Life History Diversity of Juvenile Coho Salmon and Its Contributions to Adult Returns in the Salmon River (Oregon)

Changes in management of coho salmon in the Salmon River basin on the central Oregon coast has afforded a unique opportunity to study the response of a population to the cessation of hatchery releases and to extensive estuarine wetland restoration. The study in Salmon River is timely because ecological interactions between hatchery and wild fish are implicated in reduced survival of wild fish, decreased productivity of wild coho with increasing hatchery releases, and sharply declining productivity of salmonid populations with increasing abundance of hatchery spawners.  Salmon River provides an opportunity to experimentally evaluate life history diversity and productivity as recovery proceeds in the absence of hatchery influence. Restoration of most of the historic habitat in the tidally influenced portion of the basin has reestablished connectivity between freshwater and estuarine environments, permitting opportunity for diverse life history expression.  From previous and new surveys initiated in 2008, we compare coho salmon population structure and life history characteristics during three distinct periods – pre-hatchery (1974-77), hatchery (1990-2008), and post-hatchery (2009 to present). Current findings indicate low salmon survival from egg to parr and parr to adult in the hatchery period.  The timing of adult migration and spawning in the basin has been truncated and now peaks 1.5 months earlier relative to the pre-hatchery period and compared to the timing of adjacent coastal populations.  Recruit to spawner ratio is low compared to neighboring coho salmon populations.  On a positive note, we observe a variety of juvenile migration and rearing patterns in the wild population that exploit available freshwater and estuarine habitats.  For example, subyearling and yearling coho salmon use restored and natural estuarine wetlands, particularly in the spring and winter.  PIT tag returns and juvenile life history reconstructions from the otoliths of returning adults reveal considerable phenotypic diversity in the naturally produced Salmon River population. Adult and juvenile responses to the cessation of the hatchery program and the implications for life history diversity and recovery of coho salmon will be discussed.