Factors influencing juvenile Chinook Salmon growth in a modified floodplain landscape

Anthropogenically modified floodplains, such as flood bypasses, have increasingly received attention because of their potential to provide multiple societal functions including flood control, agriculture and ecosystem services. In many highly developed landscapes, these components of the contemporary riverscape represent remnants of historically vast seasonal floodplains. Characterizing the ecological role that these habitats play in the growth and life-history of Chinook Salmon, Oncorhynchus tshawytscha, is important for improving management of this imperiled species. Previous research indicates a high potential for off-channel floodplain habitats in these flood bypasses to support high lower trophic productivity and ideal growth conditions for juvenile Chinook Salmon. However, due to interannual variability in precipitation and dynamic hydrologic conditions inherent to the Mediterranean climate of California, the range and reproducibility of resulting habitat quality conditions requires additional growth experiments to inform conservation actions. To provide such context, we conducted 45 enclosure experiments, defined as a distinct location and year combination, spanning 4 habitat types: wetland, agriculture, river channel and canal channel within the Sutter Bypass and adjacent Sacramento River, Feather River, and Butte Creek. Experiments were conducted during wet (2019), dry (2020) and critically dry (2021) water years. During each enclosure experiment, we measured juvenile Chinook Salmon growth rates, water quality parameters (e.g. water temperature, dissolved oxygen, conductivity, pH, and chlorophyll-α), zooplankton density, and salmon diets. This high spatiotemporal replication and trophic level coverage allowed characterization of habitat quality for juvenile Chinook Salmon under a wide range of environmental conditions. During experiments, we also sampled wild juvenile Chinook Salmon to assess habitat usage of the Sutter Bypass and Butte Sink. Across all years, we found distinct water quality conditions and zooplankton communities associated with off-channel habitats. Wetland habitats supported the fastest salmon growth rates in all years (range 0.36-0.98 mm˖day-1), but magnitude of difference was less in the wet year. River channel habitats displayed the lowest heterogeneity in salmon growth response both spatially and temporally (range 0.15-0.37 mm˖day-1), while agriculture and canal channel habitats of the Sutter Bypass displayed higher heterogeneity in salmon growth (range 0.31-0.68 and 0.08-0.89 mm˖day-1 respectively). A boosted regression tree model with abiotic and biotic factors indicated the importance of key prey items and primary production for salmon growth response. Findings suggest that the growth potential of juvenile salmon is 1) variable through time and dependent on hydrologic conditions and 2) variable through space and strongly dependent on prey availability in various habitat types