Indian Journal of Geo-Marine Sciences Vol. 43(5), May 2014, pp. 755-771 Reproductive biology, trophodynamics and stock structure of ribbonfish Trichiurus lepturus from northern Arabian Sea and northern Bay of Bengal S Ghosh, M V Hanumantha Rao, P Rohit 1 , K Rammohan & G Maheswarudu Visakhapatnam Regional Centre of CMFRI, Ocean View Layout, Pandurangapuram, Visakhapatnam 530 003, India 1 Mangalore Research Centre of CMFRI, Bolar, Mangalore 575 001, Karnataka, India [E-mail: subhadeep_1977@yahoo.com] Received 13 August 2012 ; revised 14 January 2013 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. [Keywords: Ribbonfish, Population dynamics, Spawning, Food and feeding, India] *Corresponding address : Shubhadeep Ghosh Introduction Bay of Bengal Large Marine Ecosystem (BOBLME) and Arabian Sea Large Marine Ecosystem (LME) are considered Class 1, highly productive ecosystems (>300 g cm -2 yr -1 ) 1,2 . Arabian Sea LME is one of the six LME’s in which reported landings have remained relatively constant or shown increase over the past few decades 3 . Fisheries of the BOBLME and Arabian Sea LME are multi-gear and multi-species. The number of collapsed and overexploited stocks in the BOBLME and Arabian Sea LME are on the rise, with over 80% of the landings from fully exploited stocks. Intensive fishing in recent years is the primary force driving biomass changes in the BOBLME and Arabian Sea LME which in turn are threatening their long term sustainability. These changes are well illustrated on the east and west coast of India, where high density of trawlers is inducing changes in the ecosystem 4 .