印度洋南方黑鮪的年齡成長與洄游環境史之研究

Abstract

南方黑鮪(Southern bluefin tuna, Thunnus maccoyii)是一種大型的遠洋洄游性魚類，其分布的範圍集中在南半球的南緯30-50度間，唯一的產卵場位於印度洋赤道海域的印尼爪哇島附近。為了瞭解南方黑鮪的成長及洄游環境史，本研究以耳石為材料，判讀其年齡及測量微量元素。2003年6-8月與2004年6-8月間在中印度洋(29-33°S, 67-91°E)共採集418對南方黑鮪的耳石，其體長範圍為87-177公分；年齡2-22歲。另外，2005年1-2月在印尼附近海域(10-20°S,110-120°E)共收集197對南方黑鮪的耳石，其體長分布範圍為152-205公分；年齡11-26歲。印尼海域的南方黑鮪皆為性成熟的生殖族群。比較南方黑鮪的體長體重關係及成長曲線，雌雄間有顯著性差異，性成熟之前的雌魚成長速度較快，但極限體長則是雄性較大。合併雌雄之基本資料，並利用von Bertalanffy growth equation (VBGE)計算南方黑鮪的理論成長方程式，得到之參數項t0值為-5.1047歲、K值為0.0997 year-1、L∞值為190.06公分。台灣在中印度洋所捕獲的南方黑鮪，有95%的個體其年齡介於2-10歲之間，相較於鮪魚在10歲之後才性成熟，且生命週期約40歲來看，台灣在中印度洋所捕捉的鮪魚是呈現一個年齡較小的情況。 本研究利用溶液型進樣系統(Solution-based, SB)與雷射光分析系統(Laser ablation, LA)感應偶合電漿質譜儀(ICP-MS)測量南方黑鮪耳石微量元素組成。SB-ICPMS的分析結果顯示元素組成在不同年齡群之間有顯著性差異，表示南方黑鮪隨著成長而改變棲息環境；LA-ICPMS所測到耳石核心到邊緣的微量元素的時序列變化，顯示耳石邊緣的元素組成在中印度洋及印尼海域之間有顯著性差異，但兩者的核心部位的元素組成卻有很高的相似性。這些證據證明兩個不同海域的南方黑鮪是源自相同產卵場。由此可見耳石微量元素指紋圖是研究南方黑鮪洄游環境史的一項有利工具。

Southern bluefin tuna (SBT, Thunnus maccoyii) is a large, oceanic migratory fish. It is found throughout the southern hemisphere mainly in waters between 30 and 50 degrees. The only known spawning area is in the Indian Ocean southeast of Java, Indonesia. To understand the growth and migratory environmental history, this study used the SBT otolith to determine the age and analyze the trace element in otoliths. In 2003 and 2004 a total of 418 otoliths were collected from the SBT caught by Taiwanese longliners in the central Indian Ocean (29-33°S, 67-91°E) .Their fork length ranged from 87-177 cm and ages were between 2-22 years old. In addition, a total of 197 otoliths were collected from the SBT caught in the spawning ground (10-20°S, 110-120°E) by an Indonesian observer in January and February 2005. Fork length of the SBT ranged from 152-205 cm and ages were between 11-26 years old. The samples collected from Indonesia were sexually matured spawning individuals. The length-weight relationship and growth curve of SBT was significantly different between female and male. Before the age of sexual maturity in approximately 10 years old, female grew faster than male, but male had the larger asymptotic length. The growth parameters of the von Bertalanffy growth equation were estimated to be t0=-5.1047 yrs, K=0.0997 yr-1, and L∞=190.06 cm, using all regional and sexual data. The catch of Taiwanese SBT fishery was mainly composed of 2 to 10 years old, accounted for about 95% of the total catch. The SBT caught by Taiwanese fishery was skewed to young individuals based on the facts that the life span of SBT was approximately 40 years old, and the age at maturity was around 10 years old. This study used otolith microchemistry as a natural tag to retrieve the past migratory environmental history and larval origin of the SBT in the Indian Ocean. The otolith compositions were measured via solution-based (SB) inductively coupled plasma mass spectrometer (ICPMS), and laser ablation (LA) ICPMS. The result of SB-ICPMS showed that the otolith elemental compositions were significantly different between age groups. This indicates that SBT migrated through different environment with growth. The result of LA-ICPMS analysis indicated that the elemental composition in the otolith edge was different between the adult fish collected from central Indian Ocean and Bali Island, but the elemental composition in the core region of the otolith that corresponding to their larval and juvenile stages were similar, irrespective of the collecting locations. This supported that the SBT was a single population in the Indian Ocean.