SUMMARY

The hooked bigeye tuna’s depth was calculated by the catenary curve equation based on the identified hook number of the hooked fish sampled in random from three China tuna longliners operating in the Central Atlantic Ocean high sea area and targeting bigeye tuna from 4 th Jul. 2001 to 27th Oct. 2001.The temperature and salinity of the hooked fish’s swimming depth was calculated from temperature and salinity vertical profiles measured by the STD guided by the hook depth. From the sample data, bigeye tuna catch rates at different ranges of depth, water temperature and salinity were estimated by the frequency statistic method. The range of depth, water temperature and salinity with the maximum catch rate of bigeye tuna indicated its optimal swimming depth, water temperature and salinity range. The range of depth, water temperature and salinity with the first 3 highest catch rate of bigeye tuna indicated its common swimming depth, water temperature and salinity range respectively. The results indicated that its optimal depth, water temperature and salinity range was 240.00- 269.99m, 12.00- 12.99º, 35.00- 35.09 respectively. Its common swimming depth, water temperature and salinity range was 240.00- 329.99m,10.00～12.99º, 35.00- 35.29 respectively