Juvenile Pacific salmonid habitat use in two Puget Sound lowland rivers

Abstract

ObjectiveLarge rivers are complex, productive environments that support numerous species. However, humans have extensively modified these ecosystems, contributing to the decline of Pacific salmonid Oncorhynchus spp. populations. Salmon recovery efforts rely upon an understanding of salmonid habitat needs at different life stages, but data on juvenile salmonid habitat use within large rivers are rare due to the challenges of sampling in large rivers. To help fill this information need, we used a cataraft‐mounted electrofisher to evaluate juvenile salmonid use of natural (bar, backwater, side channel, and unarmored bank) and human‐modified (riprap‐armored bank and biorevetment bank [armored banks with added wood]) channel edge habitats in the Snoqualmie and Green rivers within the Puget Sound region of Washington State.MethodsWe electrofished over 1000 25‐m transects across seven spring rearing seasons (2016–2022) and measured transect water depths and widths of low‐velocity habitat (≤0.45 m/s).ResultBars, backwaters, and side channels were shallower and had wider low‐velocity habitat than armored banks. River flow also influenced water depth and low‐velocity width, but the relationship varied depending on the habitat type. Subyearling Chinook Salmon O. tshawytscha and subyearling Coho Salmon O. kisutch occurred more frequently and were more abundant along natural edge habitats than along armored banks, while their use of biorevetment banks was intermediate between their use of natural edge habitats and armored banks. In contrast, yearling Coho Salmon and trout occurred more frequently and were more abundant along armored, biorevetment, and unarmored banks than along bars and backwaters, but they also used side channels extensively. Habitat use shifted throughout the spring, but specific shifts varied by species or life history stage.ConclusionThese results emphasize the importance of conserving natural edge habitats, removing bank armoring, and reconnecting or restoring side channels. Additionally, our results highlight that a diversity of habitat types and conditions is necessary to support multiple species and life history stages.