Restored seasonally inundated habitat supports juvenile salmonid rearing and growth in two California Central Valley rivers

Objective River habitat restoration projects are implemented throughout western North America to mitigate the long-term effects of anthropogenic disruptions on native salmonid (Salmonidae) populations. However, relatively little is known about the target species’ physiological or behavioral responses to habitat enhancement. We constructed four seasonally inundated habitat restoration projects in the Merced (8.5 ha) and Stanislaus (2.2 ha) rivers in California’s Central Valley. We hypothesized that increasing seasonal shallow-water habitat area would increase the quantity and quality of rearing habitat, improving juvenile salmonid rearing, growth, and variation in migration timing and body size. Methods To test our hypotheses, we compared invertebrate prey abundance and juvenile density, residence time, and growth of Chinook Salmon Oncorhynchus tshawytscha in restored seasonal habitats and the unrestored main channel. Results Invertebrate prey productivity in restored seasonally inundated habitat was variable but generally comparable to that in the main channel. In both rivers, restored seasonal habitats supported increased total salmon growth through increased residence time in wet years and faster growth rates in a below-normal precipitation year and a greater diversity in out-migration timing across all study years. Conclusions This study demonstrates that expanded seasonally inundated habitats support invertebrate prey production and are used by juvenile salmon. All sites supported natal rearing and growth, but the strength of restoration response varied by site, flow conditions, and metric. Data for rearing behavior and growth parameters of juvenile Chinook Salmon can be incorporated into existing models to better predict the rearing capacity and growth and survival benefits of seasonally inundated habitat in these and other Mediterranean-climate rivers.