Abstract
The release of captive-bred animals into the wild is commonly practised to restore or supplement wild populations but comes with a suite of ecological and genetic consequences. Vast numbers of hatchery-reared fish are released annually, ostensibly to restore/enhance wild populations or provide greater angling returns. While previous studies have shown that captive-bred fish perform poorly in the wild relative to wild-bred conspecifics, few have measured individual lifetime reproductive success (LRS) and how this affects population productivity. Here, we analyse data on Atlantic salmon from an intensely studied catchment into which varying numbers of captive-bred fish have escaped/been released and potentially bred over several decades. Using a molecular pedigree, we demonstrate that, on average, the LRS of captive-bred individuals was only 36% that of wild-bred individuals. A significant LRS difference remained after excluding individuals that left no surviving offspring, some of which might have simply failed to spawn, consistent with transgenerational effects on offspring survival. The annual productivity of the mixed population (wild-bred plus captive-bred) was lower in years where captive-bred fish comprised a greater fraction of potential spawners. These results bolster previous empirical and theoretical findings that intentional stocking, or non-intentional escapees, threaten, rather than enhance, recipient natural populations.
Highlights
The active management of populations to mitigate against anthropogenic change or increase opportunities for commercial or recreational exploitation occurs for many species [1,2,3]
While previous studies have shown that captive-bred fish perform poorly in the wild relative to wild-bred conspecifics, few have measured individual lifetime reproductive success (LRS) and how this affects population productivity
Evidence suggests that the deliberate or accidental escape of captive-bred conspecifics may depress the productivity of wild populations through ecological [9,10], genetic [11,12,13,14,15,16,17,18] or epigenetic mechanisms [19,20,21], as well as impacting other species [22], raising questions regarding the viability of wild populations that experience inputs of captive-bred individuals
Summary
The active management of populations to mitigate against anthropogenic change or increase opportunities for commercial or recreational exploitation occurs for many species [1,2,3]. Sea ranching programmes aim to recover all the fish released into the wild as adults as part of either a commercial fishery or an experimental scientific programme This is in contrast with ‘stocking’, where hatchery-produced fish may be released as eggs, juveniles or smolts, and intentionally allowed to spawn naturally in the wild once they have returned to the rivers as adults. Natural selection in the wild should select against wildbred individuals with high levels of captive ancestry [42], which in turn should purge hatchery alleles This purging process would still incur a demographic cost to the wild population [43], while continued influx of hatchery fish would lead to further introgression and fitness depression. We use a density-corrected measure of overall lifetime productivity (adult recruits per adult spawner) to test the prediction that population productivity is lower in years where captive-bred fish comprise a greater fraction of the potential spawning population
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