Cooperative Fish and Wildlife Research Units Program: Vermont
Education, Research and Technical Assistance for Managing Our Natural Resources

It is important to have an accurate understanding of species habitat use and demographics as managers prioritize restoration efforts and develop recovery plans for depleted salmon populations. Previous research on threatened populations of Coho Salmon has suggested that overwinter survival is a major limiting factor for population recovery in some watersheds, but these survival estimates are often confounded with emigration to alternative rearing habitats, such as estuaries. Furthermore, emigration to alternative rearing habitats can occur continuously throughout the fall and winter, which is ignored by most types of mark-recapture models. To refine previous survival estimates, we developed a multi-state mark-recapture model that allows separately estimated emigration and survival rates throughout the year. To do this, we used weekly time-varying occasions paired with discrete spatial states. We conducted extensive simulation trials to validate the use of a multi-state model on an existing 4-year PIT tag dataset in a northern California watershed. Results from our model suggest that overwinter survival rate for fish that overwinter in stream habitat and migrate as spring smolts ranged from 0.72-0.83, which is higher than expected for Coho Salmon in this region. We estimate that a substantial proportion of smaller juveniles (0.21-0.28 annually) are emigrating from upstream rearing habitat before the spring smolt migration. This implies that previous estimates of low overwinter survival of Coho Salmon could be due to high emigration rates to alternative rearing locations. Given the apparent juvenile life-history diversity of Coho Salmon, multiple emigration patterns should be considered in the design of future research, monitoring, and restoration projects.