On the Food of the Salmonoid Fish , Ayu (Plecoglossus altivclis Temminck & Schlegel). II

The present paper, as we have mentioned in the previous report, embodies our more extensive studies concerning the food of the Ayu and also its food-habit as to whether it changes or not according to the ages¤ locality, or season. In the young stage, living in the sea or lake, Ayu feeds entirely on zooplankton (Copepoda, Ostracoda, Phyllopoda etc.); and also on the similar matters until it has grown up to about 6 cm, even after its ascent of the river. And then, according to the growth, Ayu begins, in ascent of river, to take vegetative matters, and when it has grown up more than 9 cm it does not eat animal matters already and its food habit changes entirely into the vegetative ones. That is to say, there is a stage when Ayu takes as food both animal and vegetative matters during its growth about 6 cm to 9 cm in length. But, this is a normal case and there are some differences on account of the special conditions. In the previous report, we stated that the food of adult Ayu differes in accordance with the rivers, yet this difference does not merely depend on the individual difference of rivers but also it has a tendency to show a local difference, that is, the Ayu in the southern part of Japan proper takes mostly bluegreen algae, and that of in the northern part, as well as Hokki do, takes chiefly Diatoms. It is also an interesting fact that the food of Ayu seems to show a tendency of seasonal variation•|when the water temperature is high, bluegreen algae are mostly found in its food and in the case of low temperature Diatoms are found in abundance.