Observed By-catch of Taiwanese Tuna Longline Fishery in the South Atlantic Ocean

Abstract

本篇論文係根據1999年至2003年觀察員於南大西洋我國遠洋鮪釣漁船上所實際記錄的8航次（560次作業，1,836,700鉤）的資料整理分析之結果。根據標的魚種的不同，本研究將南大西洋分成主要漁獲對象為大目鮪的A區(50°W-20°E，5°N-15°S)及主要漁獲對象為長鰭鮪的B區（15°S以南，70°W-20°E)。A區主要的混獲為旗魚、鯊魚、其他硬骨魚及其他生物。混獲的鯊魚中以鋸峰齒鮫(Prionace glauca)、深海狐鮫(Alopias superciliosus)及鯖鮫屬(lsurus spp.)為主，而主要的丟棄鯊魚種類為蒲原式擬錐齒鯊(Pseudocarcharias kamoharai)。鋸峰齒鮫雌魚的體長在兩區均較雄魚為大，且大部份均未成熟。A區有較高比例的成熟個體（雌：30%，雄：23%），而B區則較少（雌：17%，雄：10%）。鋸峰齒鮫的雄魚較雌魚為多，性比與1:1有明顯的差異。鋸峰齒鮫在A、B兩區的原始CPUE分別為1.378尾／1000鉤及0.444尾／1000鉤。This paper was based on observers' records of 8 trips of Taiwanese tuna longline fishing vessels operating in the South Atlantic Ocean from 1999 to 2003. The catches of 560 operations including 1,836,700 hooks were observed. According to target species, two areas namely, area A (50°W-20°E, 5°N-15°S) targeting on bigeye tuna and area B (15°S south, 70°W-20°E) targeting on albacore, were stratified. In area A, the major by-catch is billfishes followed by sharks, other bony fishes, and others. The blue shark, Prionace glauca, bigeye thresher shark, Alopias superciliosus, and mako sharks, lsurus spp. are the dominant species of shark by-catch and crocodile shark, Pseudocarcharias kamoharai, is the major discarded shark species. In area B, the proportion of by-catch is less than that of area A. Sharks are the major by-catch followed by other bony fishes and billfishes. The blue shark, mako sharks and silky shark, Carcharhinus falciformis, are the dominant species of shark by-catch. The mean sizes of female blue sharks were larger than males in both areas and most of the sharks were immature. More mature individuals were caught (30%, 23% for females and males) in area A than in area B (17%, 10% for females and males). The sex ratio was significant different from 1:1 (P＜0.05) and males outnumbered females in both areas. Nominal CPUE in number of blue shark was estimated to be 1.378 and 0.444 per 1,000 hooks for area A and B, respectively.