Growth and reproduction of female eastern and whitebelly spinner dolphins incidentally killed in the eastern tropical Pacific tuna purse-seine fishery

Life-history parameters are estimated for two forms of the spinner dolphin (Stenella longirostris (Gray, 1828)): the eastern subspecies (Stenella longirostris orientalis Perrin, 1990) and the whitebelly hybrid. The data analyzed (n = 3138 eastern and 3099 whitebelly spinner dolphins) were collected between 1968 and 1994 from female dolphins incidentally killed in the yellowfin tuna (Thunnus albacares (Bonnaterre, 1788)) purse-seine fishery that operates in the eastern tropical Pacific Ocean. Eastern spinner dolphins averaged 79.7 cm at birth, 121.5 cm at 1 year, and 171.5 cm at asymptotic length, and had a mean annual pregnancy rate of 0.351 and a calving interval of 2.84 years. Whitebelly spinner dolphins averaged 80.2 cm at birth, 122.5 cm at 1 year, and 177.1 cm at asymptotic length, and had a mean annual pregnancy rate of 0.329 and calving interval of 3.04 years. The mean age at attainment of sexual maturity was estimated to be 8.7 and 8.4 years for eastern and whitebelly spinner dolphins, respectively, which are 3.2 and 1.7 years older than those previously published. This difference indicates that the mean reproductive output during a female's life is lower than previously expected and likely translates into lower population growth rates.