耳石穩定性同位素探討太平洋黑鮪之洄游史

Abstract

太平洋黑鮪是高度洄游性魚種，廣泛分布於東西側太平洋。作為重要經濟漁獲對象，了解其洄游史可以幫助國際漁業一致性的管理策略，以利於族群資源評估。其單一系群的漁業管理，包含不同出生地來源及洄游模式的個體所組成。太平洋黑鮪有兩個主要產卵場海域，分別位在西北太平洋以及日本海，成長至亞成體時期，部分個體採取跨洋洄游至東太平洋攝食場的洄游模式，並稱為洄游型；而其餘依舊居住在西太平洋的個體，則為定居型。因具有複雜的洄游分布與模式，不同成長階段的個體在洄游路徑中受到各國漁業捕撈，但對於太平洋黑鮪的產卵場來源以及跨洋洄游的比例還尚未完全了解。因此，本研究分析了各地捕獲太平洋黑鮪的耳石碳氧穩定性同位素，以評估各海域的出生地來源以及跨洋洄游的比例。穩定性氧同位素結果顯示整體族群以西北太平洋出生地來源為多數（78%），僅有少數為出生日本海的個體（22%），並且東太平洋族群有更高比例是由西北太平洋出生（97%）。此外，碳同位素結果顯示多數個體具有跨洋洄游之行為（90%），突顯東太平洋攝食場對於族群的重要性。本篇研究強調了臺灣東側附近的西北太平洋產卵場是族群數量的重要入添來源，更是東太平洋族群主要的來源。在評估其族群動態及漁撈量時，應考量太平洋黑鮪的不同出生地來源及洄游模式組成，可作為未來參考依據。

Pacific bluefin tuna (Thunnus orientalis, PBF) is a highly migratory species traveling long distances across the Pacific Ocean. As an economically important species, understanding its migratory ecology helps us make fishery management consistently. PBF is managed as a single stock although it has two known spawning grounds in the Western North Pacific Ocean (WNP), and the Sea of Japan (SoJ). During the juvenile stage, PBFs show different migratory patterns. Some PBFs undergo eastward transoceanic migration to the California Current Large Marine Ecosystem (CCLME) called as migrants; others remain in the Western Pacific Ocean as residents. Investigating these migratory behaviors is crucial because PBF is fished by several countries throughout their life stages. However, knowledge about PBF’s movement between spawning grounds and their transoceanic migrations remains limited. To evaluate migratory patterns and natal origins, this study analyzed otolith δ13C and δ18O stable isotope ratios of 157 PBFs caught across various geographic areas. The δ18Ooto analysis indicated that the majority of the whole population originated in the WNP spawning grounds (78%) than in the SoJ (22%). Furthermore, the eastern population had a higher proportion of individuals from the WNP origin (97%). Besides, the δ13C analysis revealed transoceanic migration as the predominance (90%) within the western population. The migratory pattern to the CCLME highlights the importance of these eastern feeding grounds. These findings highlight the significance of the WNP spawning grounds in maintaining the entire population and also contributing to the eastern population. Future evaluations of their population dynamics can consider the population mixing of PBFs from different natal origins and their migratory patterns.