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 themselves in simulated interstitial spaces was examined in the presence of surface ice, in turbid water, and in clear water. Tests were conducted in enclosures in a small Idaho stream with structures that provided five rectangular spaces varying in width and height, one circular space, and one triangular space. Space use was assessed each morning by trapping test fish inside the structures. Significantly more fish concealed themselves under clear water conditions than under either surface ice or turbid water conditions. Spaces narrower than the width of a test 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 expected 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.