Effect of group size or number of individuals in a group on the rate of oxygen consumption were studied with rainbow trout of 7-23g and medaka or Japanese killifish of 180-490rng, at 10-12 and 20-21°C. respectively.
Oxygen consumption per unit weight was lower in grouped fish or fish in a larger group than that in an isolated fish or fish in a smaller group. in both the species. The group effect was still recognized in a group of 24 trout bred in a group of 20.000 fish, while not recognized in a group of 17 trout bred in a group of 30 fish (Table 1). The group effect in medaka was still manifested in the dark. but it was much reduced compared with that in the light (Fig. 3).
These facts imply that grouped fish or fish in a large group feel easy mainly by visual contact with fellow fish and reduce their oxygen consumption, while an isolated fish or fish in a small group feel uneasy and exhibit a higher rate of oxygen consumption.