Reproductive Biology of Albacore Tuna (Thunnus alalunga) in the Western Indian Ocean

The reproductive biology of albacore tuna, Thunnus alalunga, in the western Indian Ocean
was examined through analysis of the sex ratio, spawning season, length-at-maturity (L50),
spawning frequency and fecundity. From 2013 to 2015, a total of 923 female and 867 male
albacore were sampled. A bias in sex ratio was found in favor of females with fork length
(LF) < 100 cm. Using histological analyses and gonadosomatic index, spawning was found
to occur between 10ÊS and 30ÊS, mainly to the east of Madagascar from October to January.
Large females contributed more to reproduction through their longer spawning period compared
to small individuals. The L50 (mean ± standard error) of female albacore was estimated
at 85.3 ± 0.7 cm LF. Albacore spawn on average every 2.2 days within the spawning
region and spawning months, from November to January. Batch fecundity ranged between
0.26 and 2.09 million oocytes and the relative batch fecundity (mean ± standard deviation)
was estimated at 53.4 ± 23.2 oocytes g-1 of somatic-gutted weight. The study provides new
information on the reproductive development and classification of albacore in the western
Indian Ocean. The reproductive parameters will reduce uncertainty in current stock assessment
models which will eventually assist the fishery to be sustainable for future generations.