Rat eradication in Breaksea Sound

Forest and Bird, Volume 19, Issue 1, 1 February 1988, Page 30

Rat eradication in Breaksea Sound

Wherever they reach, introduced rats spell disaster for native birds, lizards and invertebrates. The introduction of European species of rats to New Zealand was first recorded during Cook’s 1773 visit to Fiordland. A major eradication programme currently under way on two important islands in Breaksea Sound will help, at least in part, to rectify this unfortunate legacy. In this article Bruce Thomas and Rowley Taylor, of DSIR Ecology Division, Nelson, document the events leading up to the ambitious project, and discuss its significance to conservation and where it could lead from here.

ha Vy ou're dreaming!’ — this was the typical reaction back in the late 1970s to our suggestion that the small (9ha) bushclad Hawea Island that we were passing could be cleared of its infestation of Norway rats — a dream that was later to become a reality. To the north appeared the magnificent sight of Breaksea Island (170ha) rising 300 rugged metres into the Fiordland mist; a scene epitomising this wild and beautiful corner of New Zealand. A

nod towards it with the comment "‘we'll get rid of rats on that one too" invariably provoked the predictable retort ‘‘You must be joking — you're crazy.’ But we couldn't have been more serious. Today, with the success of Hawea Island behind us and preparations well under way to eliminate rats on Breaksea Island itself, we are quite confident that this major objective will be achieved.

Early Days

Our story really begins a millennium ago when, in legend, Tu-te-Rakiwhanoa took his great adze (with blade of ice!) and hacked out this rugged place. Tane, the god of the forest, clothed Fiordland’s naked slopes to provide a home for a myriad of wonderful and varied creatures, but Tawhirimatea and Tangaroa in jealous rage battered its coasts with incredible fury. Many of Tane’s children learned to survive, and even thrive, under these relentless attacks of wind and sea, but finally proved no match for what beset them with the introduction of rats and other exotic mammals. People eventually made their way to this hostile environment, at first visiting only seasonally to gather a rich harvest of kaimoana, nesting seabirds, plump pigeons and parrots, and to collect the highly-treas-

ured takiwai (Fiordland greenstone). Unfortunately, the Maori brought their dogs and rats — kuri and kiore — the first of the introduced mammals. Today Fiordland is one of the few mainland localities where kiore still persist. These aliens were bad enough, but in May 1773 a new rodent scourge from Europe was introduced when Captain James Cook tied up his rat-infested ship, the Resolution, at Pickersgill Harbour in Dusky Sound. According to one of the expedition’s naturalists, Anders Sparrman', ‘‘it is well known that rats readily swim ashore after a long sea voyage" but here ‘‘our rats (almost certainly Norway rats Rattus norvegicus) had the best chance to come ashore dryfoot by way of the bridge,’’ from the ship to the bank. He further commented ‘‘it seems a terrible thing that the European visits to these parts should be the cause of spreading the disagreeable and noxious breed of rats’, but he also pondered that perhaps the ship was benefitting the New Zealanders in giving them rats as food! Cook spent several days exploring Breaksea Sound and with his officers visited ‘‘the rocks which lay off the entrance to gather a supply of seals’’2. It is conceivable that rats became established on Hawea and Breaksea Island at this time but it is more likely to have hap-

pened after 1792 during the intense period of sealing that began as a result of Cook's reports. Richard Henry's arrival at Pigeon Island in Dusky Sound in 1894, with a brief from the Government to transfer the vulnerable ground-birds kakapo, kiwi and weka to the ‘‘safety’’ of the recently designated Resolution Island bird sanctuary, heralded a new period in western Fiordland’s history. During the 1890s, Henry saw hordes of rats throughout the area and noted that species such as kokako, piopio and saddlebacks were declining. Sadly, his beloved birds were to be even harder hit with the arrival of stoats; kakapo, little spotted kiwi, bellbirds and robins rapidly disappeared, as did the rats. Being good swimmers, stoats quickly reached Resolution Island. Understandably very dispirited, with 15 lonely years of hard work doomed to failure, this notable early New Zealand conservationist left the solitude of Dusky Sound in 1908 to become one of the first custodians of Kapiti Island>. It is only in recent years that the work begun by Henry in Fiordland has been brought closer to fruition.

