The stamina of perches in the cold and warm seasons was studied in a portable experimental plant that allows measuring the maximum swimming force of fishes under mechanical and electrical stimulation. The field experiment was carried out in Lake Angozero (South Karelia) in March and August. The temperature of lake water was 2 and 15–17°C on the study days, respectively. Fish caught within one hour were one by one measured for the swimming force. This index for different stimulations varied on average from 1.1 to 14.1 g in March and from 15.7 to 42.0 g in August, with peaks of 21 and 113 g. All experimental specimens reacted actively to the electrical stimulation, whereas reaction to touch under low temperature was either sluggish or absent. The results of the statistical comparison of the samples indicate significant differences between seasons, refuting the widespread opinion about invariable stamina of the European perch around the year. Despite the common passivity of the fish in March, some showed quite high swimming power. It is possible that such selective reactivity creates the illusion of a lower, compared to other species, seasonal variability of the stamina in perch. The stamina is closely related to the individuals size – the larger the fish, the greater its swimming force. Simulation has shown that governing factors for the rate of increase in the swimming force with increasing fish size are the seasonal component and the threshold of excitability, which, due to subjective reasons, is higher and more variable under mechanical impact in comparison with anodic reaction.