The ecological effects of the release and management of large numbers of mallard Anas platyrhynchos for recreational hunting

Millions of Mallard Anas platyrhynchos are artificially reared and released annually in the USA, Europe and, at particularly high densities, the UK, for recreational shooting. These releases and associated management practices have both positive and negative ecological impacts—locally through direct effects and more broadly through ecological interactions. These may affect wild conspecifics, other waterbirds, and the wider aquatic ecosystem. While we have a moderate understanding of the ecological effects of releasing other gamebirds (e.g. pheasants Phasianus colchicus , red-legged partridge Alectoris rufa ), knowledge about Mallard rear-and-release impacts is limited. Our systematic review found little relevant evidence, especially regarding high-density UK releases, but identified three key patterns. First, released Mallards often interbreed with wild conspecifics. If hybridisation reduces survival or reproductive fitness, it could harm wild populations, though field and population-level evidence is limited. Second, released birds exert direct effects on release areas. Aside from eutrophication, evidence for impacts like resource competition, disease transmission, predation, and support of generalist predators is limited. Third, management by gamekeepers or landowners – such as wetland creation, supplementary feeding, and predator control - has associated effects on other waterbirds and aquatic species more generally. However, motivations for these actions may relate to other gamebird or wild waterbird shooting, making net ecological impacts hard to assess. We call for markedly more research into Mallard release and management effects and identify five key knowledge gaps that that need addressing to provide a robust evidence base for stakeholders, policymakers, and legislators about this widespread, culturally significant, and economically important activity. • Millions of Mallard are artificially reared and released annually across the world • We review their effects on wild conspecifics, other waterbirds and aquatic ecosystems • Evidence is currently limited and often weak • Releases and associated management have both positive and negative ecological impacts • We call for markedly more research into Mallard release and management