Effects of hanging ice dams on winter movements and swimming activity of fish

Formation of a hanging dam, a large, thick sub-surface accumulation of fraz il ice that blocked river flow, altered physical environmental conditions d ramatically and common carp Cyprinus carpio and brown trout Salmo trutta ev acuated the pool in which the dam developed. A mean of 80.1% of the pool vo lume was filled with frazil ice. Mean and bottom water velocities in the po ol increased from 6 to 27 cm s(-1) and from 4 to 21 cm s(-1), respectively, when the hanging dam formed, and water depth decreased from a mean of 2.25 to 0.45 m. Activity levels of fish leaving the pool varied. Two carp had h igher swimming activities when in the overwintering pool, compared to the p eriod when they were out of it, and a third showed opposite results. (C) 20 00 The Fisheries Society of the British Isles.