Eradication of rabbits and mice from subantarctic Enderby and Rose Islands N. Torr Department of Conservation, P.O. Box 29, Te Anau, New Zealand. Current address: 64 Mokonui Street, Te Anau, New Zealand. E-mail: [email protected] Abstract In 1993 rabbits (Oryctolagus cuniculus cuniculus) were eradicated from Enderby (700ha) and Rose (80ha) islands in the New Zealand subantarctic Auckland Island group. This was achieved by a widespread poison campaign followed by an intensive second phase which included hunting with a dog, spotlighting and trapping. During the poison campaign a helicopter was used to apply a cereal pelleted bait incorporating the anticoagulant toxin brodifacoum to both islands. Mice (Mus musculus), which were present on Enderby, disappeared during the poison campaign and appear to have been eradicated during this phase. The potential impacts to non-target species were assessed prior to the operation. Although the poisoning had a notable short-term impact on skua (Stercorarius skua lonnburgi) numbers there has been no obvious long-term impact on any non-target species. Rabbits and mice were the last of several introduced mammal species to be removed from Enderby and Rose. Without them the unique ecological values of these islands have a chance to recover. Keywords Eradication; rabbits, Oryctolagus cuniculus cuniculus; mice, Mus musculus; Auckland Islands; Enderby Island. INTRODUCTION Plan for these islands, to eradicate all alien animals as soon as is feasible (Penniket et al. 1987). Goats were eradi- The Auckland Islands are an uninhabited subantarctic cated from Auckland Island between 1989 and 1991 (A. group lying 460 km south of New Zealand, at approxi- Cox pers. comm.). The majority of cattle were removed mately 50°40’ S, 166°08’ E. The group consists of two from Enderby in 1991, with eradication being completed large and five smaller islands and numerous small islets by the rabbit eradication team in 1993. In 1991, the feasi- (Fig. 1). The two largest are Auckland Island (46,000 ha), bility of removing rabbits from Enderby and Rose Islands which rises to a maximum altitude of 664 m, and Adams was investigated. Based on the results of that investiga- Island (9900 ha), which rises to 667 m. At the northern tion, a programme aimed at the total removal of rabbits end of the group, around Port Ross, there are four smaller, low-lying islands, the largest two of which are Enderby (700 ha) and Rose (80 ha) Islands (Fig. 1). The Auckland Islands are gazetted as a National Nature Reserve (Re- serves Act 1977) and are of international ecological im- portance because of their particularly diverse and unique biological communities, which include many endemic spe- cies of plants and animals such as Auckland Island teal (Anus aucklandica aucklandica), snipe (Coenocorypha aucklandica) and Auckland Island rail (Rallus pectoralis aucklandica) (Penniket et al. 1987). They are also an important breeding ground for marine mammals and seabirds including New Zealand sea lion (Phocarctos hookeri) and wandering albatross (Diomedea exulans). Since their discovery in 1806, the islands have been sub- jected to significant human impacts. These include the introduction of a variety of alien mammals, at first by seal- ers or as food for castaways, and later during attempts at farming. Some of these animals have died out naturally, but in recent times Auckland Island has continued to be host to pigs (Sus scrofa), goats (Capra hircus), cats (Felis catus), and mice; Enderby Island to cattle (Bos taurus), rabbits, and mice; and Rose Island to rabbits (Fig. 2) (Taylor 1968, 1971). Fig. 1 The Auckland Islands showing the Since 1987, New Zealand’s Department of Conservation location of Enderby and Rose Islands, and has actively pursued a policy, set out in the Management other places mentioned in the text. Pages 319-328 In Veitch, C. R. and Clout, M. N. (eds.). Turning the tide: the eradication of invasive species. IUCN SSC Invasive Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. Turning the tide: the eradication of invasive species from both islands was instigated. During the investiga- Rose slightly higher at 48 m. Except for two sandy beaches tion and planning for this operation it became apparent on Enderby and a few sections of steeply sloping shore- that there could be an opportunity to eradicate mice from line, both islands are almost completely surrounded by Enderby at the same time as rabbits. This was adopted as coastal cliffs. These rise to over 30 m on their northern a secondary aim of the eradication programme. and western shores, but are generally lower on the south- ern and eastern coasts. Apart from about 30 ha of sand The proposed programme presented some challenging fea- dunes on Enderby Island, both islands are covered in a tures, including: thick blanket of peat, which in many areas is waterlogged. v the difficulty of carrying out an eradication operation in an isolated situation far (460 km) from