Abstract
Abstract Many salmonid populations are of conservation concern, and the release of hatchery-produced juveniles is a frequently used measure to alleviate declines and increase harvest opportunities. While such releases may be of conservation value for some populations, stocking may also decrease the effective population size and subsequently impose additional strain on already threatened populations. In this study, we assessed how the cohort-wise effective number of breeders in five populations of Atlantic salmon (Salmo salar) were affected by supplementation. Altogether, 19 cohorts were studied (2–7 cohorts per population) by estimating the proportion hatchery-released individuals and the effective number of wild and captive breeders in each cohort of the respective populations. We show that the effect of releasing captive-bred individuals varies both between populations and between years within the same population. A Ryman–Laikre effect—where the effective number of breeders has decreased as a consequence of supplementation—was observed for 11 cohorts. We discuss how supplementation can be adapted to optimize the effective population size, demonstrate that evaluation of supplementation can be reliably achieved, and show that supplementation programmes that lead to high proportions of hatchery-origin fish on spawning grounds are more likely to induce a Ryman–Laikre effect.
Highlights
Supplementary stocking—here defined as the release of captive bred individuals into wild populations—is a frequently used measure to alleviate population declines and increase harvest opportunities for around 180 marine and anadromous species worldwide (Kitada, 2018)
We have evaluated the contemporary effects of supplementation on the effective population size in five different populations of Atlantic salmon where supplementation programmes have been ongoing for several decades
Because the Ryman–Laikre effect is related to the proportion hatchery-released fish in the populations, we investigated the relationship between proportion hatchery-released fish and NeTotal/NeWild using linear regression in R version 3.5.3 (R Development Core Team, 2018)
Summary
Supplementary stocking—here defined as the release of captive bred individuals into wild populations—is a frequently used measure to alleviate population declines and increase harvest opportunities for around 180 marine and anadromous species worldwide (Kitada, 2018). A pedigree-based method may involve genetic assignment of adult spawners to either their broodstock parents or to wild parents such that the relative contribution from hatchery-production and natural production can be compared (Christie et al, 2012b) This approach requires genetic samples from the adult population that include all phenotypes, smolt ages, and sea ages (Hutchings and Jones, 1998) for the evaluated cohorts. Our study shows how the proportion of hatchery-released fish in a population may influence the likelihood of negative population genetic effects; a response that is likely to be a concern for many salmonid supplementation programmes
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