Reconnecting a Fragmented River: Movements of Westslope Cutthroat Trout and Bull Trout after Transport Upstream of Milltown Dam, Montana

Abstract

Milltown Dam annually impedes the migration of 11 fish species, and data suggest that fish that migrate to the dam do not spawn once their migration is impeded. Densities of westslope cutthroat trout Oncorhynchus clarki lewisi and bull trout Salvelinus confluentus are low near Milltown Dam, but their annual presence at the dam suggests the potential to reestablish fluvial life history forms and enhance local or upriver populations. Operating a fish trap at Milltown Dam in 2000 and 2001, I captured westslope cutthroat trout and bull trout, implanted 42 with radio transmitters, and transported them upstream of Milltown Dam. I monitored the implanted fish to determine posttransport movements and spawning. Most of the fish captured continued upstream to spawn, and many migrations exceeded 100 km after transport. Several fish encountered pre- or postspawning problems, including entrainment in irrigation ditches, predation by nonnative northern pike Esox lucius, angler harvest, and fallback over Milltown Dam. These movements elucidate the need for comprehensive watershed-level restoration and measures to mitigate the continued effects of Milltown Dam. Whereas habitat restoration may contribute to the long-term recovery of depressed populations, adding spawning adults provides immediate benefits to the upriver populations. Because bull trout numbers in the Clark Fork River system are very low, the addition of a few individuals could significantly enhance recruitment and aid recovery. The success of capture, transport, and spawning suggests that this is an effective, albeit labor-intensive way to enhance or restore spawning individuals to populations of westslope cutthroat trout and bull trout that have been fragmented by Milltown Dam until permanent fish passage is provided.