アユの産卵生態-III

Abstract

On the depth of water of the spawning grounds of the Ayu, several observations have been reported, but there are noticeable discrepancies among them and the reason for the discrepancies has not yet been pursued thoroughly. The present study was undertaken to secure information on this problem with special regards to the relationships between the size of spawners and the depth of water of the spawning grounds. The study consists of observations carried out in the Sagami River in Kanagawa Prefecture and experiments performed in rearing ponds of the Freshwater Fisheries Research Laboratory at Hino, Tokyo. In the field observations, the depth of water of five representative spawning grounds in Sagami River and the size of spawners sampled there were measured. In the experiments, artificial spawning beds of different depths were prepared in the ponds and the depth of the beds where spawners of different sizes laid their eggs were determined (Fig. 1). The results obtained were as follows; 1) At natural spawning grounds, large spawners tend to gather about deep streams, and small spawners shallow streams (Table 1). 2) In experimental ponds, small spawners less than 13.6cm in length spawned exclusively on beds 50cm or less in depth. While large spawners more than 15.Ocm exercised no definite selection on the depth of spawning bed (though they often spawned on deep beds more than 50cm in depth). 3) Besides the depth of water, the size composition of bottom gravel has a significant effect on the spawning of the species, small spawners tend to select bottom of smaller gravel as their spawning beds than large spawners; generally in spawning grounds of the “shallow stream type” the current is less rapid and the gravels are smaller than in the case of the “deep stream type.” Accordingly it may be said that shallow streams have conditions suitable to requirements of small spawners, and vice versa (ISHIDA, 1961).