Microhabitat conditions and habitat configuration influence juvenile salmonid use of off-channel features in the Cedar River, Washington

Abstract Objective Off-channel habitats are crucial for freshwater rearing of juvenile Pacific salmon Oncorhynchus spp., but many of these river floodplain habitats have been lost to human development. While there is a clear need to restore and reconnect off-channel habitats to help recover imperiled salmonid populations, uncertainty exists regarding which characteristics to prioritize when constructing off-channel habitats to maximize the salmonid rearing capacity of these features. To address this uncertainty, we quantified the relative influence of microhabitat conditions and habitat configuration on juvenile salmonid densities within off-channel features of the Cedar River (Washington State). Methods We measured microhabitat conditions (water velocity, water depth, cover, and large wood density), habitat configuration (habitat type, distance from the main-stem river, and feature width), and salmonid densities at randomly selected sample units within side channels and backwaters. Result Water velocity and depth were strongly associated with subyearling Chinook Salmon O. tshawytscha, subyearling Coho Salmon O. kisutch, and yearling Coho Salmon densities, although the relationships varied among the species–age cohorts and some relationships changed during the spring sampling season. Cover and wood density were also associated with salmonid densities, but the relationships were weaker than for velocity and depth. Habitat configuration was associated with salmonid densities as well, although the relationships varied among the species–age cohorts and between habitat types. For example, within backwaters, subyearling Chinook Salmon densities were highest close to the main-stem river but yearling Coho Salmon densities were highest far from the main stem. Additionally, subyearling Chinook Salmon and Coho Salmon densities were highest within intermediate-width side channels, while yearling Coho Salmon densities exhibited a more complex response to side channel width. Conclusion Our results indicate that both microhabitat conditions and spatial configuration influence juvenile salmonid densities within off-channel habitats. These results also suggest that restoration practitioners should carefully weigh trade-offs among design alternatives because different habitat configurations will differentially benefit certain species or life stages.