The effect of depth of operation and soaking time on catch rates in the experimental tuna longline fisheries in Lakshadweep Sea, Indi

Most of the marine fish landings from the Indian waters are from the fishing operations in the coastal shelf area, especially from the shallower region ranging from 5 to 100 m depth (Rao, 2010). Heavy demand for seafood in domestic and international markets underlines the need for increasing the marine fish production. Catch trends indicated that the production from the coastal fisheries is almost stagnant and point towards the need for harvesting unexploited or under exploited oceanic fish resources. Present fleet size of the distant water fishing vessels is very less in spite of India’s vast EEZ of 2.02 million sq km and two Islands groups, viz., Andaman, Nicobar and Lakshadweep. The estimated potential yield of oceanic tuna resources is 2.78 lakh tonnes (Pillai and Jyothi, 2007). Potential of total tuna resources in Lakshadweep Islands is estimated at about 50,000 tonnes (Pillai et al., 2006). A total tuna landing in India in 2010 was 60,512 tonnes along the mainland and 7,883 tonnes in Lakshadweep. The landings trends of these high values fishes indicated a further scope for the expansion of the fisheries.

The oceanic tuna fishery of the Indian Ocean is contributed mainly by four species viz., yellowfin tuna, southern bluefin tuna, albacore tuna and bigeye tuna (Joseph, 1972). Longline operations are in its infancy state in India. Surface longline gears can operate at a range of depths, and hooks placed at different depths can have different fishing efficiencies, depending on the target species and its behaviour. With better knowledge of the relationship between hook depth and foraging behaviour of the targeted fish, catch rates could be improved by placing the majority of hooks at the depth range favoured by the target species. Fishing efficiency of the longline gears are influenced by minor changes in the gear configuration such as type of terminal gear and depth of hooking operations (Broadhurst and Hazin, 2000). Deep setting of the longline gear found to be very effective to reduce the sea turtle bycatch (Shiga et al., 2000; Beverly et al., 2009).

Marine mega faunal bycatch is a serious concern in longline fisheries which needs serious attention (Lewison et al., 2004; Diaz, 2005; Garrison, 2007).Major group of animals contributing to the marine mega faunal bycatch are sharks and cetaceans (Gilman et al., 2008; Mandelman et al., 2008; Milian et al., 2008; Mangel, 2010). The depth at which longline fishes is mainly influenced by the gear configuration, primarily by the length of mainline between floats (baskets), sagging rate and parameters such as wind and currents (Suzuki et al., 1977; Boggs, 1992). Tuna shows an aggregation nature near the floating objects which can be effectively utilised by vertical longline operations in the floatsams or FADs (Naeem and Latheefa, 1994).A successful fishing and catch rates greatly depends on the soaking time. The effect of soaking time on catch rates vary considerably between species to species. Soak time during dusk showed higher overall catch rates (Ward et al., 2004). Experimental longline operations were initiated in the Lakshadweep Sea to tap the unexploited oceanic tuna fishes. This paper discusses the effect of depth of operation and soaking time on the overall catching performance and species selectivity in the longlines operated.