Assessing contributions of coldwater refuges to reproductive migration corridor conditions for adult salmon and steelhead trout in the Columbia River, USA
Diadromous fish populations face multiple challenges along their migratory routes. These challenges include suboptimal water quality, harvest, and barriers to longitudinal and lateral connectivity. Interactions among factors influencing migration success make it challenging to assess management options for improving conditions for migratory fishes along riverine migration corridors. We describe a migration corridor simulation model which integrates complex individual behavior, responds to variable habitat conditions over large areas, and is able to link migration corridor conditions to fish condition outcomes. Our model, developed within HexSim, is built around a mechanistic behavioral decision tree that drives individual interactions of fish with their spatially-explicit simulated environment. Outcomes of the migration corridor simulation model applying alternative scenarios of refuge availability show that cold water refuges can provide substantial relief from exposure to high water temperatures, but do not substantially contribute to energy conservation by migrating adults. Cooling of the Columbia River reduced reliance on cold water refuges and allowed for slight reductions in migratory energy expenditure. This modelling approach provides a framework for assessing the sufficiency of coldwater refuges, but final determination will be dependent upon water temperature management goals and species recovery targets.