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Island Rat Eradications
History and Development


"Over the last four decades the eradication of rats from islands around New Zealand has moved from accidental eradication following the exploratory use of baits for rat control to carefully planned complex eradications of rats and feral cats on large islands. Introduced rodents have now been eradicated from more than 90 islands." In this article Jude Gilbert explores the history and development of toxin-based eradication methods pioneered in NZ.

Successful methods used on islands range from bait stations and silos serviced on foot to aerial spread by helicopters using satellite navigation systems. By applying lessons learned from previous operations the size, complexity, and cost-effectiveness of the eradications has gradually increased. The islands now permanently cleared of introduced rodents are being used for restoration of island-seabird systems and recovery of threatened species such as large flightless invertebrates, lizards, tuatara, forest birds, and some species of plants. The most ambitious campaigns have been on remote subantarctic Campbell Island (11,300 ha) and warm temperate Raoul Island (2,938 ha), both aimed to provide long-term benefits for endemic plant and animal species including land and seabirds." 1

Great Barrier Island at 28,000 hectares would be by far the largest to date and have more logistical problems than most because it is inhabited, has several endangered species, and has large areas used for farming. However, with quarantine measures in place, domestic pets and farm animals accounted for, at risk species protected, water sources managed, and the community committed, it is considered technically feasible to eradicate rats here.

So how has it been done before? The table contains a number of examples:

Rakino is an example of an inhabited island close to a large city that has successfully eradicated rats. The island has 35 permanent residents, 75 dwellings, is partially farmed, and is 146 hectares in size. This island of mainly kikuyu pasture with coastal pohutukawas was cleared of rats using brodifacoum in bait stations set up in a 50 x 50m grid over the entire island. To ensure full cover there were 3 lines around the coastal cliffs, and offshore rock stacks were also baited. There were 750 bait stations and these were buried in the stock paddocks to reduce risk to cattle. They were refilled constantly for 3-4 weeks and then every three weeks until the bait was no longer taken. Overall this took about 6 months and used about a half tonne of bait. On-going monitoring using tracking tunnels confirms the island has remained rat free despite the number of people coming and going from the island.

In total, successful NZ rat eradication campaigns have re-created around 20,000 hectares of pest-free habitat. The aerial spread of poisons on offshore islands has been remarkably successful. None of the campaigns has yet failed to eradicate rats on the first attempt, although there have been some problems with mice. (The subject of a later article)

Literature on island eradications indicates that currently the most effective eradication method is the widespread use of a second generation anticoagulant, generally brodifacoum or other similarly based poison. The spread has to be incredibly thorough—bush, pasture, coastlines; every building, old mine shaft, cave, rock stack and small outlying island needs to be covered. In researching material for this article the writer, not an ardent fan of toxin use, has been convinced that a very cautious and carefully considered approach to eradicating rats from Barrier using this poison must be taken. There are few other options that have proved effective available currently, but research is underway to find rat-specific poisons which would reduce many of the serious issues discussed later in this article. In the time that is taken for the community to sufficiently discuss and debate the issues possibly a better method of eradicating rats may be discovered. However, using the methods available now on the Barrier would probably mean that bait would be applied in bait stations in inhabited and farmed areas, where endangered vulnerable species such as pateke range, and through an aerial spread from helicopter using GPS in rural/bush areas. The Island would be baited systematically, most likely twice with a 2-4 week break in between during the winter when rat numbers are at their lowest. The weather is a huge factor in the timing of these drops. New satellite navigation systems allow great accuracy with placing baits from a helicopter and ensuring there are no gaps. Brodifacoum is also effective in culling mice, feral cats, and rabbits but all these species will take much longer to eradicate with follow- up programmes. The full detail of an island-wide eradication will be thoroughly investigated in the proposed technical feasibility study the Trust intends to commission.

One of the key rationales behind the GBI Trust’s vision is that the perpetual use of poisons and or trapping such is practised currently on Barrier probably involves more environmental risks and many more animal deaths than a brief eradication campaign. There is growing concern about the long-term use of second generation anticoagulant toxins as used in such baits as Talon and Rid Rat. Research is showing how persistent they are in the food chain. Most people here use brodifacoum-based baits on their properties and have been doing so for years.

