台灣宜蘭地區日本鰻及鱸鰻生活史之比較

Abstract

鰻魚為東亞重要養殖經濟魚類之一，目前全世界有十九種淡水鰻魚被發現，在台灣地區則有至少四種鰻魚出沒，主要為日本鰻和鱸鰻，另外還有呂宋鰻及太平洋雙色鰻，其中僅日本鰻為溫帶型。 鰻魚為降海洄游型的魚類，在海中產卵，在河流中成長。溫帶鰻在準備降海時外觀會有明顯的改變如體色、眼徑等；相較於溫帶鰻，對於熱帶鰻的銀化過程及生活史研究卻相當有限。 本研究為了能進一步的剖析熱帶鰻的後期銀化生活史，係調查2010年12月至2013年初於宜蘭地區各河川流域所採集鱸鰻成魚樣本，利用耳石年輪及鍶鈣比了解其年齡和洄游生活史、測量其外觀特徵及計算各項生理指數，以及利用組織切片觀察黃鰻、銀化前期鰻及銀鰻生殖腺形態之變化，並且與共同生存於宜蘭地區河流流域之日本鰻做比較。 本研究結果指出，日本鰻與鱸鰻大致可利用相同的判定方法來分別不同銀化階段的鰻魚，在鱸鰻中亦發現，在逐漸成熟的過程中，其眼徑會增大、腸道則會縮小，和日本鰻擁有類似的變化。鍶鈣比分析結果則發現出日本鰻的棲息地主要可分為三種型態：1.淡水型(Sr/Ca<4x10-3)(46%)2.河口型(Sr/Ca>4x10-3)(15%) 3.兩側洄游型(38%)，其中則以兩側洄游型及淡水型居多，而鱸鰻的棲地以淡水型為主(100%)。在雌鰻生殖腺組織切片中發現，日本鰻在銀化過程中卵的形態變化明顯可分為三個主要的時期，從不成熟至較成熟為核仁染色期、周邊核仁期及油滴期，而鱸鰻母鰻的卵則沒有上述的變化。 本研究發現: (1)鱸鰻與日本鰻在銀化的過程中皆可發現其眼指數、生殖腺指數及胸鰭指數有上升的現象，而消化道指數則有下降的現象。肝指數沒有明顯的變化; (2)相較於日本鰻，鱸鰻卵的型態發育程度較日本鰻來得慢而日本鰻卵徑會有顯著增加而鱸鰻的卵徑在三個時期皆沒有顯著差異; (3)鱸鰻多居住於淡水，而日本鰻則具有彈性的棲地利用，這可能是因為長期演化造成的結果。

Eel is one of the most economically important fish for aquaculture in East Asia. There are 19 freshwater eels found around the world. In Taiwan, at least 4 species of eels have been identified. The two most abundant species are the temperate type A. japonica and tropical type A. marmorata. The other two tropical species are A. bicolor pacifica and A. luzonensis. Anguillids are one of the catadromous fishes. They spawn in the ocean and grow in the river. When temperate eels are about to descend into the ocean, there would be significant changes in their morphometry exterior like body color and ocular diameter. Compared to temperate eels, previous studies on the silvering process and life history (specifically post-migratory life history) of tropical eels are quite limited. This study aims to understand the silvering history of the tropical eel, A. marmorata. We collected samples of A. marmorata and A. japonica from December 2010 to March 2013 in rivers in the Yi-lan region of Taiwan. Otolith annuli and Sr/Ca ratios are used to determine their ages and migratory life history. Measurement of morphological characters and calculation of various physiological indexes are also employed. Observation of changes in the gonad forms for eels in the yellow eel stage,the pre-silvering stage and the silver eel stage by histology is also conducted. A comparison is also made with A. japonica coexisting in the rivers in the Yi-lan region. In this study, we classify A. marmorata into three stages during silvering like those of A. japonica. And the results also show that A. marmorata eels have similar variations during silvering like enlargement of the eyes, increase in GSI, OI and FI and degeneration of the digestive tract. From analysis of Sr/Ca ratios, we find that the habitats of A. japonica could be divided into three types, namely freshwater (Sr/Ca<4x10-3) (46%), brackish (Sr/Ca>4x10-3) (15%), and catadromous (migratory between fresh water and river mouth) (38%). Most of the A. japonica eels are freshwater types while A. marmorata are found to only stay in fresh water environments. Furthermore, morphological changes of the gonad are investigated. In the female A. japonica, oocyte development progresses from the chromatin nucleolus stage in the yellow phase eel, through the peri-nucleolus stage in the pre-silver phase eel and to the oil-drop stage in the silver phase, but the phenomena are not clear in A. marmorata. In this study we find that: (1) Silvering indices increase in OI, GSI, FI and decrease in DSI have been observed in both eels; The HSI is constant through all phases of both species. (2) Compared with A. japonica, the development of oocytes of female A. marmorata is slower than that of A. japonica in the same stage; (3) A. marmorata eels are found to stay in fresh water while A. japonica eels have more flexible behavior of habitat use. The possible reason for the phenomena is long-term evolution.