Reproductive biology, trophodynamics and stock structure of ribbonfish Trichiurus lepturus from northern Arabian Sea and northern Bay of Bengal

Reproductive biology, trophodynamics and stock structure of ribbonfish Trichiurus lepturus landed by trawlers along northern Arabian Sea and northern Bay of Bengal was studied during 2007-2010. Average annual catch along northern Arabian Sea and northern Bay of Bengal was 42649 t and 31944 t. Mean length was significantly higher in the former region. Growth in adults from northern Bay of Bengal was allometric and from northern Arabian Sea was isometric and growth rate in both the regions was significantly different within sexes and between sexes and indeterminates. Length weight relation for males and indeterminates were significantly different between both the regions. Overall sex ratio was 1.33 in northern Arabian Sea and 1.22 in northern Bay of Bengal. Length at first maturity was 61.2 cm in the former region and 52.9 cm in the latter region. Peak spawning season was December-March with more mature females encountered in Northern Arabian Sea. Total fecundity ranged between 23756 and 208300 along northern Arabian Sea and 21672 and 156695 along northern Bay of Bengal. The mature ovaries from both the regions contained maturing (0.6 mm-0.7 mm) and mature ova (1.2 mm-1.3 mm). Fin fishes dominated by clupeids, sciaenids, carangids and scombrids were the most preferred prey, followed by prawns and cephalopods. Feeding intensity was more in northern Arabian Sea than northern Bay of Bengal. Juveniles fed mostly on prawns and cephalopods while adults preferred finfishes. Fishes with empty stomachs were encountered in high numbers in both the regions. Feeding intensity was higher in older fishes in northern Bay of Bengal and in juveniles and very large fishes in northern Arabian Sea. The von Bertalanffy growth equation along northern Arabian Sea was L-t = 131.6 [1-e-0.15 (t + 0.0740)] and along northern Bay of Bengal was L-t = 114.4 [1-e-0.28 (t + 0.0564)]. Along northern Bay of Bengal, reduction in fishing mortality from 0.81 to 0.73-0.76 will help in rationally exploiting the stock but along northern Arabian Sea, fishing mortality is below optimum and has to be increased from 0.18 to 0.34 for optimally exploiting the resource.