從耳石日成長輪及微化學探討西北太平洋區鱸鰻之生活史特性

Abstract

為了瞭解西北太平洋熱帶性鱸鰻之加入動態和初期生活史以及菲律賓水域同種鰻的洄游環境史。本研究調查2005-2009年台灣東部秀姑巒溪河口玻璃鰻的種類組成，結果發現鱸鰻的玻璃鰻數量最多，占總捕獲量的98.4%；另外有少量的太平洋雙色鰻 (1.6%) 及日本鰻(<1%)。熱帶性鱸鰻的玻璃鰻的主要出現季節為春夏天，但幾乎終年都可發現；而太平洋雙色鰻的玻璃鰻的主要出現季節為秋天；溫帶性日本鰻的玻璃鰻的出現季節則是在冬天。 透過東北亞地區不同河口所捕獲的鱸鰻和日本鰻玻璃鰻的耳石日周輪分析顯示，柳葉鰻變態的日齡及早期成長率對於此兩種同域分布鰻魚的分離洄游及緯度分布扮演重要的角色。成長快、變態早的鱸鰻柳葉鰻較早加入到菲律賓；而成長慢、延遲變態的日本鰻柳葉鰻繼續向南(經由明答那峨海流)及向北(經由黑潮)飄送。另一方面，日本鰻柳葉鰻抵達菲律賓水域時尚未到變態階段，所以也不會洄游到河口，而是繼續向北散佈。這可能就是日本鰻很少在菲律賓發現而鱸鰻在菲律賓數量很多的原因。 電子微探儀(EPMA)分析顯示，採集於菲律賓呂宋島東部河川的鱸鰻耳石鍶鈣比數據和之前所報導的日本及越南的鱸鰻以及日本鰻的洄游環境史非常不同。在耳石鰻線標記之後到耳石邊緣的鍶鈣比都低於4 x 103，顯示菲律賓呂宋島東側的鱸鰻在鰻線階段進入淡水溪流後一直到被捕獲的黃鰻階段為止都悽息在淡水環境中。此結果和台灣的鱸鰻一樣，在黃鰻階段都只棲息在淡水中。而日本鰻在黃鰻階段的棲地利用則較為彈性，可在海水、鹹淡水和淡水之間洄游。種間競爭、環境喜好性以及生產力可能是造成鱸鰻和日本鰻的棲地喜好性不同的原因。本研究之發現可提供鰻魚資源保育之參考。

The present study investigated the recruitment dynamics and early life history of the tropical eel Anguilla marmorata in the northwestern Pacific as well as the migratory environmental history of the eel in the Philippine waters. Analysis of the species composition of the recruiting glass eels in the estuary of the Hsiukuluan River, Eastern Taiwan from 2005-2009 revealed that A. marmorata was the most dominant eel species making up to 98.4% of the total catch while there were very few A. bicolor pacifica (1.6%) and A. japonica (<1%). Tropical eel species A. marmorata recruited mainly to the estuary during spring to summer but can be found year-round while A. bicolor pacifica recruited mainly during autumn. The temperate species, A. japonica, recruited mainly during winter. Examinations of the otolith daily growth increments of A. marmorata and A. japonica glass eels collected from various rivers and estuaries in East Asia from 1992-2008 indicated that age at metamorphosis and early growth rate seem to play an important role in the segregative migration and latitudinal distribution of these two sympatric eel species in the northwestern Pacific. Faster-growing and earlier-metamorphosing leptocephali of A. marmorata recruited earlier in the Philippines while its slower-growing, delayed metamorphosing leptocephali dispersed southward (via the Mindanao Current) and northward (via the Kuroshio Current). On the other hand, the A. japonica leptocephali which arrive in the Philippine waters are apparently too young to metamorphose and migrate towards the estuaries so it will continue to drift northwards. This must be the reason why Japanese eels are seldom found in the Philippines while A. marmorata occurs in abundance. The Sr:Ca profile in the otoliths of yellow-stage A. marmorata collected in the river of eastern Luzon, the Philippines in August 2008 revealed that it’s migratory environmental history is quite different from that previously reported from Japan and Vietnam and from A. japonica. Electron probe microanalyzer showed that after the elver check, the Sr:Ca ratio until the otolith edge were less than 4 x 10-3, indicating that after recruitment, A. marmorata just stayed in freshwater until capture, which is similar to that of A. marmorata in Taiwan. On the contrary, A. japonica has a more flexible migratory behavior in the yellow stage. It can migrate among seawater, brackish water and freshwater in the yellow eel stage. Interspecific competition, environmental factors and the productivity of the environment may play an important role in the habitat preference of A. marmorata throughout its species range. The findings of this study can provide the information for the eel conservation and management.