Conservation hatchery effects on the abundance and spawn timing of natural steelhead populations: a replicated, before-after-control-impact experiment in the Hood Canal watershed

A 17-year, replicated, before-after-control-impact experiment involving seven natural steelhead populations from the same geographic region compared three conservation hatchery-supplemented populations to four non-supplemented (control) populations. Spawn timing varied among the populations with the average median spawn day ranging from late February to the middle of May. Among-population diversity in spawn timing persisted throughout the study period, and there was no evidence for changes in spawn timing across the study period for either the control or supplemented populations. For the supplemented populations, releases of hatchery-reared smolts and adults from the conservation hatcheries caused a substantial increase in the number of redds constructed. After supplementation ended, redd abundance returned to levels only slightly greater than before supplementation. In contrast, the control populations that received no hatchery steelhead had moderately lower redd abundance in the after period compared to the before period. Taken together, the results suggest that the conservation hatcheries contributed to a marginal improvement in adult abundance relative to the control populations. This and the few previous studies that have included non-supplemented reference populations suggest that neutral-to-positive population-level demographic benefits should be expected for carefully planned conservation hatchery programs.