Reproductive potential of Yellowfin Tuna (Thunnus albacares) in the western Indian Ocean

Abstract

The reproductive biology of Yellowfin Tuna (Thunnus albacares) in the western Indian Ocean was investigated from samples collected in 2009 and 2010. In our study, 1012 female Yellowfin Tuna were sampled: 320 fish on board a purse seiner and 692 fish at a Seychelles cannery. We assessed the main biological parameters that describe reproductive potential: maturity, spawning seasonality, fish condition, and fecundity. The length at which 50\% of the female Yellowfin Tuna population matures (L-50) was estimated at 75 cm in fork length (FL) when the maturity threshold was established at the cortical alveolar stage of oocyte development. To enable comparison with previous studies, L-50 also was estimated with maturity set at the vitellogenic stage of oocyte development; this assessment resulted in a higher value of L-50 at 102 cm FL. The main spawning season, during which asynchrony in reproductive timing among sizes was observed, was November February and a second peak occurred in June. Smaller females (<100 cm FL) had shorter spawning periods (December to February) than those (November to February and June) of large individuals, and signs of skip-spawning periods were observed among small females. The Yellowfin Tuna followed a ``capital-income�� breeder strategy during ovarian development, by mobilizing accumulated energy while using incoming energy from feeding. The mean batch fecundity for females 79-147 cm FL was estimated at 3.1 million oocytes, and the mean relative batch fecundity was 74.4 oocytes per gram of gonad-free weight. Our results, obtained with techniques defined more precisely than techniques used in previous studies in this region, provide an improved understanding of the reproductive cycle of Yellowfin Tuna in the western Indian Ocean.