Exploitation and movements of yellowfin tuna (Thunnus albacares) and bigeye tuna (T. obesus) tagged in the north-western Coral Sea

John Hampton and John Gunn

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Abstract. Yellowfin tuna (Thunnus albacares) and bigeye tuna (T. obesus) were tagged and released in the north-western Coral Sea in 1991 and 1992. Over the next five years, recaptures were reported by Australian longline vessels fishing in the release area, and by industrial tuna fleets fishing in the adjacent western Pacific region, thus demonstrating clear links between the tuna stocks in these areas. Some southerly movements of yellowfin, in particular, further suggested links with stocks supporting the longline fishery in the south-eastern Australian Fishing Zone.

Bigeye tuna tag returns and catch-per-unit-effort by Cairns-based longliners showed a strong seasonal signal, peaking in mid-year. Yellowfin tag-return data displayed a similar, but weaker, seasonal pattern. The data were analysed using tag-attrition models with seasonally variable catchability and with two assumptions regarding changes in targeting of the two species by longliners during the study. Under both assumptions, the local exploitation rates for yellowfin are low: about 0.07 in 1996. For bigeye, the local exploitation rate in 1996 may have been as high as 0.30, warranting a cautious approach to further fishery expansion.