Tuna resource exploration with tuna longline in the South China Sea, Area IV: Vietnamese waters

Abstract

The survey was designed to be carried out only in the central part of the whole area which was considered as deep sea or oceanic zone. Depth of water is mostly more than 1,000 m deep. The exploration area was covered from latitude 7°-30.0’ N to latitude 16°-00.0’ N and longitude 110°-00.0’ E to longitude 112°-30.1’ E covering around 72,000 square miles. Surface temperature varied from 27.4° C to 30.1° C, shallow thermocline layer is still the characteristic of South China Sea fishing ground. It was detached from 15-50 meters depth then water temperature decreased gradually until at the depth of 130-200 m with water temperature around 15.3° C. Three tuna were caught during the survey of M.V. SEAFDEC and many were caught on local fishing boat by both type of fishing gear tuna longline and drift gill net. They are mostly skipjack tuna Katsuwanus pelamis (Linnaeus, 1758) and yellowfin tuna Thunnus albacares (Bonnaterre, 1788) others catch were dolphinfish Coryphaena hippurus (Linnaeus 1758), wahoo Acanthocybium solandri (Cuvier, 1831), shortfin mako shark Isurus oxyrinchus (Rafinesque, 1809), Bigeye thresher shark Alopias superciliosus (Lowe, 1839), swordfish Xiphias gladius (Linnaeus, 1758), Bigtooth pomfret Brama orcini (Cuvier, 1831) and the most abundance was lancetfish Alepissaurus borealis (Gill, 1874). Tuna resource was found more abundance in the middle part of the survey area than the upper and lower. Their swimming layer was around 50 m to 90 m depth.