Biological Surveys

Much of Fiordland was reserved as a National Park in 1904, and attracted a steady

stream of visitors, but the remoteness of the fiords meant that scientific studies there were few and generally confined to occasional, privately-arranged visits by enthusiasts. However, with the advent of the 16.4m Fiordland National Park research vessel Renown, a concerted effort to document coastal Fiordland’s natural history began in 1974 when Park staff and the Ecology and Botany Divisions of DSIR embarked on a series of biological surveys in Doubtful, Breaksea and Dusky Sounds. In the early 1970s, the search was on to find a suitable place to shift the last remnants of the once-thriving Fiordland kakapo population. Stoats had been found on all the larger islands in the Park, but they were not known from Breaksea Island. Reports of good numbers of robins there were encouraging, since they had virtually disappeared from western Fiordland, but during a brief preliminary inspection in 1974 Sir Charles Fleming found signs of rats on both Breaksea and Hawea Islands. The presence of Norway rats on Breaksea Island was easily confirmed when a survey team later that year saw them regularly, even by day, in most habitats. Trapped rats were completely cannibalised overnight, and one evening three were caught in a single snap-trap in ten minutes. Intermittent monitoring since

has shown large fluctuations in the size of the population. On one occasion, the only traces of rats were old burrows full of cobwebs, and sea shells many metres from the shoreline — but on later visits rats and their sign were commonplace. Historical records show that during the last century Norway rats were widespread on many Fiordland islands, but that they disappeared almost completely after the arrival of stoats. Nowadays off the coast of southern New Zealand they are found only on islands that stoats have not reached. The original survey revealed very old rat sign on some other islands in Breaksea and Dusky Sounds, but rats were subsequently caught only on Breaksea and Hawea. On the positive side, it was clear that rats had not reached nearby Wairaki Island, and that Breaksea, Hawea and Wairaki were among the very few islands in Fiordland that were outside the swimming range of stoats. None of the three islands showed sign of deer. The potential value of these islands if they could be made rat-free was obvious, and in spite of popular opinion that complete eradication was impossible, ideas

began to germinate. Novel methods of rodent eradication such as releasing male Stoats on to islands were suggested, but could not be tried in situations like Breaksea Island where the robins in particular would be at risk. New and more effective poisons were becoming available, and by the early 1980s Wildlife Service, Ecology Division and Lands and Survey were beginning to succeed with rat poisoning campaigns on some small islands in the Hauraki Gulf and Marlborough Sounds. There was now a need for a more closely monitored programme to give information on the benefits and costs of rat eradication. Although we now had the ability to clean up small islands, could we manage a large one? Hawea and Breaksea Islands seemed ideal to provide these answers but there was also much, much more to be gained. A mere 300m away from Hawea is Wairaki Island, the type-locality of Fiordland’s endemic skink Leiolopisma acrinasum. These skinks are no longer found on Hawea or Breaksea Island, yet on sunny days Wairaki's rocks virtually ooze lizards as they

emerge from the cracks to bask. This island gem is also home to at least two undescribed species of large weevils, known nowhere else. The long-term survival of this fascinating fauna could be assured by ridding the adjacent islands of rats. Compared with rat-free islands nearby, Hawea also has a reduced distribution and density of breeding seabirds, such as broadbilled prions and sooty shearwaters. The importance of restoring one small piece of coastal Fiordland to something of its former pristine state cannot be overemphasised. Successful Eradication Programme With a financial grant and logistic support from Fiordland National Park, preparation for the eradication of rats on Hawea Island began in October 1985. Tracks were established and experimental and control plots were set up on Hawea and nearby islands to monitor any changes in the populations of the land and breeding seabirds, invertebrates, intertidal fauna, and vegetation once rats were eliminated. Using the m.v. Re-

nown as a comfortable base wasn't without its problems, and resulted in one broken wrist and several bad frights during the daily landing and pick-up operations in the all-too-frequent heavy seas. In April 1986, two Talon (brodifacoum) poison baits were put in each of the 73 plastic drainage-pipe tunnels that had been placed on a 40m grid over the island. These tunnels were necessary to protect native birds from the poison, and were checked and replenished daily. By the third day all baits were being taken and rats were seen waiting at some of the tunnels for replacement baits. After the fourth day the take of bait declined and on the last day of the trip, 12 days after poisoning began, none was touched. On the next inspection, six weeks later, only two more baits had gone, and there has been no new sign of rats on Hawea Island since. Many changes, such as large numbers of seedlings and uneaten fruits littering the forest floor, are already noticeable. Obviously, this part of the programme has been a complete success.

