The effects of unintentional supplementation on abundance and productivity of an endangered Chinook Salmon subpopulation

Objective The effects of unintentional supplementation from strays from a neighboring conservation supplementation program were evaluated in Nason Creek, a watershed in the Columbia River basin that endangered spring Chinook Salmon Oncorhynchus tshawytscha currently inhabit. Methods We used spawning ground surveys to assess adult age, sex, distribution, abundance, and productivity and a before–after-control–impact design to evaluate the effects of hatchery-origin fish in Nason Creek. Results Adults from hatchery strays made up 14% to 86% (mean = 56%) of the annual spawning escapement in Nason Creek during the unintentional supplementation period. Abundance of total spawners (both hatchery- and natural-origin fish combined) increased by 57% during the period of unintentional supplementation (2001–2016) relative to abundance before large straying events occurred (1981–2000). However, the adult productivity and the abundance of natural-origin spawners that were produced from the supplemented population was nearly halved during the period of unintentional supplementation (2005–2016) and productivity decreased with higher proportions of hatchery-origin spawners. The increases in total spawners were not statistically significantly different from those for reference populations that were not supplemented, but the decreases in natural-origin spawners were significantly different from those for reference streams. Hatchery-origin strays were generally younger (e.g., jacks) and spawned in lower elevations than natural-origin spawners. Conclusions We suggest that abundance of natural-origin spawners was reduced because of a reduction in productivity that was associated with high levels of straying from a neighboring hatchery supplementation program. The reduction may have been from a combination of factors including spawn site selection, age at maturity, hatchery domestication, or density dependence but unlikely from differences from the nearby population in genetics, which were very similar to those of adults in Nason Creek. This study demonstrates that additional hatchery-origin spawners from conservation programs may not increase natural-origin spawners in some populations even when they are very closely related and in low abundance.