Variables influencing stream-type juvenile Chinook Salmon density within floodplain habitat in the Skagit River basin, Washington

Abstract Objective Despite decades of restoration work, Chinook Salmon Oncorhynchus tshawytscha in the Pacific Northwest remain under the protection of the U.S. Endangered Species Act (ESA). Chinook Salmon in the Skagit River basin play a vital role in the abundance and recovery of the Puget Sound Chinook Salmon Evolutionarily Significant Unit, which is currently listed as threatened under the ESA. The stream-type juvenile (STJ) life history pattern of Chinook Salmon in the Skagit River has higher ocean survival to the adult stage (i.e., productivity) than that of parr or fry out-migrants, and improvement in STJ Chinook Salmon habitat could increase abundance and diversity in the Skagit River basin. Our objective was to provide recommended ranges of variables shown to influence habitat selection in floodplains by STJ Chinook Salmon. Methods Using field observations from 70 sites within the Skagit River basin, we developed generalized linear mixed-effects models across three seasons in floodplain habitats to correlate variable ranges with densities of STJ Chinook Salmon. Result Model accuracy varied by season (summer: R2 = 0.24; winter: R2 = 0.56; spring: R2 = 0.54), and significant parameters included velocity, substrate, depth range, and distance to the closest connection with the main stem. Additional significant factors included wood cover, maximum water temperature, velocity range, and interaction of the ranges of velocity and depth. Recommended ranges for habitat variables associated with the highest densities of STJ Chinook Salmon include depths of 40–68 cm, velocities of 0.06–0.33 m/s, substrate sizes of 3–36 mm, and distances of 33–119 m to the main-stem connection. Water temperatures associated with high juvenile densities varied by season (winter: 4–6°C; summer: 9–14°C). Conclusion Our recommended ranges for habitat variables can be used to refine designs for river restoration projects intended to improve habitat for juvenile Chinook Salmon and other salmonids in the Pacific Northwest.