群れの中の個体の行動について-I

Abstract

The present paper deals with the psychological influence of the fish number on the feeding behavior of a fish in shoal under rearing conditions. Surf-fish (Ditrema temmincki) used in this study is a coastal weak shoaling species and is devoid of a stomach. The author regarded the feeding rate (amount of food intake/body weight) as an index of the change in the pattern of the behavior. Accordingly, in the first place, the time required to reach satiation by an individual fish and the tendency of food intake by the surf-fish in the day time were examined. Subsequently, the relationships between fish number and feeding rate of an individual fish in shoal were examined. The results obtained are summarized as follows: 1) The change in the feeding rate of an individual fish within 60 minutes after 23 hours of food deprivation is shown in Fig. 1. At the beginning of the experiment, the rate registered a high value, and then decreased radically within 30 minutes. After that, it decreased gradually with the lapse of time. 2) Surf-fish showed the tendency to take food continuously after satiation (Fig. 2). 3) The feeding rate of an individual fish increased radically with increasing the number of companions in a rearing tank (Fig. 3). 4) On the contrary, in the experiment to seperate the members in a shoal consisting of 40 fish, the average feeding rate in shoal consisted of from 30 to 15 members which maintained the maximum value (14%), and decreased gradually. Under 15 individuals, the feeding rate fell in accompaniment with the decrease in the members in the shoal (Fig. 4). In view of the results described above, it may be concluded that the increase in the number of companions removes the factors that oppress the appetite of fish. In shoal, the psychological situation of an individual fish changes so as to give full scope to its physiological ability, and the fish becomes psychologically satisfied to attain a larger rate of satiation. However, the rate of satiation of fish in shoal does not always show a constant value, even if the physical and chemical conditions are at an optimum. This implies that in rearing conditions the fish changes its behaviour in accordance to its previous experiences.