Lingering genetic evidence of North American mallards (Anas platyrhynchos) introduced to New Zealand

Introduced species are becoming part of the landscape around the world. Unfortunately, in many cases, the exact source population for these introduced species is not known, which can hamper their proper management. Genetic investigations can shed light on the introduction process and we used the New Zealand mallard (Anas platyrhynchos) population as a case study to demonstrate the insights that genetics can provide. The mallard (Anas platyrhynchos) was introduced to New Zealand from the United Kingdom (UK) and the United States (USA) for recreational hunting in the late 19th and early 20th centuries. We used mitochondrial DNA (mtDNA) sequencing to detect any enduring genetic evidence of the two small US-sourced introductions, both of which came from the same game farm. If the US-sourced introductions were pivotal in establishing mallards in New Zealand, as has been suggested, we expected that the North-American-specific haplotypes (type-B) would be common in New Zealand’s present-day mallards. From a nationally distributed sample of 122 mallards, we identified 11 mallard mtDNA haplotypes, comprising 10 type-A haplotypes but only one North-American-specific haplotype, which was shared by six ducks. Mallards displayed low nucleotide and haplotype diversity. We also detected weak genetic structure between North and South Island populations (F_ST = 0.0961). We conclude that the concerted breeding and release of mallards between 1940 and 1960 that followed the US-sourced introductions was fuelled largely by descendants of previous UK-sourced introductions. Furthermore, we speculate that some of the US-sourced mallards may have been descended from game-farm mallards imported from Europe and therefore may not have been representative of wild US ducks.