臺灣近海太平洋黑鮪年齡組成與洄游生活史研究

Abstract

太平洋黑鮪（Thunnus orientalis）是大型且高度洄游性魚類，有複雜的洄游生活史，長期遭受過度捕撈導致族群數量已大幅減少，最新的資源評估認為，現存的太平洋黑鮪的生物量雖處於歷史低點，但近年資源量有逐漸增加情形。研究黑鮪的年齡組成與洄游生活史有助於了解黑鮪族群動態並提供漁業管理之參考依據，因此本研究利用耳石估計黑鮪的年齡，建立年齡體長轉換表（Age-Length Key），並推估出2018和2019年臺灣近海捕獲太平洋黑鮪全漁獲年齡組成，結果顯示2018年的年齡平均值為11.4 ± 2.9歲，大致呈單峰分布，以9-12歲為高峰群，2019年的年齡平均值為11.1 ± 3.6歲，同樣呈單峰分布，以8-12歲為高峰群，2019年捕獲的7-8歲黑鮪數量比2018年大幅增加。另外，藉由分析耳石穩定性氧同位素(δ18Ootoltih)與脊椎骨第一環半徑，來判別臺灣近海捕獲黑鮪的孵化場，δ18Ootoltih的分析結果顯示，在臺灣近海捕獲的黑鮪，大部分都孵化於西北太平洋孵化場，約佔85%，少部分孵化於日本海孵化場約佔15%，另外測量97個脊椎骨的輪紋，有33尾個體是屬於第一環半徑較短的窄群，表示這33尾個體孵化場可能來自日本海(34%)，而有64尾個體屬於第一環半徑較長的群體，可能是來自西北太平洋的孵化場(66%)。從結果顯示，兩個實驗方法皆可判斷太平洋黑鮪的孵化場，耳石穩定性氧同位素分析結果較為準確但耗時，而使用脊椎骨判別則快速但鑑別力較低。最後利用耳石穩定性碳同位素(δ13Cotoltih)判別黑鮪跨越東西太平洋的洄游行為，結果顯示91個體中有28尾的δ13Cotoltih值介於-11.41‰至-7.6‰，其餘的63尾δ13Cotoltih值介於-7.94‰至-5.29‰，前者可能是長期棲息在西北太平洋的個體，比例約為30%，而後者可能是曾經洄游到東太平洋者，比例為70%。本研究發現太平洋產卵場的貢獻度高於日本海產卵場，且跨洋洄游的黑鮪比例高於只棲息在西北太平洋的個體。

Pacific bluefin tuna (Thunnus orientalis, PBF) is a large and high-valued species with a complex migratory life history. The population of PBF has been greatly decreased due to long-term overfishing, and the latest stock assessment shows that its biomass reached the historically low level but gradually increase in recent years. The study of age composition and migratory life history is important to better understand the PBF population dynamics and provide useful information for fishery management. Therefore, this study aimed to estimate the age composition and reconstruct the migratory life history of the PBF by examining otolith annuli and stable isotope compositions. The results of age structure showed an unimodal distrubition with the mean age of 11.4 ± 2.9 and 11.1 ± 3.6 year in 2018 and 2019, respectively. The former was dominant by 9-12 age class, and the latter was dominant by 8-12 age class. The number of 7-8 year-old PBF increased significantly in the catch of 2019 compared with that in 2018. In addition, the analysis of δ18Ootoltih suggested that most PBF caught in Taiwanes offshore waters was hatched in the Nansei Islands spawning ground (85%), and a small part was hatched in the Sea of Japan (15%). In addition, 97 vertebras were examined and the distance from the focus to the first annulus (r1) was measured, in which 33 individuals showed a relatively shorter radius of the first ring, indicating the natal origin in the spawning grounds of the Sea of Japan (34%), while 64 individuals showed a longer radius of the first ring, suggesting the natal origin in the spawning grounds of the northwestern Pacific Ocean (66%). Although both approaches can distinguish the spawning origin of Pacific bluefin tuna, δ18Ootoltih analysis is more accurate but time-consuming compared with the measurement of vertebral radius which is fast but the discrimination ability is lower. In addition, the analysis of δ13Cotoltih values suggest that 28 individuals (30% of 91 samples) with the values between -11.41‰ to -7.6‰ might be the residents in the Northwest Pacific, while 63 individuals (70% of 91 samples) with δ13Cotoltih values between -7.94‰ to -5.29‰ might have migrated to the East Pacific Ocean during the juvenile to young stages. This study suggested a higher contribution of the Pacific spawning grounds than that in the Sea of Japand. In addition, juvenile PBF that had migrated to the East Pacific Ocean showed higher contribution to the spawning adults in the Pacific Ocean than the counterpart that showed whole-life residence in the northwestern Pacific Ocean. These results provided further insight into the populaton dynamics and can benefit the sustainable management of the PBF.