An Eradication of Rats

Forest and Bird, Issue 294, 1 November 1999, Page 36

An Eradication of Rats

KATHY OMBLER

KATHY OMBLER looks at progress in reclaiming our island wildlife sanctuaries from these introduced killers.

he Pied Piper of Hamelin really is just a fairy tale compared with the real life achievements of conservation staff and scientists who are ridding New Zealand’s offshore islands of rats. These introduced animals are the enemies of birds, lizards, insects and plants, and their removal has resulted in a spectacular recovery in natural values on some of our most precious reserves. To date rats have been eradicated from 54 of our offshore islands, ranging from small rock stacks to the rugged 1965 hectares of Kapiti Island. Presently, Kapiti ranks as the largest rat eradication success in New Zealand and already the reserve is showing dramatic recovery signs. Stitchbird numbers have rocketed (to a 78 percent survival rate for young birds, compared with 33 percent previously). Saddleback numbers have increased from nine pairs to 20, where previously they were declining. Anecdotal evidence reports an increase in lizards and invertebrates on Kapiti. A partly completed vegetation study has already noted increased numbers of native seedlings, many growing in some areas for the first time on record. Breeding seasons must surely improve for the bush parrot, kaka, considering that pre-poison monitoring showed half of their nests were preyed on by rats. Like other rodent-free, offshore islands, Kapiti plays a key role in safeguarding populations of New Zealand’s threatened wildlife. Several threatened species are already on the island. The way is now clear to transfer more.

During the 1990s, expertise for the eradiction of rats has developed so much that the Department of Conservation is now planning to eradicate rats from several major islands, including Little Barrier in the Hauraki Gulf, Tuahua or Mayor Island in the Bay of Islands, Raoul Island in the Kermadec group, and Campbell Island in the subantarctic. The story of rat eradications combines developing technical expertise with personal determination. Patience and extended sojourns in cold, wet and rugged environments have all played a part. Key examples of the developing techniques can be traced in the increasingly difficult challenges met. ugged Breaksea Island (170 hectares) R:: on the Fiordland coast. Norway rats were exterminated there in 1988. Rowley Taylor of the then Department of Scientific and Industrial Research led this operation, the first on such a major-sized island. The method used was a ‘rolling front’ of bait stations placed 40-50 metres apart. The island was cleared in sections with the ‘rolling front’ advancing progressively along the length of the entire island.

Volunteers cut tracks and baited stations. After the eradication, South Island saddleback, yellowhead and a rare weevil were released on Breaksea. Also the Fiordland skink, self-reintroduced from a nearby rock stack, has become quite abundant. Until 1997, Norway rats infested Ulva Island (250 hectares) which lies in the Paterson Inlet of Stewart Island. These rats were exterminated, again by laying a ‘rolling front’ of bait stations. Ulva was divided into three sections, but the workload was doubled when one section had to be re-poisoned. The Ulva eradication was regarded as a trial run for Kapiti, and confirmed the unsuitability of the labour-intensive, ‘rolling-front’ method for larger islands. On Kapiti, for example, up to 12 ‘sections’ would have been needed and the operation would have taken more than a year, encompassing a rat-breeding season. Another problem on Ulva was that weka took baits and ate poisoned rat carcasses. Because of this weka were taken off Kapiti during the poisoning. Ulva was always considered difficult because it is so close to Stewart Island. There is easy boat access and, depending on tides, it is believed to be possible for rats to

swim from Stewart Island. Two rats have since been sighted (one on Ulva, one swimming just offshore). Both were caught and killed by DoC staff Efforts to eradicate the Pacific rat or kiore from the Mokohinau group, northwest of Great Barrier Island, began in 1991. Kiore eradication was begun on five islands, ranging from one to 56 hectares, by aerial spreading of poison bait. This, and subsequent aerial poisoning in 1994 on three of the Mercury group east of the Coromandel Peninsula, confirmed the success of aerial operations. Rodent-free status of the entire Mokohinau group was confirmed in May this year with the completed eradication of 18-hectare Fanal (Motukino) Island. The threatened Mokohinau skink and tuatara are now likely to be reintroduced. Four years of feasibility trials, and consultation with three iwi (including landowners of the northern end of the island), were undertaken prior to ridding Kapiti of Norway rat and kiore. Work began with aerial poisoning in 1996. Two drops of poison bait were made to ensure total coverage. Pre-poison trials established that the Kapiti population of kiwi, kakapo, kaka, stitchbird, saddleback and kokako would not be vulnerable. However, during the poisoning, takahe, brown teal and some weka were held in pens on the island and more weka were transferred to the mainland. A new population of North Island robins was deliberately established on nearby Mana Island. After extensive post-poison monitoring, Kapiti was declared rat free in January 1999. Poisoning to eradicate kiore from Whenua Houwu/Codfish Island (1560 hectares), was carried out in 1998 and the island is now undergoing ‘post-poison’ monitoring. Two aerial drops were made, as well as intensive ground poisoning in the habitat areas of fernbirds and kakapo. Kakapo and 25 fernbirds (of a unique subspecies) were taken off during the poisoning, and 250 short-tailed bats were held in aviaries on the island. After the eradication, kakapo were returned and Campbell Island teal released. Fernbirds are likely to be returned from nearby Putauhina Island when they have bred up to a self-sustaining population. T icnoni the history of rat eradication islands, volunteers, including ornithologists and Forest and Bird members, have worked alongside specialist staff. In the Hauraki Gulf, Forest and Bird member Mike Lee personally completed the eradication of rats from Rotoroa Island (90 hectares) and nearby rockstacks as part of a

