Reproductive Dynamics of Yellowfin Tuna, Thunnusalbacares (Bonnaterre 1788) Exploited from Western Bay of Bengal

The reproductive biology of Thunnus albacares was studied from the western Bay of Bengal during 2012–2015. Fork length ranges for males and females were 37–186 cm and 33–173.5 cm with a mean of 97.98 cm and 87.46 cm, and the length composition between sexes differed significantly. The sex ratio was 0.68, indicating a preponderance of males in the catch; and this dominance by males was significant in a few months and at fork lengths of 60.0 cm and above. Length at sexual maturity for females and males were 79.91 cm and 81.32 cm. The percentage of spawning females (48.98 ± 3.15 – 63.46 ± 3.98) and gonadosomatic index for female (1.24 ± 0.10 – 1.50 ± 0.05) and male (0.83 ± 0.06 – 1.05 ± 0.08), which differed significantly between months, were higher during October–November and March–April, signifying it to be the peak spawning season. However, spawning was found to occur around the year. Absolute batch fecundity ranged from 0.043178 to 2.670221 million oocytes. Relative batch fecundity was 28.98 ova per gram body weight. The relation between absolute batch fecundity and fork length (FL) and weight was Log F = -1.8247 + 3.7022 Log FL and Log F = 0.1053 + 1.2990 Log W. Ova diameter distribution in spawning ripe ovaries indicated multiple modes demonstrating asynchronous gonadal development and batch spawning. The present study on the reproductive biology of Thunnus albacares is the first in-detailed comprehensive one, from the western Bay of Bengal and would contribute significantly to its stock management.