Food Web Structure in Habitat Engineered to Improve Chinook Salmon Spawning Success in a Central Valley, California River

River habitat restoration often represents a fundamental component of management and recovery efforts for anadromous salmonids, yet the ecological effects of many commonly applied techniques have not been adequately evaluated. In cases where restoration efforts are meant to improve conditions for a single focal species, post-project monitoring of non-target ecological attributes is often altogether absent. Furthermore, even when post-project monitoring is conducted, restoration efforts designed to improve conditions for one specific river-dependent life stage may neglect how such management actions affect others. We assessed the food web structure and ecological attributes of resident fish populations in habitat engineered to improve conditions for Chinook salmon relative to unimproved habitat immediately upstream (the reference reach). Our focal restoration project was a 2.7-km channel realignment and gravel augmentation project implemented in the Merced River, California. Stable isotope analyses suggest that the dominant primary producer supporting benthic macroinvertebrates shifts from filamentous algae in the reference reach to attached diatomaceous algae in the restored reach. Prickly sculpin are smaller and more densely populated in the restored reach but exhibit a homogeneous diet throughout the river. Sacramento pikeminnow were less densely populated, possessed elevater condition factors, and disproportionately consumed large prey such as fish and gastropods in the restored reach. Juvenile Chinook salmon exhibited a diet of heterogeneous macroinvertebrate taxa in the reference reach, with most energy derived from the net-spinning cadissfly Hydropsyche. In contrast, restored-reach juvenile salmon derived nearly all energy from the drift-prone grazing mayfly Baetis. Analysis of salmon otoliths suggests that growth rates are equivalent or moderately greater in the restored reach. Although the food web structure and population attributes of fishes appear to differ between the restored and reference reaches, the engineered channel appears to be capable of supporting healthy populations of non-target fishes. The engineered habitat also seems sufficient to support juvenile salmon. Future efforts should assess habitat quality within the restored channel, as complex habitat such as large woody debris, undercut banks and boulders that act as flow refuges are largely absent and may be limiting further recovery of fish populations.