1. The idea of the inflection point of water temperature, d2T/dt2 (here, T: temperature and t: time), was applied to an analysis on the correlation between the feeding activity of goldfish and the changes of water temperature, especially taking into consideration both the time factor in the biological clock of the fish and the stimulus of water temperature for fish feeding. The results thus obtained were compared with those obtained by the use of the average water temperature and the coefficient of water temperature variation respectively.
2. The experiments were carried out for five to seven consecutive days a month during the period from March 1957 to February 1958, at the greenhouse of the Faculty of Fisheries, Hokkaido University. Feeding activity of the goldfish was recorded by an automatic feeding apparatus. For measuring the water temperature an automatic recording thermometer was used.
3. Feeding activity of the goldfish almost ceased under the condition of water temperature below 7 or 8°C. A plus correlation between the total daily feeding frequency and the average daily water temperature was clearly found so far as the water temperature ranged from about 9 to 25°C. However, even if in two consecutive days the average daily water temperature was almost the same, it was not rare that the feeding activities for the two days were found to be clearly different from each other; generally, more active feeding was recorded on a day when the coefficient of water temperature variation was larger. So, it is presumable that in such a case changes of the water temperature with high fluctuation accelerated the feeding activity as stimuli for the fish.
4. Throughout the present experiment, almost all of the daily feeding was found to be recorded at the time of the rising phase of water temperature, that is, the temperature showed the plus value in its changing rate (C°/Hr). In both the autumn and spring months, the maximum feeding frequency was often found to occur at just the time of the inflection point of water temperature. Especially, in late autumn as the water temperature fell, the peak of the feeding was found to correspond clearly with the time of the inflection point of water temperature. In the summer months, however, such a tendency was found to be very rare. 5. It seems that when the fishes were acclimatized to a relatively high temperature, the effect of change of the water temperature on fish feeding was rather inconspicuous, but when the fishes have adapted to the condition of low temperature such as in late autumn, they became more sensitive even to a slight change of the water temperature as a strong stimulus. Generally speaking, however, the peak of the fish feeding is found to occur at the time of the inflection point of water temperature. Such an idea of the inflection point may be applied also to any ecological and physiological studies.
