It is well known that the Ayu spawn on river beds of gravel and their eggs adhere firmly to the pebbles, but little information is available on the size of gravels on which the eggs are laid and the requirements of spawners for the size composition of gravels of spawning bed.
The present study is undertaken to these problems. For this purpose, on one hand, the size of gravels sampled from natural spawning beds in two well-known spawning grounds of Sagami River in Kanagawa Prefecture were measured and the numbers of eggs attaching to the gravels of various size were counted. On the other hand, the relationships between the size of spawners and that of gravels on which they spawn were pursued in experimental ponds (Fig. 1).
The results obtained were as follows:
1) At the natural spawning grounds, most of the eggs were laid on small gravels 10mm or less in diameter (Table 1).
2) It was commonly observed in the ponds that the spawning fish measured 13.3-15.9cm in length flocked to the artificial spawning beds made of relatively small gravels, viz., 20mm or less in diameter, and spawned there. Among them the larger spawners occasionally laid their eggs on larger gravels 10-20mm in diameter, but the smaller ones exclusively spawned on smaller gravels 10mm or less in diameter (Table 2).
3) From these observations it is assumed that the Ayu tend to gather for spawning round the stream bed riched with small gravels and spawned there, and it is more clearly observed in smaller fish. The fact mentioned above seems to relate to the spawning behaviour of the fish. The Ayu have a habit of moving gravels by their movement in spawning, accordingly it is assumed that they prefer to beds of gravels which they can move compartively easily.