以耳石穩定性同位素與年輪探討太平洋黑鮪的年齡與成長以及其來源

Abstract

太平洋黑鮪 (Pacific bluefin tuna, Thunnus orientalis)為台灣近海漁業的重要漁獲對象，但近年來黑鮪的族群量急劇下降，造成我國的黑鮪漁獲量銳減，故必須制定更適切的管理措施來保護這項資源，而了解其年齡與成長的狀況及幼魚的來源對制定相關的管理政策是非常重要的基礎資料。故本研究利用2015年所新發表的太平洋黑鮪耳石年齡判讀手冊來對黑鮪進行年齡判讀，以求得更精確的黑鮪年齡與解析黑鮪的成長狀況，並將年齡搭配對應的尾叉長資料建構Age-Length Key，以此了解我國於西北太平洋海域所捕獲的黑鮪之年齡組成。此外我們也以耳石中的穩定性同位素組成來判斷黑鮪是來自哪一個孵化場。2011-2014年間，於東港與南方澳漁港共採集688對太平洋黑鮪的耳石，其尾叉長範圍為137-271公分；體重範圍則為58-435公斤。利用耳石判讀年齡的結果相當一致，平均的百分誤差 (APE)與變異係數 (CV)為0.74 %與1.05 %，範圍為4-28歲，所得的黑鮪von Bertalanffy growth equation的成長參數值如下：L∞值為249.0公分、k值為0.179 year-1、t0為-1.383歲，而根據Age-Length Key所估算的歷年 (2002-2013年)捕獲平均年齡可看出，2002-2007年間，我國在西北太平洋海域所捕抓的黑鮪平均年齡為10-12歲，而8-10歲的年級群則佔較大比例；但2008-2013年，16-18歲的年級群取代8-10歲的年級群，捕獲的黑鮪平均年齡也增加到16-17歲，造成此現象的原因可能為日本的沿近海漁業捕捉大量幼鮪，使入添量減少所致。另外藉由耳石的穩定性同位素分析發現 (n = 30)，我國所捕獲的黑鮪大部分 (83.3 %)都來自西北太平洋黑潮流域的產卵場，此結果也指出太平洋黑鮪有高度Natal homing的現象，但兩產卵場也會有族群交流的現象，因為仍有少部分 (16.7 %)的個體可能來自日本海的產卵場。研究結果顯示，我國太平洋黑鮪漁業與日本的黑鮪漁業息息相關，未來應加強與日本學者的學術合作，並藉由逐年收集更多的黑鮪耳石樣本，來了解年齡組成的年間變化以及更進一步解析兩產卵場對我國漁業之貢獻程度，期望對黑鮪進行有效的管理以達永續經營。

Pacific bluefin tuna (Thunnus orientalis, PBF) is an important target species of Taiwanese neritic fisheries, but their stock has been declining in recent years. Age, growth and the natal origin of PBF are important life history characters for effective management. This study aimed to reconstruct the age composition of the PBF landed in Taiwan by reading their otolith annuli. In addition, we evaluated the natal origin of PBF by the analysis of otolith stable isotopic composition. A total of 688 otoliths were collected form Tungkang and Nanfang-ao ports from 2011 to 2014. The estimated ages ranged from 4-28 years and different readings on the samples were consistent with, the mean APE and CV of 0.74 % and 1.05 %, respectively. The generated von Bertalanffy growth equation of PBF had L∞ = 249.0 cm, k = 0.179 year-1, and t0 = -1.383 years. The mean age of PBF landed in Taiwan were 10-12 years old in 2002-2007 then the values gradually increase to 16-17 years old between 2008-2013. The change of age composition of the spawning stock might due to overfishing of young PBF by Japanese neritic fisheries. In addition, most (83.3 %) PBF analyzed (n = 30) were hatched in the northwestern Pacific Ocean indicated by the otolith δ18O profiles. This result suggested that PBF have high fidelity of natal origin. A small group (16.7 %) of PBF caught by Taiwanese longliners might hatch in the Japan Sea. Therefore, genetic flow of PBF shall exist between the two spawning grounds and this species shall be managed as a single population.