Connectivity in a montane river basin : salmonid use of a major tributary in the Smith River system

Abstract

The Smith River is a popular recreational sportfishery in western Montana, but salmonid abundances there are relatively low and limited by high summer water temperatures and low discharges. Smith River tributaries may serve as thermal refuges and also as important spawning and nursery areas. Tributaries unaltered by anthropogenic disturbances may be especially important. If so, maintaining connectivity between the main-stem river and its tributaries would be essential. Moreover, an understanding of salmonid habitat use and management in a stressed system could help identify potential climate change adaptation strategies and tactics. My goal was to determine the roles of a major undisturbed tributary in the life histories and movements of salmonids in a montane river basin. My focus was on Tenderfoot Creek, a remote, unaltered major tributary to the Smith River. A PIT-tag detection network monitored the seasonal movements of rainbow x cutthroat hybrid trout, Mountain Whitefish, Brown Trout, and Brook Trout. Abundances were estimated by electrofishing and snorkeling. Despite thermally stressful conditions in the Smith River, no tagged fish were directly observed using Tenderfoot Creek as a thermal refuge, although such use probably occurred at the confluence within the Smith River. Interchange between Tenderfoot Creek and the Smith River was common for Brown Trout, Mountain Whitefish, and rainbow x cutthroat hybrid trout and consisted mostly of spawning migrations. Some large, presumably dominant Brown Trout appeared to establish permanent territories within Tenderfoot Creek. Spawning effort by Mountain Whitefish and rainbow x cutthroat hybrid trout was high; about 7,568 Mountain Whitefish were observed in spawning aggregations in autumn and estimated abundance of rainbow x cutthroat hybrid trout juveniles (N = 25,127) was much higher than that of other taxa. Brown Trout also spawned in Tenderfoot Creek (159 redds counted in 2011 and 2012), and Brook Trout spawned in side channels and tributaries. Tenderfoot Creek is heavily used by Smith River fishes for spawning; maintaining its connectivity and habitat quality is therefore beneficial to recruitment to the Smith River fishery.