New Zealand Journal of Ecology (1999) 23(2): 207- 214

Brodifacoum residues in target and non-target species following an aerial poisoning operation on Motuihe Island, Hauraki Gulf, New Zealand

Research Article
J. E. Dowding 1,*
E. C. Murphy 2
C. R. Veitch 3
  1. P.O. Box 36-274, Christchurch 8030, New Zealand
  2. Science and Research Unit, Department of Conservation, Private Bag 4715, Christchurch, New Zealand
  3. 48 Manse Road, Papakura, New Zealand
*  Corresponding author

Aerial poisoning using Talon(R) 7-20 baits (active ingredient 20 ppm brodifacoum) was carried out on Motuihe Island, Hauraki Gulf, during the winter of 1997. The operation aimed to eradicate Norway rats (Rattus norvegicus) and house mice (iMus musculus) and to reduce rabbit (Oryctolagus cuniculus) numbers significantly. We studied the diet of feral house cats (Felis earns) before the operation, then monitored the impact of the operation on them to determine whether secondary brodifacoum poisoning caused a reduction in their numbers. Large numbers of rabbits remained after poisoning; this and other observations suggested that insufficient bait had been applied to achieve the aims of the programme. Cat mortality, based on radio- collared animals, was 3/14 (21%). Cats on Motuihe Island appeared to eat rabbit muscle tissue in preference to internal organs; brodifacoum concentrations are lower in muscle than in liver and we suggest that rabbits may be a poorer vector than rodents for secondary poisoning of cats. We also confirmed the risk of an aerial brodifacoum application to a range of non- target bird species. Mortality of pukeko (Porphyrio p. melanotus) and paradise shelducks (Tadorna variegata) was 49% and 60%, respectively. No mortality of New Zealand dotterels (Charadrius obscurus) or variable oystercatchers (Haematopus unicolor) was detected. Twenty-nine individuals of 10 bird species (five indigenous and five introduced) were found dead after the operation and livers of all of them contained residues of brodifacoum; toxin levels averaged 0.84 mu g per g (range 0.12-2.31 mu g per g).