Juvenile Chinook salmon use of sandbar willows in a large-scale, simulated riparian floodplain: microhabitat and energetics

Outmigrating, juvenile Chinook salmon Oncorhynchus tshawytscha, with access to floodplains (e.g., Yolo Bypass California, USA), grow faster than those restricted to the main channel of the Sacramento River. How these young salmon might use rooted, vegetative structure (e.g., to decrease energy expenditures) while holding positions in flowing water on floodplains and flooded riparian zones is unknown. We conducted daytime experiments in a large (24.4 m long) flume containing a planted area (9.76 m × 1.22 m) of sandbar willows, Salix interior. Flume water was maintained at 1.5 m depth and 16 °C over a 15–90 cm s−1 test velocity range. Fish were videoed using 19 cameras to determine positional behavior, including their depth, use of vegetation, and tail-beat (body-undulation) frequencies (TBFs). These TBFs were replicated with similarly-sized salmon in a calibrated, Brett-type swimming respirometer, where oxygen consumption rates were measured. Using these laboratory measurements, we estimated their swimming velocities and energetic costs associated with occupying sandbar willow habitats in the flume. As flume velocities increased and the leafy canopies of the willows were bent over from the flow, salmon occupied deeper water, among the thick stems of the willows, and maintained their positions. Even at the highest (90 cm s−1) nominal flume velocities, their estimated swimming velocities were only 35.6 cm s−1, within the bottom 15 cm of the water column. This resulted in unchanged energetic costs, compared with those estimated at lower nominal water velocities. The use of vegetated (e.g., with sandbar willow common to the riparian zone) floodplains, rather than non-vegetated ones, can potentially provide energy-saving, growth-promoting daytime habitat for migrating juvenile salmonids during river-flow periods that include floodplain inundation.