Eat, but Don't Get Eaten- Quantifying the Energetic Benefit of Columbia River Estuary Restoration to Juvenile Salmon

The loss of wetland and tidal marsh habitat in the lower Columbia River and its estuary has been cited as a major factor in salmon decline. Restoration of shallow backwaters is proceeding, but the benefit of such restoration to juvenile salmon is poorly understood.  These habitats are important to juvenile salmon, particularly subyearling Chinook salmon, which rear in shallow estuarine habitats prior to ocean entry. Backwaters are particularly important for subyearlings because they have a protracted ‘rear-as-they-go’ migration strategy, are smaller relative to other salmonids, and thus are more vulnerable to predation. But like other juvenile salmonids subyearlings generally need to attain growth prior to ocean entry. Subyearlings are found in warmer backwater habitats in the spring when mainstem temperatures are cooler-presumably to optimize growth. But by summer, as water temperatures exceed 20ºC, they are largely excluded from backwaters and only a few persist there. The USGS participated in a restoration and monitoring action in the Columbia River Estuary. Effectiveness monitoring indicated that the project was a success in creating additional rearing habitat for juvenile salmon, particularly subyearlings. The metrics we examined confirmed the presumed, general benefits of habitat restoration- that restoration benefited fish because they accessed the restored habitat, adequate food resources existed there, and predation risk was low. Bioenergetics modeling suggests that while favorable temperatures in backwaters result in increased growth opportunity, the energy content of the observed prey differences relative to mainstem habitat is very important. Overall, the energetic benefit derived by subyearlings accessing backwaters was small. This underscores the importance of refugia from predators as a benefit of restoration for juvenile salmon.