EFFECTS OF POND STOCKING RATE ON COHO SALMON SURVIVAL AT 2 LOWER COLUMBIA RIVER FISH HATCHERIES

Coho salmon (Oncorhynchus kisutch) were reared at six pond stocking ra tes at each of two Columbia River hatcheries in 3 years to determine i f hatchery stocking rate affects postrelease survival. The low end of the stocking range was about 85% of the recommended rate, given elevat ion, water temperature, and release sizes. Upper ends of the range wer e somewhat in excess of normal production procedures. Samples of each group were coded-wire-tagged at release, and marine survival rates wer e estimated from tag recoveries. Scale sample measurements were used t o estimate lengths at release for age-2 and age-3 adults returning to the hatcheries, and thereby determine whether high stocking rates diff erentially affected postrelease survivability of small and larger juve niles. There was no demonstrated effect of stocking rate on survivabil ity for release-date density indices ranging from 0.037 to 0.43 and fl ow indices ranging from 1.13 to 5.10 over all years at both hatcheries . Scale analysis indicated that small and large juveniles at release s urvived at similar rates regardless of pond stocking rates. Sexually m ature 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.