The Past as Prelude to the Future for Understanding 21st‐Century Climate Effects on Rocky Mountain Trout

Abstract

Bioclimatic models predict large reductions in native trout across the Rocky Mountains in the 21st century but lack details about how changes will occur. Through five case histories across the region, we explore how a changing climate has been affecting streams and the potential consequences for trout. Monitoring records show trends in temperature and hydrographs consistent with a warming climate in recent decades. Biological implications include upstream shifts in thermal habitats, risk of egg scour, increased wildfire disturbances, and declining summer habitat volumes. The importance of these factors depends on the context, but temperature increases are most relevant where population boundaries are mediated by thermal constraints. Summer flow declines and wildfires will be important where trout populations are fragmented and constrained to small refugia. A critical information gap is evidence documenting how populations are adjusting to long‐term habitat trends, so biological monitoring is a priority. Biological, temperature, and discharge data from monitoring networks could be used to develop accurate vulnerability assessments that provide information regarding where conservation actions would best improve population resilience. Even with better information, future uncertainties will remain large due to unknowns regarding Earth's ultimate warming trajectory and how effects translate across scales. Maintaining or increasing the size of habitats could provide a buffer against these uncertainties.

Potential future climate effects on mountain hydrology, stream temperature, and native salmonid life history

“…However, the intercatchment comparison among Star, Lynx, and Lyons East creeks demonstrates that T s patterns can differ under similar T a conditions, presenting a conceptual model for how thermal regimes may respond to hydroclimatic change in southwestern Alberta. Many studies suggest increases in future summer T s and thermal fragmentation in headwater streams during summer months (Isaak et al 2012b;Isaak and Rieman 2013;Jones et al 2013). While the approach used in this study likely produced conservative estimates of summer T s change, our results support Tague et al (2007).…”

“…Stream thermal regimes are likely to respond to changes in hydroclimatic regimes (Cristea and Burges 2010), as the capacity for streams to store heat is inversely proportional to volume (Poole and Berman 2001;Webb et al 2003). Degradation of thermal habitat as a result of anthropogenic and natural disturbance, coupled with competition from introduced species, will likely further reduce habitat availability for native salmonids in the Rocky Mountains (Meyer et al 1999;Isaak et al 2012a;Isaak et al 2012b;Jones et al 2013). Therefore, it is important to understand how hydrologic and thermal regimes of mountain streams will respond to climate change.…”

Bull Trout Trends in Abundance and Probabilities of Persistence in Idaho

“…It may be surprising that Bull Trout abundance has been stable or increasing across much of Idaho for the last several decades, considering that (1) climate change has been warming stream temperatures in portions of the Bull Trout's range in Idaho since at least 1980 (Isaak et al 2010 and (2) Bull Trout are expected to be among the fish species with the greatest sensitivity to climate change in western North America (Rieman et al 2007;Isaak et al 2010Isaak et al , 2012 due to their need for cold water temperatures and large patches of connected habitat (Rieman and McIntyre 1995;Dunham and Rieman 1999;Wenger et al 2011). While it likely that continued stream warming will at some point become measurably detrimental to Bull Trout, the disconnect between more than three decades of climate-induced stream warming and the positive trend in abundance for many Idaho Bull Trout populations over the same period is paradoxical.…”

Full Issue PDF Volume 38, Issue 3

“…The ability to accurately project hydrologic conditions at the fine scale using macroscale models is limited (Wenger et al 2010). Therefore, short-term management priorities might include (1) biological monitoring to determine whether and how fast distributions are actually shifting, (2) development of better hydrologic estimates through additional empirical monitoring and finer-scale modeling, and (3) establishment of stream temperature monitoring sites (Isaak et al 2012). …”