Estimating Juvenile Salmon Estuarine Carrying Capacities to Support Restoration Planning and Evaluation

Abstract

Estimating juvenile salmon habitat carrying capacities is a critical need for restoration planning. We assimilated more than 4500 unique estimates of published juvenile densities (e.g., fish/m2) in estuarine and floodplain habitats. These density data were categorized by species and life stage, habitat type, seasonal period, and geographic region to develop frequency statistics (e.g., 25th and 75th percentiles, or quartiles). These frequency statistics were then used in a habitat expansion approach to estimate carrying capacities based on habitat extent. We demonstrate the habitat expansion approach by applying the quantiles of observed juvenile Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) densities (fish/ha) to spatial data describing current, historical or potential, and predicted (based on seal level rise) habitat extents for 16 coastal Oregon estuaries to estimate carrying capacities. Current carrying capacities based on 75th percentile springtime (Apr–Jun) densities ranged from 2902 to 33,817 fish/delta for Chinook salmon and 2507 to 20,206 fish/delta for coho salmon. Historic carrying capacities during the peak rearing period (spring) ranged from 3869 to 71,844 fish/delta for Chinook salmon and 3201 to 38,337 fish/delta for coho salmon, representing a 3 to 72% loss in Chinook salmon capacity and 2 to 67% loss in coho salmon capacity. Estimated carrying capacities were predicted to decline by 2 to 54% with 1.4 m of sea level rise in systems that are projected to lose vegetated tidal wetland habitat, while a 1 to 320% increase in capacity was predicted for systems that are predicted to increase in area with sea level rise. Finally, we demonstrate how the carrying capacity estimates can be used to estimate changes in juvenile Chinook and coho salmon capacity following restoration, which can be used to both design and evaluate restoration projects.