Seasonal changes in the day and night activity of adult perch, Perca fluviatilis L.

Perca fluviatilis L. were caught by gill nets every 3 h over a 24 h period monthly from February 1975 until January 1976. The numbers caught increased with rise in temperature (correlation coefficient r = 0.86 P < 0.001). During the winter months perch migrated into deeper water although some fish were still caught in shallow water (1–6 m). Perch were active at very low light intensities. However changes in visual surface irradiance (400–700 nm) and water clarity caused inverse changes in activity. Only June did not conform to this pattern and it was presumed that spawning behaviour was of primary importance. It is suggested that increase in eutrophication with its effect on water clarity enables fish to become more active during the day in the summer months. They are thus able to utilise food sources to a maximum at an optimum period for feeding and growth.