Effects of Salmon Supplementation in Idaho: 20 Years Later, What Do We Really Know?

A common response to declining salmon populations is to supplement natural spawning with hatchery fish, but post-supplementation population responses have never been assessed. We conducted a long-term assessment of supplementation (1992-2014) using a three-phase, control-impact design in two sub-basins. Redd densities (redds per km; RPK) generally increased throughout the study in both sub-basins. From Phase 1 to Phase 3, Salmon sub-basin RPK increased from 3.68 (SD 7.08) to 5.20 (SD 4.43) in treatment streams and 1.02 (SD 1.21) to 3.62 (SD 3.15) in control streams. In the Clearwater sub-basin RPK went from 0.52 (SD 0.88) to 2.29 (SD 2.42) in treatment streams and 0.34 (SD 0.49) to 2.31 (SD 1.57) in control streams. Treatment streams in the Salmon sub-basin displayed a relatively large RPK increase from Phase 1 to Phase 2 (supplementation; 3.68 (SD 7.08) to 5.71 (SD 7.53)) but remained relatively stable during Phase 3 (post supplementation), which could be due to several factors. Both treatment and control streams in the Clearwater sub-basin had almost identical RPK values over all phases, likely due to straying and lack of control structures. Our results suggest supplementation can provide a short-term demographic boost but the effect does not persist after supplementation ceases.