The Behaviour of Minnows ( Phoxinus Phoxinus L.) in A Light Gradient

Abstract

Minnows in a light gradient tank were almost constantly active, but were restricted in their movements by light above a certain intensity. The value of the limiting intensity differed for individual fish but lay between o·2 and 0·002 m.c. These values are of the same order as the threshold for light stimulation of the pineal complex in blind minnows, described by Scharrer. It is suggested that the pineal might be the receptor for the light limit, and evidence supporting this suggestion is discussed. Hungry minnows were not limited by light, and spent a considerable percentage of the observation time in the lightest parts of the tank. After feeding they assumed normal limited behaviour. In mid-April, prior to spawning, minnows examined in the gradient were extremely active and showed little sign of a light limit. The mean swimming speed of fish in mid-April was 159 % faster than that of fish observed in December.