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 w inter move ments and habitat use by bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarki lewisi in two headwa ter streams in the Bitterroot River drainage, Montana, that varied mar kedly in habitat availability and stream ice conditions. Bull trout an d cutthroat trout made extensive (>1 km) downstream overwintering move ments with declining temperature in the fall. Most fish remained stati onary for the remainder of the study (until late February), but some f ish made additional downstream movements (1.1-1.7 km) in winter during a low-temperature (less than or equal to 1 degrees C) period marked b y anchor ice formation. Winter movement was more extensive in the mid- elevation stream where frequent freezing and thawing led to variable s urface ice cover and frequent supercooling (<0 degrees C). Habitat use of both species varied with availability; beaver ponds and pools with large woody debris were preferred in one stream, and pools with bould ers were preferred in the other. Trout overwintered in beaver ponds in large (N = 80-120), mixed aggregations. In both streams, both species decreased use of submerged cover following the formation of surface i ce. Our results indicate that (1) continued activity by trout during w inter is common in streams with dynamic ice conditions and (2) complex mixes of habitat are needed to provide suitable fall and winter habit at for these species.