Biophysical Potential for Anadromous Life-History Expression of Steelhead in the Santa Ynez River, California

Conditional-strategy models for O. mykiss predict that expression of anadromy is contingent on juvenile growth rates. In coastal California, rapid growth in the first or second summer appears to trigger outmigration, and this life-history pathway depends primarily on three ecological factors: high food availability, favorable water temperatures, and connectivity with other freshwater habitats. We used this perspective to analyze biophysical potential for anadromy in the Santa Ynez River system, which currently supports native rainbow trout but few steelhead.  For the mainstem river below a large impassable dam, we used process-based models to (1) reconstruct river temperature patterns for a recent warm year and average year; (2) assess thermal potential for rapid summer growth; and (3) ask if dam releases altered thermal potential downstream. For the wilderness tributary system above the dam, we examined summertime thermal and drying patterns, by fitting geostatistical models to temperature data collected at 71 sites in the summer of a dry year. We present the implications of this ecological assessment for expression of anadromous life-histories of O. mykiss the Santa Ynez River.