太平洋黑鮪成魚於西北太平洋產卵場之族群動態

Abstract

太平洋黑鮪（Thunnus orientalis）洄游於太平洋東西岸與日本海，為我國近海重要的漁獲對象，近年來因族群量下降，造成漁獲量銳減。黑鮪的年齡與成長是評估其漁業資源的必要生物參數，因此本研究利用漁業資料與耳石估計黑鮪的年齡和成長，另外也分析耳石與肌肉穩定性同位素，以探討台灣附近海域的黑鮪洄游動態。結果顯示，在台灣附近海域，2016和2017年主要以5-15歲年輕的個體佔產卵系群的多數（約78%），耳石穩定性氧同位素顯示西北太平洋產卵場之產卵群體大部分孵化於西北太平洋海域（87%），少部分孵化於日本海（13%），且肌肉穩定性氮同位素顯示這些產卵群體在產卵季前，極少數由太平洋東岸攝食場直接進入西北太平洋產卵場繁殖（7%），大部分是來自其他海域，例如西北太平洋溫帶海域的覓食場。這些產卵群體在4-7月產卵季平均體長逐漸上升，大型個體（≥ 230公分）相較於小型個體有較長的出現時間（< 200公分），然而雌雄比例在出現時間長短上並沒有差異。2013年以來，年輕個體逐漸加入產卵群體，並在2015年後對於我國黑鮪漁獲量有很重要的貢獻度。年輕個體因為擁有相對長的繁殖壽命，取代年邁的個體對於資源的延續有良好的發展，太平洋黑鮪的管理措施著重在減少0-1歲幼魚的死亡率，預期5-6年後，將會有更多的年輕成熟個體加入到西北太平洋產卵場。

Pacific bluefin tuna (Thunnus orientalis, PBF) is an economically important species in Taiwanese fisheries. The total landing of PBF has been largely decreased suggesting the decline of the population size in the recent year. The age and size composition are important information for fishery management of the fish. Therefore, this study aimed to estimate the age composition and growth of the PBF by reading otolith annuli. In addition, stable isotopes of otoliths and muscles were analyzed to infer the natal origin and nursery grounds before the spawning seasons, respectively. The results showed that the PBF landing in Taiwan were consisted of the younger individuals aged 5 to 15 years (78%) and the elder fish aged 16-29 years (22%) in 2016 and 2017. Most individuals spawning in the northwest Pacific spawning grounds were hatched in the northwest Pacific Ocean (87%) and fewer individuals were hatched in the Sea of Japan (13%). Very few spawning adults (7%) migrated directly from the feeding grounds in eastern Pacific Ocean to the northwest Pacific spawning grounds while most of spawning adults might migrate from the feeding grounds in the northwest Pacific Ocean, such as Japanese waters. The mean length of adult PBF gradually increased from April to July and individuals with larger size (≥ 230 cm) have longer occurrence in the spawning grounds than the fish of small size (< 200 cm). The sex ratio did not differ druing April to July. The results suggested that younger spawners have higher contribution to the spawning stock since 2013 and the year class of 2007-2008 will continue to reproduce for more a decade. The current fishery management is to reduce the fishing mortablity of the PBF aged 0-1 year. It is expected that more young adults will join the spawning groups in the northwestern Pacific Ocean in 5-6 years.