池田湖の越年アユについて

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A study on the life history of land-locked ayu Plecoglossus altivelis was made in Lake Ikeda, Kagoshima Prefecture, from 1981 to 1986.Like amphidromous ayu, the life span of the land-locked fish in this lake is usually only one year and most of them die after spawning in autumn. However, a few individuals of land-locked ayu passing over the second winter are found in the lake.The specimens counting 7, 810 of ayu in the lake were collected during this study and 73 of them were thought to be the over-wintered ones, because they had a conspicuous spawning mark on the embeded part of their scales. None of 0+ fish had any such scales. The amounts of fat tissue around viscera of mature ayu decreased in the spawning season from early October to late Novermber. Also, the condition factor of ayu caught in November showed very wide range from 0.6 to 1.7. The fish whose condition factor was less than 0.9 had exteremely shrunk stomach without any food, whereas the fish of more than 1.3 in condition factor had normal stomach containing a small amounts of diatoms and blue-green algae. All specimens of 1+ fish, which were collected after December, had recovered their fat tissue and begun to grow again. Thus, it is suggested that the survival of ayu after spawning season depends on the degree of stomach shrinkage and adequate intake of food. Some of them grew 150mm in standard length or more by the next autumn.The specimens of mature 1+ fish which were caught at a spawning ground in October suggests that some of the over-wintered ayu in the lake are able to attain maturity in second spawning season.
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池田湖の越年アユについて

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