魚類耳石微化學的研究與應用(2/3)

Abstract

本研究以耳石之鍶鈣比 (Sr: Ca ratios) 來探討大眼海鰱Megalops cyprinoides 之洄游環 境史及洄游行為。大眼海鰱為海洋魚類，成魚在海水中產卵。孵化後之柳葉形仔稚魚隨著 海流漂送至河口域成長。但變態為幼魚之後，它們的棲地選擇似乎非常複雜，從河口、紅 樹林到溪流都能發現它們的蹤跡。而利用耳石鍶鈣比正是研究魚類洄游現象的最佳工具。 首先，以實驗來證明大眼海鰱耳石鍶鈣比和環境中水體鹽度的關係。結果發現大眼海鰱柳 葉魚飼養於0‰ 的鹽度下，其耳石鍶鈣比會從海水柳葉魚階段的8‰ 降至4‰ 以下，飼養 於10‰ 及35‰ 的鹽度下耳石鍶鈣比則維持在4-8‰ 之間。根據此實驗數據進行野生大眼 海鰱洄游行為之調查時發現大眼海鰱在幼魚階段可在淡水或海水中成長, 有些個體則會在 淡海水間移動,但成魚最終都會回到海水中產卵。此研究結果顯示：(1)、耳石鍶鈣比可用 於探討大眼海鰱洄游環境史。(2)、大眼海鰱在幼魚階段的洄游行為及棲地選擇是隨機性 的。

Pacific tarpon Megalops cyprinoides are widely distributed in the tropical and subtropical areas of Indo-Pacific Ocean. They spawn in the coastal waters, and then their larvae drift with currents to estuarine nursery ground approximately 25 -30 days post-hatching. Young Pacific tarpons were found in variety of environment, such as stream, estuary, inner bays, and mangrove. The strontium (Sr) concentration is approximately 100 fold higher in sea water than in freshwater, and Sr/Ca ratios in fish otolith were positively correlated to ambient salinity; thus the ratio was widely used to study the fish migration between seawater and freshwater. To study the migratory environmental history of Pacific tarpon, Sr/Ca ratios in otoliths of the fish, including leptocephalus, young and adult, collected from Gongshytyan Creek and Tadu Creek estuary were examined by Electron Probe Microanalyzer (EPMA). Sr/Ca ratios in otoliths of the fish at leptocephalus stage were higher and similar among individuals. But the patterns of Sr/Ca ratios beyond juvenile stage were very complicated, indicating that the fish after metamorphosis from leptocephalus may migrate among different environment. If the fish stayed in the estuarine and marine environment Sr/Ca ratios in otolith were greater than 4‰, but less than 4‰ if they moved to freshwater. These demonstrated that the migratory environmental history of the tarpon can be retrieved from the otolith Sr/Ca ratios and the habitat selection of the fish beyond juvenile stage is facultative.