Breaksea Island The Next Step After the Hawea Island experience it was much easier to make a commitment, with confidence, to a rat eradication programme on Breaksea Island, but logistically such an operation will be much harder. Breaksea Island is a comparatively huge chunk of land, covered with beech and podocarp forest, with thick coastal scrub along seaward cliffs, and it has several inaccessible cliff areas and inshore rock stacks. Undaunted, the DSIR and Fiordland National Park personnel involved felt the idea was far too important to abandon. It has been designated a National Park Centennial Project for Fiordland National Park — but with no special funding. Approaches for large-scale financial support have been made, and so far ICI (NZ) Ltd have made a major contribution by donating 500kg of Talon 50WB poison. As well, the Ministry of Works and Development provided a surplus building, and Southern Lakes Helicopters Ltd donated flying time to help transfer it to Breaksea Island. This has now been made into a fine base camp. By far the greatest commitment has gone into the construction of a network of con-

tour tracks to provide ready access to all parts of Breaksea Island. Unexpected help: came early in 1987 when two teams of Operation Raleigh Venturers, young people from all over the world, spent several weeks under canvas in difficult conditions, marking and cutting the first of these tracks. Since then volunteers from all over New Zealand have been working alongside Park staff on the massive task, getting to and from the island with help from Fiordland Travel Ltd and Electricorp to cross Lake Manapouri, and then by m.v. Renown. The Future The tracks on Breaksea Island and the 1000 bait tunnels should be in place by the end of summer 1988. The poisoning will probably begin in May 1988 and should take about a month to complete. At the end of the initial campaign the tunnels will need to be kept baited with poison until at least the following winter, and regular checks made for rodent sign. This will be done in conjunction with the continuing programme to evaluate the conservation benefits of eliminating rats from Hawea Island. The progress of several pairs of robins recently transferred from

Breaksea to Hawea Island will also need to be followed, and eventually Fiordland skinks and rare insects can be transferred to this now rat-free haven. Similar possibilities exist for Breaksea Island once the rats are eliminated, but this project has other far more important implications. A recent 2-year poisoning campaign by Wildlife Service/DoC Eastern Region against rats and rabbits on similarsized Whale Island has apparently been successful*. However, the method used there of broadcasting baits is not appropriate on Breaksea or other islands where sensitive fauna would be placed at risk. The comparatively safe, well-controlled eradication techniques being developed at Breaksea Sound are now helping in the preparation of contingency plans for important island Nature Reserves (such as The Snares) that are at risk from invasion by rodents. Perhaps the next major campaign against rats could be on Kapiti Island, where there is already an excellent system of tracks, used during the eradication of possums. References 1 Sparrman, A., 1953. A Voyage Round the World with Captain James Cook in H.M.S. Resolution. Robert Hale Ltd., London. 2 Beaglehole, J.C., 1974. The life of Captain James Cook. Adam and Charles Black, London. 3 Hill, S., Hill, J., 1987. Richard Henry of Resolution Island. John Mcindoe, Dunedin. 4 K. Owen, personal communication. Acknowledgements We thank the many people who have assisted in this project but special mention must be made of the Fiordland National Park staff, and Ron Peacock in particular, without whom the Breaksea Island programme would never have got underway. Dedication Sir Charles Fleming, 1916 — 1987 This article is dedicated to the memory of Charles Fleming KBE, FRS, who took a keen interest in the Breaksea rat eradication programme. In a letter to BWT (September 1986) he wrote, ‘‘The prospect of progressive improvement of conditions by following up your success on ‘‘OG2" (Hawea Island) with a real attempt on Breaksea Island is stimulus for me to live another decade!’’; but he died on 11 September 1987 before this was finally achieved. Undoubtedly a rangatira of New Zealand natural sciences, he has left a written legacy that will influence scientific thinking in this country for many generations to come. Haere ra te tohunga me oO matauranga, haere, haere, haere. Bruce Thomas has taken part in many biological surveys in western Fiordland since 1974, and is particularly interested in the ecology and conservation of New Zealand lizards — including the rare Fiordland skink. Rowley Taylor has long been concerned with the impact of introduced mammals on the New Zealand flora and fauna, and more recently has studied the dispersal of stoats and rodents to offshore islands and the practicalities of their eradication and control. Both authors have carried out field studies on a wide variety of native and introduced fauna throughout New Zealand, including offshore and outlying islands, and also in Antarctica. They derive great satisfaction from working at the interface of basic research and conservation management.