Abstract
In large river basins, migratory fish populations are threatened by the combination of hydropower and climate change. With river temperatures rising and hydropower development increasing globally, the longstanding monitoring programs for threatened Pacific salmon populations in the Columbia River Basin present an opportunity to study these impacts over extended time scales. We fit a statistical model to 20 years of PIT-tagging data to jointly model the effects of temperature and dam operations (spill management) on the movement of Steelhead (anadromous Oncorhynchus mykiss) during their adult pre-spawn migration. We modeled the relationship between these factors and behaviors that pose mortality risks, including natal tributary overshoot (ascending a dam upstream of a natal tributary) and non-natal tributary use. We then used the posterior distributions of model-estimated parameters to predict the homing success of fish to natal tributaries under different climate and hydropower scenarios. Across the populations in our study, movement decisions were consistently thermally influenced, with temperature having a negative relationship with natal homing and a positive relationship with both natal tributary overshoot and non-natal tributary use. Another consistent finding across the populations in our study was that higher overshoot rates were associated with lower homing rates. Despite data limitations associated with the PIT-tag array network, we found evidence for population-specific benefits of winter spill on natal homing success, which is currently being implemented to assist the downstream migration of overshooting Steelhead. We demonstrate how integrating the effects of climate and hydropower management actions with movement ecology provides powerful insights into how species may respond to future scenarios. In our case study, we found that pre-spawn mortality of Steelhead is likely to increase with future climate change due to temperature-driven interactions with the hydrosystem, but there is potential for hydropower managers to partially offset these impacts.