Improvements on the timing for maturation induction in Japanese eel
Date Issued
2015
Date
2015
Author(s)
Chan, Chih-Yao
Abstract
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.
Subjects
second stage induction
oocyte stage
cannulation
natural fertilization
BWI
Type
thesis
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