WINTER CONCEALMENT BY SUBYEARLING RAINBOW-TROUT - SPACE SIZE SELECTION AND REDUCED CONCEALMENT UNDER SURFACE ICE AND IN TURBID WATER CONDITIONS

The proportion of rainbow trout (Oncorhynchus mykiss) concealing thems elves in simulated interstitial spaces was examined in the presence of surface ice, in turbid water, and in clear water. Tests were conducte d in enclosures in a small Idaho stream with structures that provided five rectangular spaces varying in width and height, one circular spac e, and one triangular space. Space use was assessed each morning by tr apping test fish inside the structures. Significantly more fish concea led themselves under clear water conditions than under either surface ice or turbid water conditions. Spaces narrower than the width of a te st fish with extended pectoral fins and spaces taller than the height of a test fish with dorsal fin extended were used less than would be e xpected if space use was random. The frequency with which two or more fish occurred together in the same space was similar to that expected if fish occurred together at random. Fish rarely returned to the same space on consecutive nights.