Hicks, M., Austin, C. and Armstrong J., 2023. Juvenile Salmonid Rearing Potential in Seasonally Disconnected Floodplain Habitats in the Skagit River, WA. Oregon State University, Corvallis, OR. pp. 46.

Our understanding of seasonally disconnected floodplains and how they function to
support rearing salmonids in the Pacific Northwest is limited. Past studies have found that these
dynamic habitats may provide high-risk, high-reward trade-offs for rearing fish. High prey
densities and optimal temperatures allow for accelerated growth, but low water levels in the
summer can cause fish to become trapped in potentially suboptimal or lethal conditions. We
used a Relative Elevation Map (REM) to identify potential seasonally disconnected floodplains
in the Skagit River basin in Washington state. After ground-truthing sites and evaluating fish use
and habitat characteristics in 2021, we selected a subset of ten sites for more rigorous assessment
of rearing potential in 2022. This included measurements of site connectivity, water level and
temperature, fish abundance and size distribution, and fish diet. Coho salmon (Oncorhynchus
kisutch) were the dominant salmonid species and we observed changes in fish size distribution
suggestive of ontogenetic habitat shifts and migration. Larger age-1 fish were replaced by a
smaller age-0 cohort in the spring, and these fish continued to rear until July when sites became
disconnected from the mainstem. Once sites were disconnected, water temperatures increased
and water levels decreased throughout the summer. We input temperature and fish diet data into
bioenergetics models to quantify growth potential at a range of temperature conditions and
estimate prey consumption. While temperatures in seasonally disconnected floodplains generally
remained more optimal than the mainstem, some sites became suboptimally warm (approaching
20°C in the warmest sites) in late summer and three dried completely before reconnecting to the
mainstem in the fall. Contrary to expectations, instantaneous rations (i.e., diet contents) were
low, as were bioenergetics estimates of fish prey consumption (calculated from growth, water
temperature, and prey energy density). Inferred age-0 cohort growth rates became negative in
late summer, which may reflect size-selective mortality of larger fish from bird predation. Our
findings suggest that seasonally disconnected floodplains in the Skagit River basin may pose a
risk of mortality from desiccation in late summer, but do not necessarily provide the
opportunities for accelerated growth that we predicted. Further research to elucidate patterns of
fish use in these habitats would help to inform potential restoration efforts.