The Role of Streamflow and Land Use in Limiting Oversummer Survival of Juvenile Steelhead in California Streams

Increasing human pressures on freshwater resources have led to global declines in fish populations and have made the protection of instream flows critical to the conservation of riverine ecosystems. However, uncertainty in predicting ecological responses to flow variability has hindered implementation of successful environmental flow management. An improved understanding of the relationships between streamflows and Pacific salmon Oncorhynchus spp. population persistence is particularly needed in semiarid regions such as California, where streamflows during the dry season are highly variable and increasingly threatened by withdrawals to meet human water demands. To examine the effects of summer low flows on a threatened salmonid species, we analyzed 9 years of count data for juvenile steelhead O. mykiss from nine stream reaches in four coastal California watersheds. We used a Bayesian modeling framework to examine the relative influences of streamflow, land use, and habitat quality on juvenile steelhead oversummer survival. An estimated mean survival of 30% suggested that the dry season is a significant period of stress for juvenile steelhead rearing in tributary streams. The models indicated that the magnitude of summer flow and the duration of low-flow conditions were both important in explaining interannual variation in juvenile survival rates, which showed a consistent increasing trend with higher early and late-summer flows among all sites. The results also suggested an adverse effect of intensive agricultural land use on juvenile steelhead populations, while local habitat suitability had a positive effect. Although additional research is needed to identify the mechanisms by which flow influences fish survival in the dry season, these findings indicate that water quantity is a potential limiting factor to juvenile steelhead survival in coastal California watersheds and suggest that protection of summer streamflows is important for the conservation of threatened salmonid populations.