Post-spawn migrations of hatchery-origin Oncorhynchus mykiss kelts in the Central Valley of California

We used acoustic telemetry to study the post-spawn movement of Oncorhynchus mykiss kelts released in April 2005 and 2006 from the Coleman National Fish Hatchery, Anderson, CA. Following release, O. mykiss kelts demonstrated both anadromous and non-anadromous life histories, with some fish alternating life history strategies between years. Anadromy was most common, characterized by a short-term residence near the release site, followed by sustained downstream emigration once initiated. O. mykiss kelts demonstrating anadromy arrived at the Golden Gate Bridge from April to mid-July. Repeat spawning migrations of anadromous O. mykiss kelts began from late-September through October of the year of release. High fidelity back to Battle Creek was observed, occurring from late-September through November. While most O. mykiss kelts were anadromous, at least 10 % remained in freshwater, or residualized. O. mykiss kelts that residualized demonstrated two distinct patterns of movement: 1) residency near the release location, and 2) potamodromy. Overall survival was high with 36 % and 48 % of O. mykiss kelts released making a repeat spawning migration and demonstrating iteroparity in 2005 and 2006, respectively. Increase in body lengths of O. mykiss kelts that returned to the Coleman NFH were significantly greater for anadromous fish, compared to fish that residualized, but survival was higher for fish that residualized. Release of hatchery O. mykiss kelts could result in both positive and negative genetic and ecological effects to hatchery- and naturally-producing salmonids. We believe the benefits of releasing O. mykiss kelts at the Coleman NFH, including increased numbers and size of fish in the recreational fishery and genetic and demographic benefits to the hatchery brood stock outweigh the limited risk to natural populations that would result from predation and competition of the relatively small number of O. mykiss kelts that resided in fresh water.