If the only method available that is effective is brodifacoum and the community agrees that the risks are outweighed by the benefits then this Trust proposes we do it once and do it well. With effective quarantine and monitoring systems in place we should be able to minimise the risk of reinvasion, and quickly handle any reinvasion by the odd ship-borne individual. If we have been lucky enough to keep Norway Rats, possums and mustelids from making their way to the Barrier then perhaps with a great deal of vigilance we can keep rats off once eradicated. The interest and involvement of the community here is vital.

There are a number of issues that are important to consider in any eradication process and particularly important on Barrier. The following are quoted from referenced papers and reflect current knowledge.

Impact on water supplies

"Brodifacoum is most unlikely to be found in water even after aerial application of baits for rodent control on offshore islands. Brodifacoum is not mobile in soil and is extremely insoluble in water (<10 mg/L water at pH 7). When baits disintegrate, brodifacoum will be likely to remain in the soil, where it will be slowly degraded by soil micro-organisms. The half-life in soil varies from 12 to 25 weeks depending on the soil type. Microbial degradation will be dependent on climatic factors such as temperature, and the presence of species able to degrade brodifacoum. Since brodifacoum remains absorbed in soil when baits disintegrate, only the erosion of soil itself would see any brodifacoum reaching water, and even then the brodifacoum would be likely to remain bound to organic material and settle out in the sediment."

In any case it would be a sensible precaution for people to cover their water sources if possible at the time of an aerial drop.

Non-target effectsLoading the bait distribution hopper.  Little Barrier kiore eradication.

The reports from many eradication programmes note a variety of non-target bird deaths, however, post-eradication monitoring indicates in almost all situations that the toxin had no deleterious effect on breeding and most losses were rapidly made up through more successful nest outcomes following the removal of rats. It is thought that 58% of all bird nest predations are caused by rats.

"Brodifacoum has the potential to cause both primary and secondary poisoning of non-target species. However, as with the other vertebrate pesticides, the adverse effects of brodifacoum on wildlife are dependent more on how baits are used and the behaviour of non-target species than susceptibility of individual species to the toxin. Baits in bait stations are less accessible to non-target species than baits on the ground. Secondary poisoning of birds is likely where target species (e.g. rabbits and rats) are a major part of the diet (e.g.. brown skua and harriers).

Despite these distinctions, a wide range of small and large birds have been found dead from primary or secondary poisoning after field use of brodifacoum in New Zealand: saddlebacks, blackbirds, chaffinches, house sparrows, hedge sparrows, silvereyes, song thrushes, paradise shelducks, Australian magpies, robins and western weka.

More than 90% of pukeko on Tiritiri Matangi Island were killed after aerial distribution of Talon® 20P for eradication of kiore. Some introduced ground-feeding bird species such as brown quail, blackbirds, house sparrows, and common mynahs on Tiritiri Matangi Island were also decimated. However, despite deaths of some individuals, populations of other bird species have been less affected. For example, on Red Mercury Island, all nine little spotted kiwi with radio transmitters were still alive 1 month after aerial distribution of Talon. On Tiritiri Matangi Island, little spotted kiwi, North Island saddlebacks, and North Island robin populations were not detrimentally affected by aerial distribution of Talon."

"There is no published data on the acute toxicity of brodifacoum to reptiles or amphibians. Reptiles, at least, are known to be susceptible to brodifacoum. However, in New Zealand, lizard numbers increased after use of Talon to eradicate rabbits and rats on Stanley Island and rats on Tiritiri Matangi Island."

"Invertebrates have been seen eating baits containing brodifacoum, and residues of brodifacoum have been found in beetles collected from bait stations containing Talon intended for rats on Stewart Island. It is considered that invertebrates are unlikely to be directly killed by brodifacoum. Contaminated invertebrates may pose a risk of secondary poisoning to insectivorous vertebrates. However, recent studies have shown that brodifacoum does not persist in wetas. If there is a similar lack of persistence in other invertebrates, then the risk of secondary poisoning via invertebrates would be short-lived." 2

Main non-target native species at risk on GBIs are banded rail, pateke, pukeko and moreporks.

