Re-introduction of bull trout in the upper Willamette River basin, USA

Summary

Bull trout (Salvelinus confluentus), federally threatened in the US and extirpated from three of four historic drainages in Oregon's upper Willamette River basin by the 1990s, survived only in the McKenzie River when an interagency Working Group began reintroducing wild-caught, out-migrating age-0 fry from a healthy donor population in Anderson Creek, chosen to minimize donor-population impact and preserve high genetic diversity. Between 1993 and 2005, fry were transferred to newly reconnected tributaries after impassable culverts were renovated: 6,377 to Sweetwater Creek, 670 to Olallie Creek, and 10,408 to the Middle Fork Willamette River drainage, while poor fry survival in the larger Swift Creek watershed prompted a captive-rearing program at Leaburg Fish Hatchery from 2007 onward to test whether larger, hatchery-reared juveniles would fare better. Spawning by reintroduced adults was first detected in Sweetwater Creek in 2000, seven years after initial transfers, growing to 20-22 redds annually by 2006-2009, while the Middle Fork Willamette population, first confirmed spawning in 2005, has grown more slowly to only about 15 redds. Genetic analysis found reintroduced Sweetwater and Middle Fork populations retained diversity comparable to the source population, though individual hatchery cohorts showed lower diversity, indicating multi-year transfers are needed to sustain it. Major difficulties included limited donor fry availability in some years, unsuccessful direct transfers to the larger Swift Creek watershed likely due to predation, and the continuing fragmentation effects of high-head dams. The project rates itself Successful, citing established spawning populations at multiple reconnected sites after six to eight years.