Effects of stream temperature and climate change on fluvial arctic grayling and non-native salmonids in the upper Big Hole River, Montana

Abstract

The upper Big Hole River basin in southwestern Montana supports one of the last naturally-reproducing populations of fluvial Arctic grayling Thymallus arcticus in the coterminous United States. Warm summer water temperatures and negative interactions with non-native fish (brook trout Salvelinus fontinalis, rainbow trout Oncorhynchus mykiss, and brown trout Salmo trutta) have been identified as critical factors constraining this grayling population. Arctic grayling and these non-native fishes are all cold-water species with similar temperature requirements. Understanding when, where, and to what extent water temperatures are suitable for these fishes provides a physical basis for conservation planning. In chapter 2, I used a combination of thermal infrared (TIR) imagery and stationary temperature recorders to estimate water temperatures at a relatively fine spatial (every ~ 100 m of stream length) and temporal (continuous) resolution over a large extent of the Big Hole River (~ 100 km) during the warmest part of the summer in 2008. This modeling revealed considerable spatial and temporal heterogeneity in water temperature and highlighted the value of assessing thermal regimes at relatively fine spatial and temporal scales. In chapter 3, I assessed the potential effects of climate change on thermal suitability of summer water temperatures for these salmonids. Water temperature simulations projected significant warming from the 1980s through the 2060s. Despite this warming, water temperatures in some sections of stream remained below thermal tolerance thresholds through the 2060s. These stream temperature data provide a critical foundation for understanding the dynamic, multiscale habitat needs of these mobile stream fish and can aid in developing conservation strategies for fluvial Arctic grayling.