Liberation and spread of stoats (Mustela erminea) and weasels (M. nivalis) in New Zealand, 1883–1920

This paper reviews the timing and spread of weasels and stoats across the South and North Islands of New Zealand during the late nineteenth century, entirely from historical records. The flavour of the debates and the assumptions that led to the commissioning of private and government shipments of these animals are best appreciated from the original documents. I describe the sites of the early deliberate releases in Otago, Canterbury, Marlborough, and Wairarapa, and list contemporary observations of the subsequent dispersal of the released animals to named locations in Southland, Westland, Wellington, Hawke’s Bay, Auckland and Northland. Originally, weasels were landed in far greater numbers than stoats (2622 weasels and 963 stoats listed in shipment records) and, while at first they were very abundant, they are now much less abundant than stoats. Two non-exclusive hypotheses could explain this historic change: (1) depletion of supplies of their preferred small prey including birds, mice, roosting bats, lizards, frogs and invertebrates, and (2) competition with stoats. Contemporary historic written observations on the first impacts of the arrivals of weasels and stoats on the native fauna offer graphic illustrations of what has been lost, but usually failed to consider the previous impacts of the abundant rats (Rattus exulans since the late 13th century, and R. norvegicus since 1770s–90s), and cannot now be distinguished from the activities of R. rattus arriving in the 1860s–90s.