Fishery and feeding habits of yellowfin tuna (Thunnus albacares) targeted by coastal tuna longlining in the north western and north eastern coasts of
D.C.T. DISSANAYAKE*, E.K.V. SAMARAWEERA AND C. AMARASIRI
Present study analyzed the catch, effort and size distribution of yellowfin tuna (Thunnus albacares) caught by coastal tuna longlining in north western and north eastern coastal waters of Sri Lanka from January 2005 to December 2006. Further, the contents of 83 non–empty stomachs were analyzed and the indices such as frequency of occurrence (%F), percentage of number of each food (%N), percentage of wet weight of each food item (%W) and percentage index of relative importance of each food item (%IRI) were calculated for the specimens caught from north western coastal waters from September 2006 to April 2007. Weekly fishery data were collected at major landing sites in north western and north eastern coastal areas. The fishing activities were carried out using Fiberglass Reinforced Boats (FRP) with outboard engine which basically operated in coastal waters during one day trip duration. The fishery was highly seasonal and it related to the monsoon pattern. Fishing started in October and continued until April of the following year in north western coast and from May to September in north eastern area. The estimated figure for total production in north western area (1052 t in 2005 and 3313 t in 2006) was higher than that in the north eastern area (578 t in 2005 and 741 t in 2006) and it was significant in 2006 (P < 0.05). Total fishing effort in north western coast was much higher than in the north eastern coast in both years but the difference was not significant. The log transformed catch per unit effort ln (CPUE+1) values were not significantly different in the two areas. Distribution classes of the length range of yellowfin tuna was within 30 – 150 cm in the catches and dominant length classes laid within the range of 95 to 120 cm and forward shifting of dominant peaks were evident within the fishing season. In the stomachs of yellowfin tuna sampled, on average 33 prey items were found per stomach and dominant prey items were crustaceans, especially the swimming crab, Charybdis smithii followed by fishes belonging to family Sphyraenidae and Engraulididae. Loligo bartrami belonging to family Loliginidae, formed the main cephalopod prey. % IRI of crustaceans was higher than that of fish and cephalopods and this further revealed that the pelagic crab C. smithii is the most important prey item in the diet of yellowfin tuna.