Movements of and Habitat Use by Fluvial Bull Trout in the Blackfoot River, Montana

The seasonal movements and habitat use by fluvial bull trout Salvelinus confluentus in the Blackfoot River drainage, western Montana, were investigated by using radiotelemetry and snorkel surveys from May 1994 to October 1995. Twenty‐four bull trout made upriver migrations (mean distance, 63 ± 21 km), 33% of which were related to spawning. In June of both years fish began migrations, which appeared to be cued by an increase in maximum daily water temperature (to 17.7°C in 1995) and a decrease in discharge from peak runoff. Larger fish began moving at cooler temperatures and earlier dates than smaller fish. Migrations occurred nocturnally and were generally rapid (grand mean, 4.4 ± 2.2 km/d). Daily rates of migration were correlated with maximum daily temperatures. Spawning bull trout ascended tributaries in late June to early July, 67 ± 10 d before spawning. Nonspawning fish entered the lower portions of these tributaries after spawning fish and remained in them 28 ± 18 d before returning downriver in late August. While in Monture Creek, a major spawning tributary for the Blackfoot River, adult bull trout used deep pools and were positively associated with habitat units containing mountain whitefish Prosopitan williamsoni . Eighty‐six percent of migrants returned downriver to within 20 m of sites occupied in the spring. In 1994, two nonmigrating fish in the Blackfoot River used the confluence of a cold tributary, but no such behavior was observed in 1995. Results suggested that water temperature influenced the movement of fluvial bull trout and that tributary habitat was important for both spawning and nonspawning fish. Results also demonstrated the large spatial scale and diversity of habitats required to sustain fluvial bull trout populations.