鹽度對鱸鰻生長率、死亡率和滲透壓的影響及其環境適應性和棲地選擇之探討

Abstract

台灣本地的鱸鰻歷經20年的保育管制後，終於在2009年4月宣佈解除禁令，開放養殖。因經濟價值高，民間許多業者開始進行養殖。但因管制期間有關鱸鰻生態習性的基礎研究相當少，適合生長的環境條件也不清楚。尤其是鱸鰻的鹽度偏好及環境適應性，及其與日本的棲地利用之競爭關係，也有不同看法。 為了釐清鱸鰻的棲地選擇以及了解其適合其生長的條件和鹽度適應能力等。本研究進行以下四項實驗：1)日本鰻與鱸鰻幼鰻，分別在人工海水及天然海水的不同鹽度下的成長和死亡率之比較。 2)VB期鱸鰻鰻線在不同鹽度的天然海水的成長的死亡率之比較。3)滲透壓及NKA活性測定，以及4)鱸鰻耳石Sr/Ca比與鹽度之關係的建立。根據這些實驗結果，以其了解上述問題。 結果發現人工海水的鹽度從0‰增加到10‰及15‰時，鱸鰻和日本鰻平均死亡率分別從0%增加到34±10%和94±2%以及66±2%和100%(p<0.05)。由此可見兩種鰻類對人工海水鹽度極不適應。但在天然海水實驗中，兩種鰻類死亡率在四種不同鹽度(0、8、16、32‰)之間沒有顯著差異性(p＞0.05)，存活率分別為84~97％及80~97％。同時兩種幼鰻在淡水組0‰的成長率也比其他海水組(8、16、32‰)佳。VB期的鱸鰻的成長率也是淡水組比其他海水組佳，但天然海水組的死亡率隨色素發育的開始而顯著升高。淡水組的平均存活率為95.5±0.5％遠高於全海水組的54.0±9.0％。滲透壓及NKA實驗結果顯示在中期(兩個月)飼養下淡水組的滲透壓顯著較其他海水組低，且不同海水組間並沒有顯著性差異，NKA活性則海水組比淡水組高3.42倍。此外，耳石鍶鈣比可分出淡水與海水組，但海水組間則沒有顯著性差異。綜合以上結果，得知鱸鰻及日本鰻皆在淡水成長較佳，且證實鱸鰻具有鹽度適應能力，而且在台灣河川中兩種鰻可能會發生種間競爭的情形。

The giant mottled eel, Anguilla marmorata, was listed as an endangered species in Taiwan where its culture and capture was banned in the past 20 years, but just recently,the government lifted the ban in April 2009. Due to its high economic and commercial value, the giant mottled eel has been a good candidate for aquaculture. However, the biological information for culture practices and resources management is still unknown. In addition to this, the interspecific competition for habitat-use in the wild with its congeneric species A. japonica was also unclear. Meanwhile, it was believed that A marmorata inhabited high saline environment in some of the Japanese waters, which was contradictory to the general knowledge. To clarify the salinity preference, and the optimum conditions for A marmorata. Also it aims to understand the effect of salinity on mortality and growth rates of the eel. The following 4 experimental designs were formulated to understand the effect of salinity on the growth and survival rates of A. marmorata 1) compare of the growth and survival rates of A. japonica and A. marmorata elvers in both artificial and natural sea water under different salinities. 2) compare the growth and survival rate of VB stage A. marmorata glass eels in different salinities 3) compare the osmolaritic pressure and NKA activities of the elvers in different salinity in short (48hrs) and long term (2 months) acclimatization and 4) establish the relationship between different salinities and the otolith Sr/Ca ratio to understand the habitat-use of the giant mottled eel in the wild. The results indicated that in artificial seawater the mean (±SD)mortality rates of A. japonica and A. marmorata dramatically increased from 0% in 0‰ salinity to 34±10% and 94±2% in 10‰ and 66±2%、100% in 15‰, respectively. In natural sea water, the mean(±SD) survival rates in both species were not significant(84~97％ V.S 80~97％). Both species showed better growth in fresh water than saline water. A. marmorata glass eels stage also grew better in fresh water than saline water. In addition, the survival rate of A. marmorata in the first month was not different among experimented groups, but dramatically increased in the second month along with the processing pigmentation stages. These indicated that the survival rates was better in fresh water than in sea water. The osmolarity pressure was higher in sea water than fresh water, but no difference was observed among different salinities. The NKA activity of the eel was approx 3.42 folds higher in sea water than fresh water. The otolith Sr/Ca ratio can distinguish the eel living in freshwater and saltwater environment. In conclusion, the giant mottled eel has salinity adaptation abilities and both A. japonica and A. marmorata prefer fresh water than sea water. These two species may have interspecific competition in Taiwan rivers.