Effects of Pond Stocking Rate on Coho Salmon Survival at Two Lower Columbia River Fish Hatcheries

Abstract

Abstract Coho salmon (Oncorhynchus kisutch) were reared at six pond stocking rates at each of two Columbia River hatcheries in 3 years to determine if hatchery stocking rate affects postrelease survival. The low end of the stocking range was about 85% of the recommended rate, given elevation, water temperature, and release sizes. Upper ends of the range were somewhat in excess of normal production procedures. Samples of each group were coded-wire-tagged at release, and marine survival rates were estimated from tag recoveries. Scale sample measurements were used to estimate lengths at release for age-2 and age-3 adults returning to the hatcheries, and thereby determine whether high stocking rates differentially affected postrelease survivability of small and larger juveniles. There was no demonstrated effect of stocking rate on survivability for release-date density indices ranging from 0.037 to 0.43 and flow indices ranging from 1.13 to 5.10 over all years at both hatcheries. Scale analysis indicated that small and large juveniles at release survived at similar rates regardless of pond stocking rates. Sexually mature males returning at age 2 tended to have been released as larger juveniles. Males and females returning after 3 years usually had been released as smaller juveniles.