| 英文摘要 |
一、Japanese eel is an important aquaculture species in East Asia with high economic value. Although artificial propagation of the glass eel has been successful, the high-cost can not fully support aquaculture needed. Thus, the needed fries are all caught from the wild till now, resulting in a serious bottleneck for the eel aquaculture industry. Thus, to manage and recover the valuable natural eel resource is a key for the sustainability of eel aquaculture industry. This project aims to identify an efficient Japanese eel releasing method by studying the growth, maturation and migration of the released eels in the river. This year we will cooperate with Fisheries Research Institute for eel stock releasing work on Ilan River and Fongshan River. The eels will be marked before releasing. After re-capture, the dynamics of its life history and downstream migration will be evaluated. First part of the project is to cooperate with Fisheries Research Institute to complete the marking procedures. Two release activities were carried out and first time was done in 21th November, 2013. Total of 800 kg of eels at size of 1.5 individual/kg (1200 eels) were undergoing triple marking procedure, including cutting caudal fin, injection of 100 ppm oxyteracycline solution (OTC) and a chip with scanable number into abdominal cavity. Three fishing methods including net, fishing rod and electron fishing were used for recapture. By the end of October 2014, a total of 198 Japanese eel were caught in both rivers, and 20 of them were marked eels, including 13 females and 7 males. In ILan River, 85 wild eels has been recapture (18 marked eels), and 90 individuals were recapture in Fongshan River, including 2 marked individuals. The recapture rates were slightly lower than previous year, and we hypothesis that released eels not get used to the new environment so stop feeding in the beginning so the fishing is not efficient to catch eel. Total length of the recapture eels averaged at 730 ± 64 mm (range 655-930 mm), and average weight is 597 ± 178 g (range 419-1053 g). The condition factor (CF) ranged at 1.3 -1.8, which is similar to wild eel. The digestosomatic index was 0.9 ± 0.3 % which is slight lower indicating the releasing eels stops feeding in the nature environment for the time being. Ocular index is 5.0 ± 0.9 % and GSI is 0.6 ± 0.5 % indicating the releasing eels are still at yellow eel stage but not start silvering yet. We used otolith flurorescent OTC mark to back-calculate the growth after eel release. The eel grew 4.4±1.0 cm (range 2.3-5.8 cm) after released showing the eels continuous grow after been release into nature rivers. The recapture site is closed to the released site showing the eels settle in the nature river once they found suitable habitat, and some individuals migrated downstream for higher productivity. None upstream migration individuals were discovered. The calculated eel population size in Ilan River is 3675 individuals, and in Fongshan River is 8550 individuals. None tagged individuals were recaptured so the population size in Kaoping River can not be calculated. Second part of the project is to use radiometry tags to mark 30 eel individuals and to use receptor together with a GPS system to trace the eel movement in the river. After 4 scanning in every week after releasing, about half of the eels (16 individuals) remain in the location close to the releasing site without any movement. Among them, 10 eels downstream migrated to middle stream and 3 eels migrated even down to estuary, while only 2 eels moved slightly upstream. Those migration direction is close to the eel catadromous nature, and showing the eel releasing can benefit to the spawning eel population. However, the future releasing activities show put other factors, such as the food abundance and carry capacity of the river ecology system, into consideration before determine future site and amount to release. |
