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A survey of yellowfin tuna, Thunnus albacares, fishing ground was carried out on board of the Chinese longliners from September 15 to December 12, 2005 in the tropical high seas of the Indian Ocean. The depth at which each yellowfin tuna was hooked was estimated using a stepwise regression analysis of theoretical hook depth and observed average hook depth measured using a temperature depth recorder. Water temperature, salinity, chlorophyll a, dissolved oxygen and thermocline, which are important variables influencing yellowfin tuna habitats, were measured in the survey. Catch rates of yellowfin tuna were then analyzed with respect to depth, temperature, salinity, chlorophyll a, dissolved oxygen and thermocline. We suggest that the optimum ranges of swimming depth, water temperature, chlorophyll a and dissolved oxygen concentration for yellowfin tuna are 100.0-179.9 m, 15.0-17.9°C, 0.090-0.099 lg L )1 , 2.50-2.99 mg L )1 , respectively; that salinity has less influence on the vertical distribution of adult yellowfin tuna; and that yellowfin tuna are mainly distributed within the thermocline in the high seas of the Indian Ocean. Our results match the yellowfin tuna's vulnerability to deep longline fishing gear well.

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