Introduction Coldwater rivers downstream from reservoirs in Wyoming provide sport fisheries of state-wide and national significance (Braaten and Annear 1994). These regulated rivers, often referred to as tailwaters, support year-around trout fisheries. Recently, it has become apparent that the overwinter survival of small (20- to 25-cm TL) rainbow trout (Oncorhynchus mykiss) in these tailwaters has declined. Such is the case with the Big Horn River, downstream from Boysen Dam (Yekel 1990; 1994). However, research on the overwinter survival, movement, and habitat use of rainbow trout in tailwaters is limited. Only two studies on the winter habitat of rainbow trout in tailwaters have been published, both were on the Henrys Fork of the Snake River below Palasides Dam in Idaho (i.e. Griffith and Smith 1995; Smith and Griffith 1994).
The goal of this study was to identify winter habitat features that may limit the overwinter survival of naturally spawned (wild) and recently stocked (hatchery) rainbow trout (20- to 25-cm TL) in the Big Horn River downstream from Boysen Dam. Our objectives were to: (1) describe physical habitat, including the magnitude and variation in flow, air and water temperature, and ice formation during the 1995-96 winter (November through March); (2) describe the distribution and movements of small rainbow trout relative to changing physical habitat characteristics during the winter; and (3) assess the influence of ice formation on the habitat use and movements of small rainbow trout.
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