Drought Increases Trout Recruitment at the Expense of Adult Size in Tailwaters Downriver of Dams in Western North America

River regulation by dams has significantly altered thermal and hydrologic regimes across riverscapes, creating cold, clear “tailwaters” in which trout populations thrive. Trout in these novel ecosystems can provide large economic benefits, but also fluctuate widely in abundance and size over time. In a previous analysis of 29 dams in the Western U.S. we determined that trout recruitment is primarily influenced by flow operations, whereas adult size is influenced by fish density. We were, however, unable to include water temperature in these prior analyses. Here, we incorporate temperature into this regional analysis of trout dynamics and demonstrate that water temperature is inversely related to upstream reservoir capacity, the latter of which declines during drought or water overallocation conditions. Rainbow trout (Oncorhynchus mykiss) recruitment increased with maximum monthly temperature, especially if the warmest months occurred in summer. Additionally, adult rainbow and brown trout (Salmo trutta) size was negatively related to mean annual and maximum monthly temperature, respectively. Last, rainbow trout grew largest when the coldest month of the year occurred in late winter/early spring rather than fall/early winter. These results suggest that drought-induced temperature increases in tailwaters of the Western U.S. may enhance trout recruitment at the expense of adult size.