13 Biology and fishery of the bullet tuna, Auxis rochei (Risso, 1810) in Indian waters S. JASMINE, PRATHIBHA ROHIT, E. M. ABDUSSAMAD, K. P. SAID KOYA, K. K. JOSHI, S. KEMPARAJU, D. PRAKASAN, M. N. K. ELAYATHU AND MANJU SEBASTINE Central Marine Fisheries Research Institute, Kochi - 682 018, Kerala, India. e-mail: jasminevzm@rediffmail.com ABSTRACT Auxis rochei, popularly known as the bullet tuna, is the smallest of all tuna species available in Indian waters. Its distribution is reported from all maritime states but it forms a fishery of commercial importance only in the south-west region (Kerala, Tamil Nadu and Karnataka) with targeted fishery being carried out in southern Kerala. The average annual catch (2006- 2010) is estimated at 3,131 t, constituting 2.8% of the total tuna landings. Commercial exploitation is mainly by gillnets and small hook and lines with the 0-1 yr old fishes comprising bulk of the commercial catch. The length-weight relationship was estimated as W=0.0076L 3.249 with no significant difference between the sexes. Size at first maturity was estimated at 23.6 cm and fecundity estimated was 12,03,258. A. rochei spawns throughout the year with peak during July-September. Zooplankton, fishes and crustaceans were the dominant food items recorded. The von Bertalanffy growth parameters estimated were L ∞ = 42.3 cm, K= 0.61 yr -1 and t 0 = -0.0337. Mortality estimates were M=1.18 and Z=5.90 and F=4.72 with a high exploitation rate of E=0.80, calling for appropriate management measures to be adopted for continued exploitation at sustainable levels. Keywords: Age and growth, Auxis rochei, Bullet tuna, Exploitation, Food and feeding, Maturity Introduction Auxis rochei (Risso, 1810), known popularly as the bullet tuna, is the smallest among all tuna species in the world. Fish of the genus Auxis, are epi/meso-pelagic, having a worldwide distribution in tropical and subtropical waters with a seasonal coastal distribution in temperate and tropical areas (Uchida, 1981; Collete, 1986). The occurrence of the species has been recorded along both the coasts of India but it is harvested on a commercial scale mainly in the south-west region. Very little is known on the fishery and biology of A. rochei and exploitation of this coastal species along the Indian coast was minimal till the late eighties. However, with popularisation of motorised crafts, exploitation and catch by these crafts registered a steep increase. Along southern Kerala, A. rochei has become a major species in the fishery (Gopakumar et al., 1994) and is exploited on a commercial scale throughout the year. The average annual catch (2006-2010) is estimated at 3,131 t contributing 1.7% to the total tuna landings. Several investigators (Uchida, 1981; Rodriguez-Roda, 1983; Grudtsev, 1992; Bök and Oray, 2001; Macias et al., 2005; Macias et al., 2006; Palandri et al., 2008; Valeiras et al., 2008; Kahraman et al., 2010) have studied age determination, length at first maturity, gonado-somatic index, sex ratio, and the spawning period of the species. Indian J. Fish., 60(2) : 13-20, 2013 Fishery, spawning areas and the behaviour patterns of the schools have also been investigated from other parts of the globe (Yoshida and Nakamura, 1965; Ishida, 1971; Yesaki and Arce, 1994; Sabatésand Recasens, 2001; Oray et al., 2005; Oray and Karakulak, 2005). Studies on the biology of A. rochei exploited in India are limited and include those of Silas (1969), Muthiah (1985), Gopakumar and Sarma (1989) and James et al. (1993). The present study was taken up to get detailed information on the fishery, biology, growth and exploitation of bullet tuna on a national level. Material and methods The study was conducted for a period of five years during 2006-2010. Data on catch, effort, species composition of tuna and biological information of A. rochei were collected at weekly intervals from major landing centres on the Indian mainland coast following Stratified Random Sampling Technique developed by CMFRI. Length measurements (fork length, FL) were taken at the landing centres and were raised to the monthly/annual catches. This formed the basic data for estimating growth and population structure using length based models. The length-weight relationship as suggested by Le Cren (1951) was calculated by regression analysis. The sex and gonad maturity stages were assessed from structure and size of gonads and classified as per the ICES scale adopted by