Age, growth, and reproductive patterns of the Pacific white-sided dolphin (Lagenorhynchus obliquidens) taken in high seas drift nets in the central North Pacific Ocean

Biological data were collected from 341 Pacific white-sided dolphins (Lagenorhynchus obliquidens; 171 males and 170 females) taken in high seas drift nets in the central North Pacific Ocean in June-September of 1990 and 1991. Age was determined for 149 dolphins (73 males and 76 females) by counting dentinal growth layer groups and measuring postnatal dentine thickness. Reproductive status was determined for 76 females and 71 males. Ten females were sexually mature (3 pregnant and 7 lactating), as were 11 males. The estimated average length at birth was 91.8 cm (SE = 0.4787 cm; n = 4). The estimated gestation period was 11-12 months. The sex ratio was not significantly different from 1:1. The youngest sexually mature male was 10 years of age and the oldest sexually immature male 11 years. The youngest sexually mature female was 8.3 years of age and the oldest sexually immature female 11 years. The average length at sexual maturation was 173.7 cm (SE = 0.8953 cm) for males and 177.5 cm (SE = 1.5 cm) for females. Males and females reached an estimated asymptotic length of 184 and 177 cm, respectively. Estimates of early postnatal growth rates were complicated by poor growth model fit; however, both sexes appeared to grow about 4.5 cm/month during the first half year. A calving period preceded sampling during late winter and spring.