshore stock of 

 spotted dolphins were between 100,000 and 400,000 

 annually throughout the 1960's and early 1970's 

 (Smith 1983). Since 1968, research efforts by the Na- 

 tional Marine Fisheries Service (NMFS) have 

 focused on assessing the biological consequences of 

 the large incidental kill of this and other affected 

 dolphins using specimens and data collected by 

 NMFS observers aboard U.S. tuna seiners. Perrin 

 et al. (1976) presented the first comprehensive 

 description of spotted dolphin life history and 

 reproduction for specimens from the ETP. The ac- 

 cumulation of thousands of additional specimens, the 

 sharp decline in dolphin mortality (Smith 1983; Ham- 

 mond and Tsai 1983), and the improvements made 

 in estimating age since that study (Myrick et al. 

 1983) have made a new analysis necessary. 



The purpose of this paper is to estimate the 

 reproductive parameters of the female spotted 

 dolphin, based on analyses which include more data 

 and a better age estimating method than previous 



1 Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 92038. 



studies. Reproductive features of the male spotted 

 dolphin (Hohn et al. 1985) and temporal trends in 

 reproduction in the northern offshore stock (Barlow 

 1985) are discussed in separate papers. 



MATERIALS AND METHODS 



Samples 



The specimens were analyzed as three samples. 

 The "overall" sample contained about 4,700 speci- 

 mens that had been collected from 1973 through 

 1981. A second sample for which ages were esti- 

 mated contained 580 specimens selected randomly 

 from more than 3,500 specimens collected in 1973 

 through most of 1978 (the 1973-78 aged sample). The 

 randomly chosen 1973-78 aged sample did not in- 

 clude any of the specimens studied by Perrin et al. 

 (1976). The third sample (the 1981 aged sample) was 

 composed of 226 specimens which had been collected 

 in 1981 and for which ages were estimated. It in- 

 cluded almost all specimens for which ovaries and 

 teeth were collected in that year. The two aged sam- 

 ples, referred to collectively as the aged sample, are 

 subsets of the overall sample In several analyses the 

 1973-78 aged sample was divided into 1973-74 and 

 1975-78 subsamples in an effort to detect possible 

 temporal changes in reproductive rates. Only the 



Manuscript accepted June 1985. 



FISHERY BULLETIN: VOL. 84, NO. 2, 1986. 



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