アユの産卵生態-IV : 産卵水域と産卵場の地形

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Several observations have been reported on the natural features and the distribution of the spawing grounds of the Ayu, but they seem fragmentary. The present study was performed to obtain fuller information on these problems. For this purpose, the examination of the spawners and the eggs laid on stream beds and the to-pographical survey were done in 12 rivers. The results obtained were as follows: 1) The location and the extent of the spawning area were very variable, i.e., in some rivers both the distance from the mouth of the river to the spawning area and the extent of the latter were less than 10km, and in others they were over 100km and 50km respectively. 2) Some certain functional relations were recognized between the average gradient (between the mouth of a river and the spawning area) of the river beds and the distance from the mouth to the spawning area or the extent of the latter. The gradient of the spawning area also changed with the gradient of the river. But the former increased in the area of the extreme small value of the latter. 3) The gradient of the river decides the speed of the current and consequently, the composition of the gravels of the stream bed. Accordingly it is assumed that some certain ranges of the speed of the current or those of the size of the gravels are essential factors for the limitation of spawning area. 4) The spawning grounds situated at a point where the current was irregular, i. e., junction, curve, sand band and bridge. 5) As the balance between the current velocity and the size of gravels is unstable at these places pointed out above, the bed varied continually, its surface being unsettled. From the observations on the spawning behaviour of the Ayu (ISHIDA 1961), it may be suggested that unstable stream beds as mentioned above are suitable sites for the spawning of the fish.
公益社団法人日本水産学会の論文

著者

石田力三
淡水区水産研究所

アユの産卵生態-IV : 産卵水域と産卵場の地形

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