Surface Water-Groundwater Exchange Increases Post-Emergent Salmonid Growth

Pacific salmon rearing conditions are highly variable due to a host of environmental factors, such as temperature, nutrient availability and discharge. Exchange between surface water and groundwater can affect food resources and energetic conditions for rearing salmon via multiple pathways, including changes in temperature and inputs of resource subsidies. These groundwater influences may have consequences for winter-rearing species, but this hypothesis has not been experimentally tested. We tested the hypothesis that upwelling flows cause higher growth rates in rearing salmon via an enclosure experiment using post-emergent hatchery Chinook salmon across a gradient of surface water-groundwater exchange in the Methow River, Washington. We measured key food web components and used a bioenergetics model to elucidate potential energy pathways. Results from the experiment and wild fish surveys revealed fish grew more in gaining (0.0265 g d^-1) than in losing segments (0.0145 g d^-1).  Fish from transient segments grew as much as gaining segments, but their condition was significantly lower (18.3% vs. 20.0 %). Lastly, vertical hydraulic gradient, temperature, and phosphorus explained most variation in post-emergent growth. Findings highlight the importance of surface water-groundwater exchange to rearing salmon and may help guide strategies for conserving and restoring habitats critical to this life stage.