Annual Performance Report, July 1, 2019 to June 30, 2020

Résumé du livre

Kokanee Oncorhynchus nerka mature earlier than most other salmonids, typically spawning and dying between age-2 and age-4. Due to short lifespan and semelparity, kokanee are often only exploited by anglers for a short period of time during their last year of life. Use of triploid (i.e., sterile) salmonids in hatchery-supported freshwater fisheries protects native stocks from genetic introgression with hatchery fish, but other benefits may include increased longevity, survival, and growth—all of which might enhance kokanee fisheries. The objective of this study was to evaluate the response of switching hatchery-supported kokanee populations from diploid (i.e., fertile) to triploid stocking as measured through changes in age-structure, annual growth, and average length. Annual gill net samples were collected in June 2012-2018 to characterize baseline population metrics and to evaluate changes after switching to triploid fish stocking. Increased longevity was achieved at both treatment populations, whereby an additional age class established after switching to triploid fish. However, despite living an additional year, no size benefit was observed for those older fish. Annual growth remained relatively unchanged between stocking periods across all water bodies. After transitioning to triploid stocking, average length of kokanee sampled each year increased in one population and decreased in the other, emulating the patterns observed in nearby control populations. Unmarked (i.e., wild) fish were abundant throughout the study at both treatment populations, which may have confounded the effects of triploid stocking, but marked fish (i.e., recaptured triploids) were larger on average than unmarked conspecifics. Taken collectively, the effects of triploid stocking appeared to be population-specific, and was likely regulated by underlying system-level growth and recruitment conditions. However, even in the absence of a size benefit, increased longevity resulting from triploidy could be useful by extending the availability of older age classes to the sport fishery. Nevertheless, our results suggest that in general anglers will not experience an increase in kokanee abundance or size structure if triploid fish are stocked.