Acoustic estimation of longline tuna abundance

The French Polynesia Economic Exclusive Zone is located in an important lon gline fishing ground for albacore (Thunnus alalunga), yellowfin (T. albacar es) and bigeye tuna (T. obesus). Longline tuna abundance estimates using co mmercial catches are particularly biased when hook depth does not coincide with the depths at which tuna prefer to swim. To avoid catchability problem s, a direct acoustic estimate of tuna abundance was made in the French Poly nesia EEZ using a 38 kHz echo-sounder with a depth range of 500 m. Several biases can influence individual tuna target selection, such as the threshol d effect, the risk of multiple target acceptance, the beam width effect and the reduction in target detection at depth. However, they all appeared to be limited in effect. Comparison with experimental longline catches shows t hat the acoustically selected targets appear to be representative of longli ne tuna distribution. A density of 1.33 fish per km(2), i.e. about 33.8 kg of tuna per km(2), was measured. Such a density is slightly greater than th e estimate based on tuna catches, as the whole tuna habitat range is not sa mpled by most professional longlining. (C) 2000 International Council for t he Exploration of the Sea.