the New Zea- Since their discovery, modification to the original vegeta- land mainland, under unfavourable climatic conditions; tion of these two islands has been quite severe, mainly v the size of Enderby, which at 700 ha, is much larger through the use of fire during failed farming attempts in than any island from which rabbits and mice had previ- the late 1800s and the presence of introduced mammals ously been eradicated (Round Island at 151 ha and Mana (Fig. 2) (Taylor 1968, 1971). Island at 217 ha respectively) (Merton 1987; Hook and Todd 1992). Along the southern and eastern sides of both islands, and v the presence of some indigenous birds, considered to covering about one quarter of their surface areas, is a belt be at risk from some of the methods used in the eradi- of southern rata (Metrosideros umbellata) forest and scrub- cation programme. land. The rest of Rose Island is predominantly covered by large areas of Poa litorosa tussock grassland interspersed This paper reports the progress of the eradication pro- with areas of short sward vegetation. On Enderby, the gramme from initial bait trials in 1991. It covers the 1993 centre of the island and about one third of its area is moor- eradication operation and the following period of moni- land dominated by the shrub Cassinia vauvilliersii and toring. cushion plant Oreobolus pectinatus interspersed with iso- lated patches of rata forest and Myrsine divaricata scrub. Description of Enderby and Rose Around the coast is a band of short sward vegetation, which Islands is extensive on the northern and western side, and up to several hundred metres wide in some places. Both islands are comparatively low-lying, with Enderby Island rising to a maximum altitude of about 45 m and Fig. 2 Historical summary of introduced mammals on the Auckland Islands (updated from Taylor 1968). The thick bars indicate when each species was present. 320 Torr: Eradication of rabbits and mice from Enderby and Rose Islands Prior to the eradication, cattle and rabbit browsing had at 5000-6000 animals. On Rose Island, density, distribu- confined the large-leafed herbaceous plants Stilbocarpa tion, and numbers of rabbits were much harder to estimate polaris and Anisotome latifolia to cliff areas and because of the thick ground cover of Poa litorosa, but the Pleurophyllum criniferum had been eliminated from both total population was probably 300-400 individuals. Esti- islands. On Enderby Island, tussock grasslands had been mated rabbit density for both islands ranged from >35 almost eliminated and replaced with a closely-cropped animals per hectare in the more favoured habitat around sward made up of a mixture of both introduced and native Sandy Bay on Enderby Island, to <2 animals per hectare plants. in less favourable habitat. In general, rabbits were more numerous on Enderby than Rose Island. Rabbit sign was History and effects of rabbits on found over all areas of both islands, including the rata for- Enderby and Rose Islands est (B. W. Glentworth pers. comm.). Rabbits were liberated on Rose Island in about 1850, dur- In addition to their impacts on vegetation, rabbits also re- ing a period of attempted settlement in the Port Ross area. duced the survival of New Zealand sea lion pups on The liberation was probably of mixed stock, or there may Enderby Island. New Zealand sea lions are found only have been other unrecorded liberations, as the population within New Zealand waters and are a threatened species. there included agouti and silver black rabbits, both with Sandy Bay on Enderby Island is an important breeding and without white markings (Taylor 1971). site for them. The numerous rabbit burrows around Sandy Bay proved a hazard to sea lion pups, which became The rabbits on Enderby were descended from 12 animals trapped in them and died. Mortality from this cause was liberated in 1865 to establish a population as food for casta- estimated as up to 10% of pups on Enderby Island in some way mariners (Taylor 1971). They came from the Accli- years (Penniket et al. 1987). matisation Society of Victoria in Australia and belonged History and effects of mice on Enderby to the French breed known as “Argenté de Champagne” (or French Blue). The rabbits on Enderby had bred true to Island this type (Taylor 1971). Argenté de Champagne is now a fairly rare breed and it was thought that the Enderby popu- Mice were accidentally introduced to Auckland Island lation might have been the last true wild population left in during the main period of sealing activity in the early 1820s the world (B. W. Glentworth pers. comm.). Before the (Taylor 1968). They probably arrived on Enderby about eradication, 49 rabbits were recovered from Enderby Is- 1850, when there was an increase of human activity on the land by the Rare Breeds Conservation Society of New island associated with a period of attempted settlement in Zealand and the Department of Conservation, and brought the Port Ross area (Taylor 1971).