Robust estimation of survival and contribution of captive‐bred Mallards Anas platyrhynchos to a wild population in a large‐scale release programme

Abstract

The survival of captive‐bred individuals from release into the wild to their first breeding season is crucial to assess the success of reintroduction or translocation programmes, and to assess their potential impact of wild populations. However, assessing the survival of captive‐bred individuals following their release is often complicated by immediate dispersal once in the wild. Here, we apply Lindberg's robust design model, a method that incorporates emigration from the study site, to obtain true estimates of survival of captive‐bred Mallards Anas platyrhynchos, a common duck species released on a large scale in Europe since the 1970s. Overall survival rate from release in July until the onset of the next breeding season in April was low (0.18 ± 0.07 se) and equivalent to half the first‐year survival of local wild Mallards. Higher overall detectability and temporary emigration during the hunting period revealed movements in response to hunting pressure. Such low survival of released Mallards during their first year may help prevent large‐scale genetic mixing with the wild population. Nevertheless, by combining our results with regional waterfowl counts, we estimated that a minimum of 34% of the Mallards in the region were of captive origin at the onset of the breeding season. Although most released birds quickly die, restocking for hunting may be of sufficient magnitude to affect the wild population through genetic homogenization or loss of local adaptation. Robust design protocols allow for the estimation of true survival estimates by controlling for permanent and temporary emigration and may require only a moderate increase in fieldwork effort.

Differences in the growth rate and immune strategies of farmed and wild mallard populations

“…[12,16]), strains selected for good performance under captivity typically exhibited decreased fitness, if were not exposed to various stressors present in natural environment. This is also consistent with low survival rates [14,23,24] and low level of genetic introgression [21,22] of farmed mallards in native populations. Altogether, phenotype differences documented by both our study and previous studies highlight the potential risk for phenotypic shift and a subsequent effect on the fitness of wild populations exposed to massive restocking.…”

“…Farmed mallards exhibit clear genetic divergence and decreased genetic variation compared to the native population [ 20 ]. Despite long-term massive restocking, native genotypes are still widely preserved in today’s natural populations [ 21 , 22 ], suggesting low survival rates for released individuals [ 14 , 23 , 24 ]. Nevertheless, presence of admixed individuals in natural populations indicates ongoing introgression of the farmed genotype into the wild gene pool [ 20 – 22 ].…”

Admixture between released and wild game birds: a changing genetic landscape in European mallards (Anas platyrhynchos)

“…Genetic monitoring and ringing can serve to evaluate the status of both wild and farmed populations and would also meet the need for better knowledge about numbers, origin, and future fate of farmed individuals after release (e.g., Champagnon et al 2016). We encourage further genetic studies on released mallards with a more targeted molecular marker set (e.g., SNP sets specifically developed to determine hybrid class assignment: Nussberger et al 2013), or full genomic resequencing (Kraus and Wink 2015), to better evaluate the rate of hybridization and introgression.…”

“…This may result in a naïve behavior affecting their survival after release. Despite high mortality rates, tens of thousands of farmed mallards survive their first year, simply because such large numbers are released annually (Champagnon et al 2016). These potentially mate with wild conspecifics in subsequent years, leading to introgression by non-local genotypes.…”

Survival of wild and farmed-released mallards: the Swedish example

“…Also, Brakhage (1953) found a 30% higher first fall mortality in released mallards compared to wild. Several other subsequent studies corroborate low survival and recovery rates in farmed-released mallards, 0.19 and 0.33 first-year survival rates for males and females, respectively, in Soutiere (1989); 5.3 times higher survival rate in wild mallards in Dunn et al (1995); and 0.18 survival rate from release to onset of breeding season in Champagnon et al (2016). Note, however, that Lee and Kruse (1973) found similar survival rates in wild and farmed mallards.…”