Visual Observation of Fish Beneath the Ice in a Winterkill Lake

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Abstract
A device is described to directly observe fish behavior beneath the ice of a shallow lake. The viewing device, a 1.9-m tall plexiglass tube (14.0 cm inside diameter) sealed at the bottom, worked on the principle of a periscope. It was frozen into place in the center of a net enclosure. A man in a darkened hut lowered a mirror into the tube to observe the fish in the enclosure.Vertical distributions and behavior of 5 adult northern pike (Esox lucius), 6 yellow perch (Perca flavescens), and 13 bluegill (Lepomis macrochirus) were compared with the levels of dissolved oxygen, free carbon dioxide, hydrogen sulfide, and water temperature. Observations in Mystery Lake, Wisconsin, were from December 29, 1968, through January 30, 1969, during a period when environmental conditions worsened beneath the ice and resulted in a winterkill.Yellow perch were the most active, northern pike the least. Bluegill remained farther beneath the ice than did the other two species. Northern pike took up residence in domes that they formed in the undersurface of the ice. Northern pike and yellow perch frequently sounded into the anoxic layers.We conclude that differences in fish behavior were significant in prolonging survival. A combination of little locomotory activity and a position immediately beneath the ice apparently favored the longer survival of northern pike over bluegill and yellow perch.

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