Seasonal crayfish activity as influenced by fluctuating water levels and presence of a fish predator

Abstract

The diel rhythm of juvenile (0+) Astacus astacus was studied experimentally with constant and dielly fluctuating water levels. In the first case the diel rhythm was mainly crepescular, but with tendencies of a phase shift towards diurnal rhythm during low light conditions. The locomotor activity increased with temperature and with the length of the dark period. In presence of a crepescular predator the crayfish decreased their light period activity.With fluctuating water levels the activity increased especially when the water level changes occurred during normal resting periods. Crayfish did not show any adaptation to fluctuating water levels by preferring covers constantly situated beneath the surface. The relative effect of the water level fluctuations increased with temperature.As the main part of the juvenile crayfish population is concentrated to the litoral zone, diel water level regulations are thought to be detrimental to crayfish populations even if the regulation amplitude is small.