Secondary poisoning

"In New Zealand, predator and scavenger populations have been monitored during five brodifacoum poisoning operations. Comparable numbers of brown skuas and New Zealand falcons, the main avian predators at risk, were seen before and after use of Talon in bait stations for eradication of Norway rats on  Island. There was no evidence of New Zealand falcons or moreporks being killed by use of Talon in bait stations for eradication of Norway rats on Breaksea Island. There was no evidence of a detrimental effect on populations of moreporks on Stanley Island or Red Mercury Island after aerial distribution of Talon for eradication of kiore. Moreporks and Australasian harriers on Tiritiri Matangi Island decreased after aerial distribution of Talon, but it is not known whether this was induced by poisoning or the removal of their major food item, rats.

The detection of brodifacoum residues in a range of wildlife including native birds such as kiwi (Apteryx spp.) (Robertson et al. 1993), raises serious concerns about the long-term effects of broad-scale field use of brodif-acoum in New Zealand. This is compounded by the recent detection of residues in a wide range of species: weka, morepork, Australian harrier, pukeko, grey duck, mallard, black-backed gull, robin, saddleback, chaffinch, mynah, magpie, and blackbird.

Recent surveys of wildlife have indicated that extensive contamination has occurred where there has been sustained use of brodif-acoum. In one survey undertaken in 2000 the livers were collected from feral pigs, feral red deer, feral cats, stoats, and weka that were shot, or trapped, except for one feral pig and six weka found dead. All the animals were killed in areas where brodifacoum was currently in use for possum and rat control. Fourteen out of 35 pigs (40%) contained no residues. The remaining 21 pigs, including one which was found dead, contained residues of brodifacoum at concentrations ranging from 0.007 to 1.78 mg/kg. The pig found dead contained the highest liver residue. Fifty-seven out of 71 cats (80%), and 98 out of 115 stoats (85%) contained residues. Concentrations in cats ranged from 0.078 to 1.84 mg/kg and in stoats from 0.008 to 1.32 mg/kg. Six weka were found dead and contained residues of between 0.11 and 2.3 mg/kg. The other 12 weka were trapped; four contained no residues, and eight (67%) contained between 0.01—0.95 mg/kg.

These recently acquired residue results reinforce earlier recommendations that pigs should not be hunted for human consumption, from areas where baits containing brodifacoum have been used for control, for at least 9 months after the application of the baits.

Indigenous New Zealand birds most at risk from feeding directly on cereal-based baits containing brodifacoum are those species that are naturally inquisitive and have an omnivorous diet (e.g. weka, pukeko, brown skua, and kea). The risk of secondary poisoning is probably greatest for predatory and scavenging birds (especially the weka, brown skua, Australasian harrier, morepork, and southern black-backed gull) that feed on target species (e.g. live or dead rats, rabbits, and possums).3

Main secondary poisoning species at risk on GBI are pigs, harriers, banded rail, pukeko, moreporks and domestic cats.


The risks of non-target mortality after pest control must be carefully balanced against the benefits. On islands where rats have been eliminated there have been broad ecosystem improvements. This includes the recolonisation by seabirds, improved seedling success, increases in invertebrates, amphibians, lizards, and birds, and a return to a balance of plants unaffected by the selective eating habits of rats. The economic benefits to the people of Great Barrier is to be investigated this year through a commissioned economic feasibility study.

It is very clear from current research that the use of a toxin to eliminate rats from Great Barrier has many risks and the potential for great benefits. As much as possible the Trust will inform, debate, and discuss these risks with the aim that at the time a decision from the community is needed that decision will come from an informed and considered place.


1 "From small Maria to massive Campbell Island ; forty years of rat eradications from NZ islands ." Dave Towns and Keith Broome.

NZ Journal of Zoology 2003

2/3 "Brodifacoum Research." Charles Eason, Centre for Environmental Toxicology, Landcare Research.

Note: Copies of these referenced publications are available for reading at the Claris Library.