The influence of multiyear drought and associated reduction in tributary connectivity on an adfluvial fish species

Multiyear drought is projected to increase in frequency and duration in arid and semiarid regions across the world, threatening native species and ecosystem function. The effects of multiyear drought are often exacerbated by human water use, which manifest in reduced lake elevation, reduced stream discharge, and disruption in hydrologic connectivity for aquatic species. Here, we demonstrate that drought-driven decreasing lake levels reduce the connectivity between tributaries and lakes by creating an elevation-explicit tributary distance map. We combined long-term fish catch data and a lake elevation time-series with our elevation-explicit tributary distance map to test whether population demographics are related to drought-driven changes in tributary connectivity. We surveyed the littoral zone of Bear Lake, Utah and Idaho, USA, from full pool to a depth of >12 m, totalling 94.86 km² surveyed. As lake elevation decreased from full pool to the lowest historical elevation, tributary channel distance increased by 3.5 km. Bear Lake Bonneville Cutthroat Trout (Oncorhynchus clarkii utah) catch per unit effort was strongly negatively correlated with tributary channel distance, though individual growth rates were not influenced by drought metrics. We predicted the response of age 0 to age 10 cutthroat under high and low cobble availability, estimating up to a 61% decline in cutthroat CPUE when tributary channel distance is at its maximum historical value. Our study provided a unique opportunity to identify quantitative linkages between climate-driven changes in habitat connectivity and an ecologically important endemic sportfish, expanding our understanding of potential pathways through which climate change may affect lentic ecosystems and fishes.