Characterization of Velocity Gradients Inhabited by Juvenile Chinook Salmon by Habitat Type and Season

摘要

Focal positions of drift-feeding salmonids are often proximate to higher velocities and may be characterized by velocity gradients. Velocity gradients result in distortion of the flow field through linear deformation and angular deformation, which aresummarized as the normal and shear strain rates, respectively. The objective of our study was to use a metric termed the exposure strain rate to quantify velocity gradients used by juvenile spring Chinook salmon Oncorhynchus tshawyts-cha across habitattype, seasons, and spatial scales. Within a habitat type, focal and mean water column velocities were measured at the position of each fish. In addition, mean column velocities were measured 0.6 and 1.2 m laterally toward the center of the channel. Threeexposure strain rates were calculated as the difference between focal and mean column velocity in the vertical scale and at 0.6 and 1.2 m lateral scale divided by the length of the smallest fish (0.4 cm) length scale for all seasons and habitat types. This allowed direct comparison of exposure strain rates for all sizes offish. The data revealed that for Chinook salmon larger than 4 cm, the distribution of vertical exposure strain rates was similar across all habitat types by season. Exposure strain rates began to vary between habitat types for the lateral scale, reflecting hydraulic differences between reach-scale habitat features. We concluded that juvenile Chinook occupied a specific shear environment independent of reach-scale habitat. Exposure strain rates described microhabitat use in a manner reflecting the habitat occupancy model for drift-feeding salmonids. For this reason, exposure strain rates provide more specific information on habitat use than focal velocities alone.