Minnows become nocturnal at low temperatures

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