利用耳石穩定性碳氧同位素組成探討鮪魚之生活史

Abstract

耳石為一種魚類自然生物礦化沉積而成的碳酸鈣結構，不會被魚體再吸收，因此隨著魚體的成長，能完整記錄環境的資訊。本實驗利用儲存於耳石內的穩定性碳氧同位素，來探討高經濟價值的鮪魚生活史，以往文獻碳同位素可以反映食性，氧同位素可以反映環境溫度。作為除了以往利用標定及漁獲資料統計外，研究鮪魚生活史的另外一種方式。 本研究目的先探討耳石內穩定同位素對於鮪魚生活史研究的可行性後，進而解釋溫帶性太平洋黑鮪(Thunnus orientalis)的生活史，及比較溫帶性鮪魚及熱帶性鮪魚大目鮪(Thunnus obesus)及黃鰭鮪(Thunnus albacares)可能呈現在同位素上的差異。以氧同位素推算的溫度結果顯示，太平洋黑鮪在孵化及仔魚時期經歷高溫，於稚魚時期溫度急遽下降隨即再上升，而後至成魚時期溫度呈現一平緩波動。此結果顯示，假設太平洋黑鮪在高溫的黑潮海域孵化，經黑潮傳輸至高緯度地區，之後因為脫離黑潮區後處於高緯度地區造成溫度明顯下降，而溫度異常回升可能是反應魚體溫度而非水溫。假設太平洋黑鮪在日本海孵化，日本海內的特殊半封閉水團的特性，較溫度影響耳石內氧同位素沉積更甚，因此呈現與實際溫度差異大的情形，之後溫度下降及再上升可能為魚體成長脫離日本海至大洋所呈現的訊號。此結果與以往對於太平洋黑鮪生活史的研究符合。熱帶性鮪魚在靠近耳石核心部分並無像溫帶性鮪魚急遽下降後明顯再上升的趨勢，推測為熱帶性鮪魚生活史侷限於熱帶區域並無像溫帶性鮪魚作大範圍洄游所致。在此部分研究另外發現大目鮪所呈現的平均溫度較黃鰭鮪更低。在同個生活範圍下，可能代表棲息深度的不同，或是種間熱調節能力差異所致。 此外也利用幼魚耳石內碳氧同位素及日輪寬試著建立太平洋黑鮪不同產卵場(日本海及黑潮)水團間差異下的辨別性，作為判定各漁區的黑鮪來源依據，了解不同產卵場對於各漁區漁獲貢獻度。本研究結果顯示兩產卵場間儲存於耳石內的碳氧同位素具有顯著性的差異，同時也反映出水團混合的情形。因此，耳石內碳氧同位素提供了另一個研究鮪魚生活史的工具。

Otolith, as a metabolically inert structure, can record the environmental history throughout fish life. This study aims to evaluate the life histories of tropic and temperate tunas by stable isotopic signature stored in otoliths. Stable 13C can reflect the diet of fishes, and stable 18O can record the environmental temperature. It can be another method to study tuna life history except for tagging or catch data analyzing. In this study, we first confirm the feasibility of stable isotope 13C and 18O stored in tuna otoliths, and then utilizing them to explain the Pacific bluefin tuna (PBF) life history, comparing the differences between tropic (PBF) and temperate (Yellowfin YFT and Bigeye BET) tunas. Profiles of 18O suggested a warm water period during the larval and juvenile stages, followed by water temperature decrease. Water temperature dramatically increased during the young stage then the values moderately fluctuated for the remaining life. This result suggested that hatched PBF stayed in tropical spawning ground, then transported by Kuroshio Current to temperate areas. PBF experienced cool water after detraining from the Kuroshio. The water temperature increased at the young stage may reflect the body temperature due to thermoregulation rather than the ambient water temperature. Supposing that PBF hatched in Japan sea, the high water temperate may represent the particular water mass rather than real water temperature. PBF experienced cool water after departing from Japan sea up to the open sea. BET and YFT show slow water temperature decrease and no apparent rising thereafter. This result indicates that BET and YFT reside in tropic areas, and reflect the different residing depth. Another aim is to set up the fingerprint of two Pacific bluefin spawning grounds (Kuroshio and Japan Sea) by 13C、18O and daily growth increments widths (DGI) in otoliths. The result show significant discrimination between two spawning grounds, and reveal the water mass mixing status.