Accounting for growth potential: A framework for leveraging water temperature data in aquatic connectivity decisions

ABSTRACT Fish move among complementary habitats to meet life history requirements, but their movements are often restricted by anthropogenic barriers. Programs to restore aquatic habitat connectivity via barrier remediation constitute a multi-billion-dollar investment, while intentional fragmentation through installation of conservation barriers remains an important management tool to protect native trout populations. Often, such programs for salmonids have focused on cold headwaters. However, a recent emphasis on the growth potential of warmer downstream reaches during non-summer seasons has ignited renewed interest in better understanding the benefits of spatial and temporal thermal heterogeneity in watersheds. Here, we use two heuristic case studies to illustrate how temperature data and growth models can more fully inform conservation decisions about barriers. One example explores the growth-related costs of protecting native Columbia River Redband Trout Oncorhynchus mykiss gairdneri from an invasion of Smallmouth Bass Micropterus dolomieu; a conservation barrier that intentionally fragments the watershed may reduce annual growth potential up to 6.4%. The other example illustrates how strategically replacing impassible road culverts may increase annual growth potential for midwestern Brook Trout Salvelinus fontinalis by 2.3% to 5.9%, based on the remediation strategy. We focus discussion on applying annual growth potential in habitats typically considered thermally unsuitable in summer to inform aquatic connectivity decisions that may enhance growth, and, therefore, fecundity and population resiliency, in coldwater fish. Our case studies demonstrate the application of readily available stream temperature data in combination with published growth models to better understand the implications of connectivity decisions for growth potential among complementary thermal habitats throughout a watershed. With this work, we provide a framework to formally account for growth potential in barrier management strategies and discuss the broader relevance and considerations for its use.