FISHERIES RESEARCH INSTITUTE,MOA,TAIWAN

The yellowfin tuna (Thunnus albacares) is one of the major species caught around subsurface fish aggregation devices (SFADs) in the waters east of Taiwan. However, its biology, particularly its feeding ecology, is poorly understood. In this study, stomach content analyses were used to examine the diet and feeding habits of 505 specimens of juvenile yellowfin tuna with fork lengths (FL) ranging from 36.5 to 97 cm collected via longline around SFADs in the waters east of Taiwan. These stomach content analyses indicated that the important prey species (in terms of %N) of juvenile yellowfin tuna were Amphipoda (20.73%), Sthenoteuthis oualaniens (12.30%), larval Gonorynchus abbreviates (8.94%), Faughnia spp. (8.57%), Decapterus macarellus (5.96%), and Auxis rochei (1.12%). In terms of %W, the important prey species were Decapterus macarellus (23.17%), Auxis rochei (21.91%), Sthenoteuthis oualaniens (8.29%),Faughnia spp. (5.17%), larval Gonorynchus abbreviates (3.38%), and Amphipoda (1.10%). In terms of the percentage of index of relative abundance (%IRI), the high values were for unidentified fishes (30.83%), Decapterus macarellus (14.97%), and Amphipoda (20.12%), followed by Faughnia spp. (10.20%). The diets of juvenile yellowfin tuna with FL of <50 cm consisted primarily of Amphipoda and Faughnia spp., while the diets of tuna with FL > 50 cm were shifted toward feeding of teleost fishes such as Gonorynchus abbreviates, Auxis rochei, and Sthenoteuthis oualaniens. Based on the stomach content analyses, the yellowfin tuna changed their diets at FL of approximately 50 cm.