Contribution of hatchery and natural origin Chinook salmon to the Lower Yuba River

Recovery of self-sustaining populations of wild salmon is a primary goal for many conservation programs. Connectivity patterns across time and space are key to understanding the demographic and genetic boundaries of a population. The impact of immigrants on local population dynamics and fitness are largely unknown, and straying rates remain largely unquantified. Here, we used otolith (“earstone”) Sr isotopes in adult Chinook salmon returning to the Yuba River in 2009 to determine the relative contributions of fish that were produced and returned to the Yuba Rivervs. produced in other rivers or hatcheries thatstrayed to the Yuba River to spawn. We observed considerable variation in otolith Sr profiles during early freshwater rearing, indicating that the surviving adults had used a diverse array of habitats and outmigration timings as juveniles. One “profile type” was characterized by a high and stable otolith core value, indicating egg development in isotopically heavy water, but which dropped to isotopically distinct values immediately after emergence, suggesting early movements and extended rearing in habitats isotopically distinct from the Yuba mainstem. This “step” was prevalent in the adult sample (38%), so had a significant impact on our natal assignments; however, we are confident that it is Yuba-diagnostic as the only plausible explanation is that egg development occurred in isotopically heavy water (of which the Yuba is the only conceivable option). Also,we have only ever seen this “profile type” in known-origin fish from the Yuba River and never from any other Central Valley tributaries or hatcheries. Otolith thermal mark analyses further strengthened our inferences, and water sampling revealed locations of potential rearing habitats in the watershed, based on isotopic values matching those observed in some of the otolith profiles. Our data indicated that the proportion of wild Yuba fish in the 2009 escapement was 57% (48-66%), with 43% (34-52%) comprised of strays from the Feather River and the Feather, Mokelumne and Merced River Hatcheries. Of the known phenotypic spring run fish in the 2009 sample, 50% had originated in and returned to the Yuba River.