Sexual Dimorphism in the Growth Rate of Southern Bluefin Tuna Thunnus maccoyii in the Indian Ocean

Abstract

Sexual dimorphism in growth rate is a special life history strategy of fishes in the evolution of sexual selection. To understand if this phenomenon exists in southern bluefin tuna (SBT, Thunnus maccoyii), the ages of 615 fish were determined from otolith annuli and the length-at-age data of SBTs were fitted by the von Bertalanffy growth equation and the growth rates for each sex were compared by the ARSS method. Otoliths were collected from tuna taken at their feeding ground in the central Indian Ocean (CIO, 29-33°S, 67-91°E) from June to August in 2003 and 2004 (n=418) and from their spawning ground in the adjacent waters of Java, Indonesia (10-20°S, 110-120°E) from January to February 2005 (n=197), respectively. The age and size composition of SBT differed among habitats. All fish collected from Java were sexually mature spawners and significantly larger and older (170.24±9.52 cm in mean (± SD) fork length and 16.51±2.98 years old) than those from CIO (120.07±17.44 cm and 5.83±2.36 years old) (all P values＜0.01). Female SBT grew faster but reached a smaller asymptotic length (K=0.109 year^(-1), L(subscript ∞)=192.38 cm and t0=-1.753 years) than did males (K=0.093 year^(-1), L(subscript ∞)=211.66 cm and t0=-1.602 years). The mean age of 10 years of SBT at first maturity did not differ significantly between sexes (P=0.094). Males were significantly larger than females after sexual maturation. Sexual dimorphism in the growth rate of southern blue fin tuna was probably due to tradeoffs in energy allocation between growth and reproduction.