日本鰻人工繁殖相關基礎面的研究

Abstract

將魚塭培育的雌性日本鰻(Anguilla japonica)在催熟的過程中之成熟度，依胸鰭的黑色化程度分為6級(0-5級)，在第4週，依據胸鰭黑色化的程度，每間隔2週，以劑量0.5-1.0 IU/g BW的人類絨毛膜促性腺激素(Human Chorionic Gonadotropin, HCG)催熟，儘量同步化雌鰻成熟度，使其在4週後，達成熟度3-4級。第4週起，體重800g以上的雌鰻，每尾每週注射4粒塘虱魚腦下垂體的研磨液，體重800g以下者注射3粒，催熟後期，配合縮短注射間隔及降低注射劑量，在注射激素後約2個月起，雌鰻可陸續達到最後成熟階段，然後以劑量30 mg/kg BW的17α-hydroxyprogesterone (17α-OHP)或2mg/kg BW的17α,20β-dihydroxy-4-pregnen-3-one (DHP)注射雌鰻，以誘發排卵或產卵。雌鰻注射17α-OHP或DHP後，以3-5尾雄鰻與之配對，雌鰻常會在100 l的小水族箱中自行產卵，無自行產卵者，輔以人工授精的方式，可達到全年均能生產受精卵的目標。 由鰻卵之受精率、能否進行皮層反應及卵之浮力，判斷受精時鰻卵之油球，應為多油球，方為正常的鰻卵。在胚體初期，胚孔無法即時閉合的胚胎，孵化率明顯較低。卵黃囊期仔魚有垂直沉降的習性，以『靜態梯形孵苗水槽』孵育仔魚，能克服卵黃囊期仔魚在育苗器中沉降的問題，可提高孵育卵黃囊期仔魚的活存率。 胚胎及卵黃囊期仔魚能適應廣泛圍的水溫範圍，原來在23℃孵育的桑椹期、原腸期及胚體期胚胎，緩慢改變水溫後，可適應18-28℃的範圍，但桑椹期及原腸期的胚胎移至18℃孵育時，孵化率明顯較低，如果C型的胚胎預先緩慢適應至26℃，則能忍受較高的水溫(30℃),1日齡的卵黃囊期仔魚能短暫適應3-32℃的水溫範圍。考慮有關鰻魚產卵及孵化受精卵操作的方便性，建議孵育日本鰻受精卵及卵黃囊期仔魚的水溫，分別設為24-26℃及26-28℃。 在鹽度25及30 ppt下孵育，孵化後之正常卵黃囊期仔魚之活存率明顯地比其他試驗組為高，卵黃囊期仔魚的沉降速度隨仔魚日齡的增加及鹽度的降低而加速，由第2日齡起，即使在鹽度30 ppt的仔魚亦會沉降至底層或躺在底部上，而鹽度20 ppt組的仔魚，由第1日齡起，即沉降至底層或躺在底部上。此外，卵黃囊期仔魚的活存率，明顯地會受橈足類之存在與否及底質類型(砂質或泥質)之影響。 仔魚培育方面，探討鰻魚仔魚懸浮在水層中攝餌(Medel Ⅰ)或在底部攝餌(Model Ⅱ)等兩種攝餌假說模式，『靜態梯形孵苗水槽』作為第一種育苗模式之育苗器，以綠藻水(Chlorella spp)、蝦片浸出液、藻類死亡沉澱所形成的藻土之發酵液、海鱺卵的研磨液、人工微粒飼料及輪蟲等作為第一種攝餌模式的餌料。第二種攝餌模式，為在易於清除底部汙物或殘餌的透明圓弧底育苗器中進行，投餵的糊狀軟性餌料，係由蝦片浸出液、海鱺卵的研磨液及維生素等添加鰻魚粉研磨而成。但無論以何種餌料或育苗模式的試驗，目前均無法使仔魚活存超過25天。由本試驗的結果顯示，影響仔魚活存的因數可能相當多，包括餌料的種類、投餌的方法、環境條件(光照度等)的控制及水質的管理等等因數，或各因數間相互的作用，均可能對仔魚的存活會有關鍵性的影響，尚待今後在此方面加強繼續探討。

