Contribution of released captive-bred Mallards to the dynamics of natural populations

The consequences of releasing captive-bred game animals into the wild have received little attention, despite their potential demographic impact, as well as costs and/or benefits for recipient populations. If restocking aims at increasing harvest opportunities, increased hunting pressure is expected, which would then be supported by either wild or released individuals. On the other hand, the wild recipient population may benefit from the release of captive-bred conspecifics if this reduces hunting pressure on the former through dilution of risk or selective harvesting of captive-bred individuals. Here, we modelled a Mallard (Anas platyrhynchos) population consisting of wild individuals supplemented by captive-bred conspecifics, a very common practice in Europe over the last 40 years. The objective was to test the effect of an increase of harvest rate on released and wild individuals, respectively. Our results show that, due to the low reproductive value of the released Mal-lards, the population was hardly affected by a change in harvest of these low performance individuals. Conversely, a 15 percent increase in harvest rate of the wild individuals would lead to a quick decline of the population. We discuss these results in the context of the Camargue population, located in the South of France, which has experienced an increase in Mallard harvest without apparent reduction of population size. We suggest that this has only been possible due to the release of captive-bred Mallards.