Early Emigration of Juvenile Coho Salmon: Implications for Population Monitoring

AbstractSalmon monitoring programs often measure juvenile production by operating migrant traps downstream of spawning and rearing areas during smolt migration. However, this approach does not account for individuals that move downstream of trapping locations prior to smolt sampling. We used a mark–recapture study with passive integrated transponder tagging to estimate the proportion of Coho Salmon Oncorhynchus kisutch juveniles, tagged in the fall in a Northern California stream, that migrated to rearing habitat downstream of a seasonally operated trap before spring smolt sampling. Emigrants were detected by using the migrant trap, located near the upstream limit of tidal influence, and continuously operated antennas located in tidal wetlands downstream of the trap. For all three cohorts sampled (2010, 2011, 2012), we identified two distinct emigration periods (not including fry emigrants that emigrated in spring at a size too small to tag): a fall–winter period, when early emigrant parr moved into a res...