Estimating Habitat Utilization for Chinook, Chum, and Coho Salmon in a Restoring Tidal Freshwater Wetland

Restoration of tidal wetlands in the lower Columbia River and estuary aims to benefit salmon during the critical juvenile migration phase. Assessing the effectiveness of restoration relies in part on integrating physical and biological constraints to habitat utilization. Physical limits are set by periods of inundation and water quality, while biological bounds are determined by species-specific migration timing and seasonal residency characteristics. The potential habitat opportunity (PHO) is the total time available salmon can access intertidal sites based on physical drivers, and the realized habitat utilization (RHU) is that subset of the PHO when salmon are residing or migrating through the system. Here we investigate the interaction of PHO and RHU in the Kandoll Farm restoration site using salmon presence and hydrographic data, and a simple tidal height-inundation curve used to compute wetted area. Physical constraints were mainly determined by tidal inundation, although variation in base flow (flooding) had a minor impact and water quality (especially temperature) likely limited salmon after June. Semidiurnal tidal inundation largely determined the percent of time salmonids could access channel, marsh edge, and marsh surface habitats, with spring-neap tidal variation modifying access times. Salmon migration characteristics varied between species and years and strongly impacted the cumulative hours salmon used the wetland habitat. On average, chum had remarkably consistent seasonal use of the wetland (929 ± 13 h), while Chinook and coho had more variable abundance patterns (962 ± 294 and 1278 ± 261 h, respectively). Overall RHU of the restoring Kandoll marsh by juvenile salmon was limited broadly by tidal inundation which constrained access to specific habitat types and more narrowly to a species-specific seasonal migration window.