Status of Westslope Cutthroat Trout in the Madison River Basin: Influence of Disperal Barriers and Stream Temperature

Abstract

We evaluated the contemporary distribution and abundance of westslope cutthroat trout (Oncorhynchus c/arki lewisi; WCT) in the Madison River basin, southwest Montana in relation to fish dispersal barriers and stream temperatures. Westslope cutthroat trout distribution boundaries were primarily shaped by natural fish dispersal barriers that excluded nonnative salmonids from upstream reaches. Most WCT populations occupied relatively short stream lengths (x= 4.51 km, SE = 1.1), and densities (x=21.9 fish >75 mm total length/100 m of stream, SE= 3.2) were generally much lower than in other drainages inside their range within Montana. Where WCT and nonnative salmonids segregated without the influence of dispersal barriers, distribution boundaries were related to stream temperature with WCT occupying colder stream reaches. Patterns of fish occurrence and stream temperature indicated that WCT have been displaced from warmer stream habitats and now occupy a narrower and colder range of stream temperatures than they did historically. Isolated populations of WCT encountered a higher and greater range of average summer stream temperatures and reached higher abundances than those populations in streams without dispersal barriers. This suggests that while colder stream temperatures may provide a competitive advantage for WCT relative to nonnative species, these habitats may be marginal due to lower individual fitness and reproductive success of WCT. Because low population sizes and isolation place many WCT populations at risk of extirpation, we recommend that WCT populations in the Madison Basin be replicated and expanded downstream to ensure their long term persistence.