university study (Forest & Bird, August 1999). On many islands, DoC and local iwi have developed good liaison in getting rid of rats. In Southland, the local Oraka Aparima rununga of Putauhina approached DoC to eradicate rats from their island and supported the relocation of fernbirds there while neighbouring Whenua Hou was poisoned. In the Marlborough Sounds, DoC is currently planning a ‘joint eradication’ of rats from the Rangitoto group of islands, involving local Maori and Victoria

Quarantine concerns o far 54 of New Zealand’s offshore islands are now rat free. But will they stay that way? The conservation director of Forest and Bird, Kevin Smith, has concerns about DoC’s procedures to safeguard these islands from re-inva-sion. ‘In my view, few of the rat-free islands have adequate quarantine procedures, he says. "Where procedures such as bag inspections are in place, they are not consistently adhered to. Quarantine standards are highly variable. ‘It is a privilege to visit these offshore islands. They are ecological treasures. To keep them that way all visitors need to follow strict biosecurity procedures to prevent accidental rodent re-introduction and to stop the dispersal of weeds.’ Another issue concerning Kevin Smith is that mice invasion of islands is more likely once rats have been eradicated. "Mice are harder to eradicate than rats and are known to readily hitchhike around with humans. This reinforces the need for stringent rodent quarantine, he says. A senior technical officer for DoC, Pam Cromarty, acknowledges there is no single set of ‘standard operating procedures’ regarding quarantine arrangements. ‘While each conservancy is acutely aware of the importance of rodentfree islands, it is probable that more formal procedures, reflecting national "best practice", and taking into account the differing legal status of the various islands, will be developed, she says.

University of Wellington. The idea is to relocate tuatara there. ampbell Island (11,216 hectares) in é the Subantarctic World Heritage Area, is one of four major island priorities now listed in DoC’s strategic planning for rat eradication — the problem species here is the Norway rat. Bait trials have been carried out recently, and an operational plan is being prepared with funds to be sought depending on the trial results. Because of the huge distance involved and the island’s wet climate, the logistics of this operation will be difficult and the cost huge. It was originally thought the presence of cats on Campbell Island would complicate the eradication, but a recent search has detected no sign of them. Also high on the priority list is Hauturu, Little Barrier Island (3000 hectares), which is infested with kiore. Some aspects of planning are underway and negotiations with iwi are progressing with a view to making this a millennium project. (See ‘Insects at Risk’, page 32 this issue.) Eradication of Norway rats and kiore, from Mayor Island/Tuahua (1277 hectares) off the Bay of Plenty coast, is proposed to take place within the next two to three years. Local iwi support the eradication.

Some trials have already been undertaken to eradicate Norway rats and kiore from Raoul Island (3000 hectares) in the Kermadecs. Eradication could be carried out within three years. The remote location of Raoul, some 1100 kilometres northwest of mainland New Zealand, will make this operation particularly expensive. Because rats and cats are present on both Mayor Island and Raoul, it is hoped the Bay of Plenty eradication will help improve techniques for the larger, more distant Raoul operation. Expertise with rat eradication in New Zealand has developed so much during the 1990s that other countries now come seeking our technical know-how. In recent years, New Zealand conservation experts

have designed operational plans for eradications in Norfolk Island, American Samoa, the French subantarctic islands, Pitcairn, Canada and Hawaii. In some cases, New Zealand experts have been personally involved in projects. Currently under consideration is the possibility of Australian conservation staff joining DoC’s proposed Campbell Island eradication, to gain experience for eradications from islands off Western Australian.

is a Wellington-based

freelance writer with an active interest in conservation and recreation.