| Research Abstract |
Fisheries biology of the stocked eel population with a fin clipped mark The eel stock in Japan has tendency of decrease recently. The purpose of this study is to discuss on strategies and counter plans to recover eel stocks using research results of population processes such as survival, growth, distribution and the other ecological characters of the stocked eel population.
7977 fish (00 group) of some months cultured eel which were clipped off a right pectoral fin by scissors were stocked into the Monobe river, Kochi prefecture, Japan, in May, 2000. And 7989 eels (01 group) without a left pectoral fin were stocked into the same river, in May, 2001. The research water length was 7 km from a mouth of the river to an upper dam. Three wooden bait traps for, eels were set at 6 points respectively. Fishery statistics by river fishermen was used to analysis catch data. The research period was until December of 2003.
Annual catch(number) of 00 group was 94,46,19,21 and the ratio to total catch
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(%) was 8.5,7.5,2.6,2.6 since 2000, respectively. The instantaneous mortality rate was about 0.5. Percent catch of annual total catch added a natural eel population in the upper water place was from 57.2 to 70.2 for four years. However the percent catch of only stocked eel was from 31.6 to 514. The range of total length of eel was 28.9 to 40.5 cm in 2000 and 36.4 to 51.3 cm in 2003. The average increment was 8.4 cm for three years. The condition factor showed very wide range, but had a tendency of increase year by year. Percent yellowish body, color was from 15.7 in 2000 to 90.0 in 2003. This means stocked eels may have naturalized in the river. Percent female was from 5.3 to 95.0 since 2000. 01 group was caught only 8 fish in 2003. This resulted limitation of fishing gear to depend on eel length.
Some conclusions were as follows, Some months cultured eel could dwell and survive in a river at least for four years. It was considered that the half of stocked eels could eat natural foods, grow, become fat and make yellowish body by naturalization. It is expected that the eel stock might increase by stocking of young eels. However rehabilitation of some suitable habitats in a river for an eel population must be maintained higher survival rate and also growth rate for eels. Less
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