Role of Stream Ice on Fall and Winter Movements and Habitat Use by Bull Trout and Cutthroat Trout in Montana Headwater Streams

—We used radiotelemetry and underwater observation to assess fall and winter movements and habitat use by bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarki lewisi in two headwater streams in the Bitterroot River drainage, Montana, that varied markedly in habitat availability and stream ice conditions. Bull trout and cutthroat trout made extensive (.1 km) downstream overwintering movements with declining temperature in the fall. Most fish remained stationary for the remainder of the study (until late February), but some fish made additional downstream movements (1.1–1.7 km) in winter during a low-temperature (#18C) period marked by anchor ice formation. Winter movement was more extensive in the midelevation stream where frequent freezing and thawing led to variable surface ice cover and frequent supercooling (,08C). Habitat use of both species varied with availability; beaver ponds and pools with large woody debris were preferred in one stream, and pools with boulders were preferred in the other. Trout overwintered in beaver ponds in large (N 5 80–120), mixed aggregations. In both streams, both species decreased use of submerged cover following the formation of surface ice. Our results indicate that (1) continued activity by trout during winter is common in streams with dynamic ice conditions and (2) complex mixes of habitat are needed to provide suitable fall and winter habitat for these species. Populations of bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarki lewisi have declined substantially throughout their native ranges (Liknes and Graham 1988; Rieman and McIntyre 1993). Though migratory (fluvial and adfluvial) life history forms were formerly common, as a result of fragmentation and loss of habitat and of competition and hybridization with nonnative salmonids, many remaining populations persist as nonmigratory ‘‘resident’’ forms confined to headwaters (Ziller 1992; Rieman and McIntyre 1993). Little is known about movement patterns and habitat preferences of these headwater populations. We focused on defining the fall and winter movement and habitat use in headwater populations of bull trout and cutthroat trout because this * Corresponding author: [email protected] 1 Present address: Bitterroot National Forest, 88 Main Street, Stevensville, Montana 59870, USA. period is considered limiting to salmonids (Cunjak 1996). Movement of stream-resident salmonids is especially common in the fall as fish seek suitable overwintering sites in response to declining water temperatures (Bjornn 1971; Cunjak 1996) and suitable spawning sites in the case of fall-spawning salmonids like bull trout (Elle 1995). Extent of movements may vary widely, from 0 to more than 100 km, depending on life history form and habitat availability (Bjornn and Mallet 1964; West et al. 1992; Brown and Mackay 1995). Several investigators have hypothesized that subsurface ice formation and groundwater influx are major factors determining habitat suitability and carrying capacity for salmonids in many icecovered streams (Maciolek and Needham 1952; West et al. 1992; Brown and Mackay 1995; Cunjak 1996). However, this hypothesis remains largely untested since most winter studies have focused on streams that remain relatively ice-free. In the ice-covered Ram River, Alberta, Brown and MackD ow nl oa de d by [ M on ta na S ta te U ni ve rs ity B oz em an ] at 1 0: 38 0 1 Ju ly 2 01 3