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. Releases of hatchery-reared smolts and adults from the conservation hatcheries caused a substantial increase in the number of redds constructed, an index of abundance. After supplementation, abundance returned to slightly greater levels than before. In contrast, the control populations that received no hatchery steelhead had lower abundance after supplementation than before. Taken together, the results suggest that the conservation hatcheries contributed to a marginal improvement 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.