Estimating Fish Passage Windows to Achieve Aquatic Habitat Connectivity for Southern California Steelhead

The National Marine Fisheries Service (NMFS) provides guidelines for establishing a minimum range of discharges (fish passage window) over which anadromous salmonids must be able to migrate past culverts, dams, and other barriers to upstream spawning habitats.  Under the current guidelines, design fish passage windows are independent of regional hydrologic conditions and vary significantly depending upon the method and data used to compute the window.  Selecting the widest possible window minimizes fish passage delay, but often creates significant engineering challenges and increases project cost.  The low abundance of Southern California Steelhead (SCS) prevents the development of regional fish passage window criteria by directly monitoring SCS migrate within Southern California’s unique hydrologic conditions.  It is hypothesized and demonstrated that understanding local hydrologic conditions and some basic fish migration behavior (and not only the total time flows fall within a fish passage window) may be crucial in estimating fish passage windows that achieve habitat connectivity.  Suggestions for further research and refining existing NMFS guidelines for SCS fish passage windows are discussed.  How establishing better fish passage windows for SCS may translate to other species of concern and the review of fish passage projects in general is also presented.