MINNOWS BECOME NOCTURNAL AT LOW-TEMPERATURES

Juveniles of several species of salmonid become almost exclusively noc turnal in winter. Their diel activity schedule is determined mainly by temperature, with the fish seeking shelter increasingly during the da y as the temperature drops. Through controlled laboratory experiments, it is demonstrated that a comparable response is found in another str eam-dwelling species, the minnow Phoxinus phoxinus. Daytime observatio ns showed that fish were found hiding in refuges on only 20% of occasi ons at 13.2 degrees C but on over 75% of occasions at 6.6 degrees C. I n contrast, the minnows remained in the water column and used the refu ges rarely at night irrespective of water temperature. As a consequenc e their activity became increasingly nocturnal as temperature dropped. It is suggested that this may be an adaptation to avoid diurnal preda tors. (C) 1998 The Fisheries Society of the British Isles.