Habitat factors affecting Sockeye salmon redd site selection in off-channel ponds of a river floodplain

摘要

Sockeye salmon Oncorhynchus nerka exhibit diverse use of rivers, lake beaches, sloughs, and floodplains for spawning. Their ability to use these habitats has been reduced by loss of freshwater habitat. We investigated redd site selection by sockeye salmon in two off-channel ponds (one naturally created and one man-made) of the Cedar River, Washington, which has lost floodplain habitats as a result of flow regulation, flood control, and urbanization. The purpose of the study was to identify habitat preferences of adult sockeye salmon in off-channel ponds and provide insights for preserving and restoring off-channel areas. We examined the influence of water depth, subsurface flow, substrate, water temperature, bank cover, detrital depth, distance to shore, and woody debris on redd site selection using geographic information systems, logistic regression, and electivity indices. Redds were most frequently constructed in areas with upwelling water, moderate water depths (10-80 cm), and gravel or cobble substrates. However, the importance of these attributes varied between ponds. Upwelling water was the most significant attribute influencing redd site selection in Wetland 79 (the naturally created pond). In upwelling areas, females appeared to relax selection for substrates and water depths. Conversely, females in Cavanaugh Pond (the man-made pond) selected redd sites based on water depth and substrate, with little regard for upwelling. Possible explanations for the differences in attribute selection between ponds are subsurface hydrological connections and pond morphology. Our results indicate that projects that enhance and reestablish surface connections between existing floodplain side channels, wetlands, and ponds may provide more beneficial habitat for sockeye salmon than created habitats. Restoration of naturally created areas may also allow sockeye salmon and other fish and wildlife species to exhibit diversity in an altered river system with limited habitat types.