Age and growth of the bigeye tuna, Thunnus obesus, in the western Pacifi c Ocean

Bigeye tuna (Thunnus obesus Lowe, 1839) are a commercially important species of tuna inhabiting the warm waters of the Atlantic, Indian, and Pacifi c oceans. They are found across the entire Pacifi c between northern Japan and North Island of New Zealand in the west and from 40°N to 30°S in the east (Calkins, 1980; Matsumoto, 1998). Adult bigeye tuna are caught mainly by longlines, but substantial numbers of juveniles are taken by purse seines. Taiwanese distant water tuna longline fl eets have operated throughout these three oceans since the late 1960s targeting albacore. In the early 1980s, the Taiwanese began equipping their longliners with very cold (below –55°C) freezers and deep longlines in the Indian and Atlantic oceans, which allowed them to target bigeye tuna for the lucrative sashimi market in Japan. In the western Pacifi c, the Taiwanese offshore longline fl eets, based in domestic (Tungkang mainly) and foreign fi shing ports, have landed more bigeye tuna than in the past. Growth studies of Pacifi c bigeye tuna conducted in the 1950s and 1960s were based either on increments between modal points in size-composition data (Iversen, 1955; Shomura and Keala, 1963; Yukinawa and Yabuta, 1963; Kume and Joseph, 1966; Suda and Kume, 1967) or on the number of annual markings (annuli) on scales (Nose et al., 1957; Yukinawa and Yabuta, 1963). Recently, Hampton and Leroy1 and Matsumoto (1998) presented preliminary results from growth studies based on otolith increment counts. No previous study had aged Pacifi c and Indian bigeye tuna from dorsal spines, alAge and growth of the bigeye tuna, Thunnus obesus, in the western Pacifi c Ocean