Early Emigration of Juvenile Coho Salmon: Implications for Population Monitoring

Abstract

Salmon 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 restored tidal wetland (early emigrants); and a spring period, when smolts emigrated (smolt emigrants). There was little movement in the intervening period. Emigration timing varied depending on the location in the basin where fish were tagged; locations in the lower main stem generally produced more early emigrants, while locations in the upper basin produced more smolt emigrants. Across locations, early emigrants accounted for 2–25% of the fall‐marked juveniles from 2010, 8–29% from 2011, and 7–13% in 2012. Smolt emigrants accounted for 15–49% of the fall‐marked juveniles from 2010, 13–14% from 2011, and 3–35% from 2012. The consistent occurrence of early emigration in this and other recent studies brings into question estimates of smolt abundance and demographic rates (e.g., overwinter and marine survival) that do not account for this life history variant. Received June 19, 2014; accepted October 23, 2014

Comparing abundance‐based and tag‐based estimates of coho salmon marine survival

“…Abundance-based and tag-based methods produced significantly different estimates of marine survival. Consistent with the hypothesis in earlier studies (Bennett et al, 2015;Rebenack et al, 2015), abundance-based estimates of coho salmon marine survival generally were higher than tag-based estimates. Early migrating juvenile coho salmon are likely a contributing cause of these differences.…”

“…Many smolt abundance estimates do not include early migrating coho salmon that migrate downstream of trapping locations before the start of smolt‐trapping operations in the spring (Bennett et al., ; Rebenack et al., ). This could result in positive bias in abundance‐based estimates of marine survival because the abundance of outmigrating smolts is underestimated (Bennett et al., ; Rebenack et al., ). However, true marine survival at these sites is not known; this study shows that the estimates differ, but does not demonstrate that either approach produces an unbiased estimate.…”

“…Deploying tags in juveniles in the autumn prior to emigration or in early emigrants rearing below trapping locations requires a much greater field effort than tagging migrating smolts at a trap. Tagging efforts outside of the smolt‐trapping season can provide estimates of the proportions of different migration life history types in the population (Bennett et al., ; Jones et al., ; Rebenack et al., ). However, these estimates of the composition of the population do not provide a means to correct marine survival estimates without more information on the fate of individuals of each life history type (e.g., estimates of how many of the early emigrants that leave in late autumn or winter survive to become smolts in the spring; Rebenack et al., ).…”

“…Many smolt abundance estimates do not include early migrating coho salmon that migrate downstream of trapping locations before the start of smolt-trapping operations in the spring (Bennett et al, 2015;Rebenack et al, 2015). This could result in positive bias in abundance-based estimates of marine survival because the abundance of outmigrating smolts is underestimated (Bennett et al, 2015;Rebenack et al, 2015).…”

“…However, both approaches are potentially biased or subject to misinterpretation, and it is not clear whether the estimates produced by the two approaches are comparable. For example, Bennett et al (2015) and Rebenack, Ricker, Anderson, Wallace and Ward (2015) hypothesised that the abundance-based approach would overestimate marine survival because most smolt sampling programmes that estimate smolt abundance miss a component of the population that migrates downstream of the trapping location outside of the normal migration time. To determine whether the two approaches produce comparable estimates, this study used both approaches to estimate marine survival for coho salmon in three coastal watersheds in Northern California.…”