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Marine survival difference between wild and hatchery-reared steelhead trout determined during early downstream migration

Melnychuk, Michael C., Korman, Josh, Hausch, Stephen, Welch, David W., McCubbing, Don J.F., Walters, Carl J.
Canadian journal of fisheries and aquatic sciences 2014 v.71 no.6 pp. 831-846
Oncorhynchus mykiss, acoustics, adults, freshwater, hatcheries, juveniles, models, mortality, rearing, rivers, salmon, smolts, sodium hydroxide, telemetry, wild fish, British Columbia
We observed large survival differences between wild and hatchery-reared steelhead trout (Oncorhynchus mykiss) during the juvenile downstream migration immediately after release, which persisted through adult life. Following a railway spill of sodium hydroxide into the Cheakamus River, British Columbia, a short-term conservation hatchery rearing program was implemented for steelhead. We used acoustic telemetry and mark–recapture models to estimate survival of wild and (or) hatchery-reared steelhead during 4 years of the smolt migration, with both groups released in 2008. After adjusting for estimated freshwater residualization, 7%–13% of wild smolts and 30%–40% of hatchery smolts died in the first 3 km of the migration. Estimated survival from release to ocean entry was 71%–84% for wild fish and 26%–40% for hatchery fish and to exit from the Strait of Georgia system was 22%–33% for wild fish and 3.5%–6.7% for hatchery fish. A calculated 2.3-fold survival difference established during the downstream migration was similar to that after the return of adult spawners, as return rates were 8.0% for wild fish and 4.1% for hatchery fish. Contrary to current understanding, a large proportion of salmon mortality in the smolt-to-adult period, commonly termed “marine mortality”, may actually occur prior to ocean entry.