日本鰻進入最後催熟時機之改進

Abstract

日本鰻在養殖環境中性腺無法發育，需藉由人工誘導方式才能促進性腺成熟，再進行雌雄交配。目前兩階段誘導技術中，第一階段為誘導性腺成熟，時間長達2至3個月，因此第二階段誘導之時機判斷相當關鍵。以往多以身體體重指數 (body weight index, BWI) 和平均卵徑為判斷依據。發現上述方法常因未能掌握適當時機而導致卵過熟 (Over-ripening)，因此本研究欲針對卵細胞內型質變化進行觀察分析。本實驗採用體重206-585 g之養殖成鰻，經海水馴化兩週開始實驗。實驗1以鮭魚腦下垂體 (salmon pituitary extracts, SPE) 和甲基睪固酮 (methyltestosterone, MT) 進行第一階段誘導，並以BWI和卵徑分佈做為進入第二階段之依據。實驗2中雄鰻在分辨出性別後施打人類絨毛膜促性腺激素 (human gonadotrophin, hCG)，而在雌鰻進入第二階段前再增加一次抽卵鏡檢，並觀察其配對行為。再利用本研究訂定之10期卵細胞型質變化，比較配對前卵細胞分佈狀況與配對結果之關聯性。 結果發現，雌、雄鰻完成第一階段誘導比例可達總實驗尾數之73.49% (n= 116)和62.50% (n= 130)，實驗2中雌鰻增加一次抽卵鏡檢後，發現進入第二階段誘導之雌鰻可由52.17%上升至70.49%。而實驗2中針對不同產卵狀況分析其卵細胞時期分佈差異時，發現5、6期占50%以上進入第二階段誘導之雌鰻個體有61.53%可成功產卵，並觀察到產卵組在兩天內卵細胞發育2個時期、BWI增加約10%，也發現同尾雌鰻有進行數次產卵行為。子代受精率為2-84%、孵化率為0-78%，幼苗在不餵食狀態下可存活至12天，但存活超過5天的數量甚少。總結來說，本研究所用的誘導方式可誘導小型成鰻至進行自然受精並獲得受精卵，但所使用的鰻魚，是否影響幼苗存活時間，仍有待進一步探究。建議未來進行鰻魚人工繁殖時，選擇BWI大於110%、卵徑700-850μm比例高於70%、5、6期占比例50%以上的雌鰻進行第二階段誘導後，可獲得較高比例可產卵之雌鰻。

The Japanese eel, Anguilla japonica, cannot complete sexual maturation in cultivated environment. We have to induce their gonads to be maturation for spawning and natural fertilization. First stage induction for maturation usually takes two to three months. The timing of entering second stage induction of maturation is critical for natural fertilization of the eel. Because oocytes tend to be over-ripening if we miss the timing of final maturation. We consider that body weight index (BWI) and average oocyte diameter are used as indicators for determines the timing of entering second stage induction is not enough. In this study, we further take ooplasm maturity which completed first stage induction into considerations, and try to determine the optimal timing for entering second stage induction. The Japanese eels for experiments were bought from commercial dealer. We used Japanese eels with a body weight ranged 206-585 g. The eels were acclimated in seawater (32-34 ‰) for two weeks before experiment started. We designed two experiments for this study. Experiment I, we used salmon pituitary extracts (SPE) and methyltestosterone (MT) for weekly injection in first stage induction and oocyte cannulation for second induction. Experiment II, we injected human gonadotrophin (hCG) for males and added one extra-cannulation and divided the morphology development of oocyte into ten stages. We also observe their mating behavior. Two experiments indicated that 73.49 % female eels (n= 116) and 62.50 % (n= 130) male eels had been successfully induced to maturation phase. The one extra-cannulation in the experiment II lead to the percentage of pairing increasing from 52.17% to 70.49%. In the experiment II, we analyzed the oocyte stage composition before pairing. It was found that if the ratios of oocytes of stage 5 and stage 6 were higher than 50%, approximately 61.53% females can spawn and get fertilized eggs. We found that BWI increased about 10% and the oocyte developed two stages within two days in spawning group. After pairing, we are amazing that one of the female release eggs three times in mating process. Fertilization rate of the egg reached 2-84% and hatching rate reached 0-78%. Larvae survived 12 days without feeding. Most of the larvae die within 5 days after hatch. Although the eel used in this study is too small, they can complete fertilization. But hatching rate is low. According to this study, we suggested that choose the females with BWI greater than 110%, ratio of oocyte diameter 700-850μm greater than 70% and the ratios of oocyte stage 5 and stage 6 greater 50% to process second stage induction would suitable.