| The yellowfin tuna(Thunnus albacares) is a species of Thunnusfound in pelagic waters of tropical and subtropical oceans worldwide. It’s one of the upper-middle-class fishes in the warming waters in oceans. Yellowfin tuna hasa huge production and its economic value is extremely high, so that it has brought the attention tothe relevant departments all over the world. The South China Seaas one of the five Pacific tuna fishing grounds, rich tuna resources. The neighboring countries have decades of development for tuna fishing in the South China Sea, but there is no large-scale development in our country due to various reasons.At present, China is committed to developingpelagic fishing in Atlantic Ocean, Indian Ocean and other Oceans. Yellowfin tuna is the main catchesamong the larger tuna speciesin the South China Sea. For the past few years,there are many sovereigntydisputes in the South China Sea. So our country also should pay more attention to the fish resources of the South China Sea.The development of fishery resources in the South China Sea is particularly important for long-term fisheries management and conservation, and the protection ofmarine rights and interests in our country. So great attention should be given tostudy the fishery resources of yellowfin tuna in the South China Sea.Two acoustical surveys were conducted to study the fishery resources of yellowfin tuna in the waters of central and southern South China Sea inthe autumn of 2012 and the spring of 2013. Biological samples were collected using a light falling-net with maximum working depth of 50 m. The survey area was about 20 square miles for acoustics statistics and we chose 40 sampling stations with light falling-net in the area. The results showed as follows.1. There were126 tails of yellowfin tunain autumn 2012 and the average fork length was 299.6 mm,of which 96.1% were juveniles with the fork length ofless than 500 mm.The dominantfork lengths of tuna were200 mm ~250 mm and 450 mm ~500 mm.2.There were 127 tails of yellowfin tuna in spring 2013 and the average fork length was 188.3mm,of which 96.8% were juveniles with the fork length of less than 300 mm. The dominant fork length was180 mm ~200 mm.3.Theyellowfin tuna juveniles with the fork length ofless than 300 mmcaughtin autumn 2012 and spring 2013 were used to analysis the relations of fork length and weight. The result was W = 6.0′10-6FL3.220.4.Theresource abundance and biomassin acoustics survey of yellowfin tuna in 2012 were7.46′108inds and 2.69′105 t respectively. The maximum densities of abundance and biomass of yellowfin tuna were 1.77×105 tinds/ nmi 2(111°E,5.5°N)and 22.75 t / nmi 2(111°E,15°N) respectively.5.The resourceabundance and biomass in acoustics survey of yellowfin tuna in 2013 were 7.27′108inds and 1.88′105 t respectively. The maximum densities of abundance and biomass of yellowfin tuna were 4.8×104 tinds/ nmi 2(115°E,11°N)and 18.3 t / nmi 2(111°E,15°N) respectively. The research results showed that yellowfin tuna mainly lived in the area from 16.0°N to 10.0°Nin autumn 2012 which was different in spring 2013,and had bigger body in autumn 2012 than that in spring 2013.So we can conclude that yellowfin tuna were abundant and widely distributedin the South China Sea. This study aims to provide the important basis for the resource development and protection of yellowfin tuna in the South China Sea.
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