NOCTURNAL EMERGENCE OF JUVENILE RAINBOW-TROUT FROM WINTER CONCEALMENTRELATIVE TO LIGHT-INTENSITY

This study examined the relationship between light intensity and the n umber of juvenile rainbow trout (Oncorhynchus mykiss) visible to a sno rkeler during February in the Henrys Fork of the Snake River, Idaho, U SA. Fish were concealed in the substratum during daylight. Emergence f rom concealment was observed from 30 to 80 min after real sunset time and began when stars were first visible (pyranometric irradiance, 4.5 x 10(-3) W-2). Densities of visible fish were negatively correlated wi th light intensity (r(2) = 0.81, P < 0.001). Later at night, densities decreased in the presence of moonlight and artificial light. Fish wer e observed to feed at night.