Survival, Contribution, and Return of Hatchery Coho Salmon (Oncorhynchus kisutch) Released into Freshwater, Estuarine, and Marine Environments

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Abstract
We released six groups of marked yearling hatchery coho salmon (Oncorhynchus kisutch) in six locations each year for five years beginning with the 1981 brood. Fish were released immediately below Bonneville Dam (control), at the Tongue Point Coast Guard Station (head of saltwater intrusion in the Columbia River), between the jetties at the Columbia River bar, in the Columbia River plume water, in coastal water approximately 19 km north and 19 km offshore of the mouth of the river, and in oceanic water approximately 38 km offshore. We found a 1.6-fold increase in the survival index (ocean catch through September 18 each year) for the fish released at Tongue Point compared with the control group. After adjusting for differences in the survival index between release groups, we found a 2.5-fold increase in the contribution to the Columbia River gillnet fishery from the fish released at Tongue Point compared with the control group. We found no significant difference between survival of the other release groups and survival of the control group. We also found that the percentage of adult fish that returned to locations other than the Columbia basin increased as the distance the fish were transported offshore increased.