The maturity of pond-reared female Japanese eels (Anguilla japonica) during the inducing maturation was divided into 6 classes (classes 0-5) based on the black coloration of the pectoral fins. Females were injected with 0.5-1 IU/g BW of Human Chorionic Gonadotropin (HCG) at 2 weeks interval in the first month in order to synchronize their maturity up to grades 3-4 within one month after beginning of hormonal treatment. Subsequently, females with body weight over 800 g were injected with a dosage of 4 catfish pituitary homogenate, and 3 catfish pituitary homogenate for those with the body weight below 800 g from the fourth week. Accompanied with shortening of injection interval and reduction of injection dosage at the later stage of induction, final maturation can be attained from 2 months after hormonal treatment. Ovulation or spawning was induced by treatment of 17α-hydroxyprogesterone (17α-OHP, 30 mg/kg BW) or 17α, 20β-dihydroxy-4-pregnen-3-one (DHP, 2 mg/kg BW). Spawning was found occasionally in 100 l aquarium by mating one female with 3-5 males. The goal of producing fertilized eggs all year around can be achieved by inducing the female broodstock to spawn in aquaria, with or without stripping. Normal fertilized eel eggs were identified as those with multi-oil droplets based on the fertilization rate, cortical reaction and their buoyancy. Those eggs with unclosed blastopore at the early embryonic stage have lower hatching rate compared with the normal one. Hatched yolk-sac larvae perform an attribute of vertical subsidence. Stocking the yolk-sac larvae in “Quiet Trapezoid Incubation Chamber (QTIC)” can overcome the problem of subsidence and improve the survival rate of yolk-sac larvae. Embryos and yolk-sac larvae can adapt to wide ranges of water temperatures. The embryos at the morula, gastrula and C-shaped stages initially incubated at 23 ℃ can adapt to the temperatures of 18-28 ℃. However, hatching rate was significantly lower when morula and gastrula stages were transferred to 18 ℃. Moreover, the C-shaped embryos can tolerate to higher temperature (30 ℃) if acclimated to 26 ℃ prior to trial. One-day old yolk-sac larvae, on the other hand, can temporarily adapt to the temperatures of 3-32 ℃. Considering other technicalities with regard to spawning and egg incubation, it is then suggested that water temperature be regulated at the range of 24-26 ℃ for incubation of Japanese eel embryo, and 26-28 ℃ for incubation of yolk-sac larvae. The survival of normal and eye-pigmented larvae was significantly higher after transferring the embryos from 30 ppt to 25 and 30 ppt compared to the other salinity treatments tested. The subsidence of yolk-sac larvae was positively correlated with age and the decrease in salinity. Most larvae in 30 ppt water moved to the lower water layer or lay on the bottom from 2 days old, while in the 20 ppt group, almost all larvae from 1 day old sank to the lower layer or lay on the bottom. Moreover, survival rate of yolk-sac larvae significantly decreased by the presence of copepods and bottom substrate (sand and silt). In aspect of larval rearing, two feeding model hypotheses were tested on water column (Model Ⅰ) and on the bottom (Model Ⅱ). The “QTIC” was used as larval rearing facility for the Model Ⅰ. The feeds prepared for the larval rearing include green water (Mainly Chlorella spp), shrimp flake fluid, fermented fluid from the algal silt, homogenate of cobia eggs, micro particulate feeds and rotifer. For Model Ⅱ, transparent round-shaped bottom vessels for easier removal of detritus and left-over feeds were used. Softly viscous feeds were made from shrimp flake fluid, homogenate of cobia eggs, vitamins and mixed with eel feed powder. Regardless of the feeds and feeding models used, the larvae could not survive for over 25 days old. From the results obtained in this study, it is therefore concluded that various factors might affect the survival of eel larvae including feeds and feeding method, environmental factors (such as light intensity etc), and water quality control. Each or combination of these factors might play a key role in the improvement of survival of eel larvae, thus further studies are recommended in this field.