Eradication of Pacific rats (Rattus exulans) from Fanal Island, New Zealand. C. R. Veitch Department of Conservation, Private Bag 68-908, Newton, Auckland, New Zealand. Present address: 48 Manse Road. Papakura, New Zealand. E-mail: [email protected] Abstract Fanal Island (73 ha) is the largest island in the Mokohinau Group, Hauraki Gulf, New Zealand. Pacific rats, or kiore, (Rattus exulans) reached these islands between about 1100 and 1800 A.D. Pacific rats were removed from all islands in this group, except Fanal Island, in 1990. An aerial application of Talon 7-20 (containing brodifacoum at 20 ppm) at a nominal rate of 10 kg/ha was made on Fanal Island on 4 August 1997 with the intention of eradicating the rats. Despite heavy rainfall immediately after the poisoning operation, and the fact that baits were not of optimum palatability, the rats were eradicated. Keywords Bitrex; brodifacoum; aerial baiting. INTRODUCTION cations they were presumed to be detrimental to natural Fanal Island (Motukino), 73 ha, is the largest and most processes (Holdaway 1989; Atkinson and Moller 1990). southerly island in the Mokohinau Group which is at the Pacific rats were removed from all islands in this group, northern extremity of the Hauraki Gulf, about mid way except Fanal Island, by the Department of Conservation between Great Barrier Island and the mainland (Fig. 1). in 1990 (McFadden and Greene 1994). Fanal Island is part of Mokohinau Islands Nature Reserve, Under the Regulations Act 1936 and the Grey-faced Pet- which is administered by the Department of Conservation rel (Northern Muttonbird) Notice 1979, Ngati Wai of Aotea under the Reserves Act 1977. It is also a wildlife sanctu- (Great Barrier) have muttonbirding privileges on all is- ary under the Wildlife Act 1953. lands of the Mokohinau Group. They approved the re- Pacific rats, or kiore, (Rattus exulans) are presumed to moval of Pacific rats from the northern Mokohinau Group, have reached these islands with Maori between about 1100 and they initiated the proposal to remove Pacific rats from and 1800 A.D. These islands were also modified by burn- Fanal Island in May 1995. ing of the forest to aid Maori food gathering and Burgess Fanal Island is surrounded by steep cliffs and has a gently Island, the next largest in the group, was cleared for pasto- sloping forested summit plateau, which rises to 134 m near ral farming associated with the establishment of a light- the northern cliffs. There are small seasonal streams in house (c. 1890) and wartime defence operations. The the three main valleys which drain to the south-west. The impacts of Pacific rats on these ecosystems are not known, vegetation is coastal forest and scrub, much of which is but from circumstantial evidence and studies at other lo- regenerating following fires in earlier times (Esler 1978; Wright 1980a, 1980b; de Lange et al. 1994). Most of the expected array of forest birds was present on Fanal but saddlebacks (Philesturnus carunculatus) were absent, presumably as a result of previous forest destruc- tion. Attempts to re-introduce saddlebacks in 1968 and 1985 failed to establish a breeding population although individuals did survive up to 15 years after liberation. During preparations for this operation no populations of fauna were identified which might be detrimentally affected by the proposed aerial bait distribution to eradicate the rats. METHODS Bait spread On 4 August 1997 an aerial application of Talon 7-20 was applied to Fanal Island at a nominal rate of 10 kg/ha. Talon Fig. 1 Fanal Island is part of the Mokohinau 7-20 is a pollard bait containing brodifacoum at 20 ppm Group situated at the northern extremity of the and the bittering agent bitrex (denatonium benzoate). Hauraki Gulf. Pages 357-359 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 The operation was run from Tawharanui Regional Park RESULTS (Fig. 1), with permission of the Auckland Regional Coun- cil Parks managers. A Differential Global Positioning No rats or rat sign was detected during the three visits in System (DGPS) base station was established on the hill- 1998, 1999, and 2000 (I. McFadden pers. comm.; P. Todd top east of Park Headquarters at Tawharanui Regional Park. pers. comm.; G. Wilson pers. comm.). It is believed that Marine Helicopters Ltd Llama helicopter took one under- Pacific rats have been eradicated from Fanal Island de- slung bait spreader bucket loaded with 825 kg of Talon spite significant rainfall over the second and third nights 7-20 bait to Fanal Island. A strong south-east wind was after the bait was spread and the presence of bitrex in the blowing. The island boundaries were logged into the bait. DGPS and the bait was then spread. DISCUSSION The DGPS was set to show navigation lines for the heli- copter at 60 metre intervals. The bucket being used was In this situation, where ground checking and a second flight spreading bait over a 120 m swath at a rate of 4 kg/ha. to spread more bait was not possible, the increased flying Thus we expected 8 kg/ha to be spread in most areas, 4 time to give closer flight lines proved an acceptable alter- kg/ha where there had been only one pass, and an average native. The overlapping flight lines ensured that there was of 10 kg/ha over the flight area. a complete bait coverage of the island with a rate of 4 kg/ha Monitoring being the lowest likely spread. Bitrex, a bittering substance added to reduce accidental Unpredicted heavy rain fell between 5pm on 5 August and human consumption of rat bait, has been shown to reduce 1am on 7 August (Fig. 2). The total fall recorded by the the consumption of bait by captive Pacific rats (Appen- automatic weather station on Burgess Island was 106 mm dix 1). We were not aware of the presence of bitrex in the with hourly rates averaging 3.4 mm per hour and reaching Talon 7-20 formulation until after the operation but it did a maximum of 9 mm per hour. There were no visits to the not affect the final outcome of the operation. island to monitor possible acceleration of bait decay due to this rain. The period of heavy rainfall was not predicted in weather forecasts or seen by weather observers until late on 4 Au- During a brief visit to the island in mid-1998 rat traps were gust (R. McDavitt pers. comm.). It is unfortunate that set for one night (I. McFadden pers. comm.). During a monitoring of bait decay was not possible and it is not visit in May 1999 snap traps were set for 400 trap nights known whether bait survived this rain or whether the rats and extensive searches were made for fresh rat sign (P. all consumed lethal doses within the first two nights. Todd pers. comm.). Further searches for rat sign were made, but no traps were set, during a visit in May 2000 The success of this operation, despite adverse weather (G. Wilson pers. comm.). conditions and unattractive bait, is evidence of the effec- tiveness of aerial application of brodifacoum baits for rat eradication from islands. Nevertheless, it is prudent that future operations be managed with such variables in their favour. ACKNOWLEDGMENTS I thank Bill Simmons and Animal Control Products for the bait supplied for trials and Ian McFadden for his tech- nical advice and reports on the follow-up monitoring. REFERENCES Atkinson, I. A. E. and Moller, H. 1990. Kiore. In King, C. M. (ed.). The Handbook of New Zealand Mammals, pp 175-192. Auckland, Oxford University Press. de Lange, P. J.; McFadden, I. and Cameron, E. K. 1994. Preliminary report of the flora and fauna of Fanal Is- Fig. 2 Rainfall recorded at the automatic land, Mokohinau Islands Nature Reserve. Unpublished weather station on Burgess Island. This is for Report, Science and Research Division, Department of the 24 hours of each day, not to 0900 hours as Conservation. at manual weather stations. 358 Veitch: Eradication of rats from Fanal Island Esler, A. E. 1978. Botanical features of the Mokohinau Appendix 1 A comparison of baits with and Islands. Tane 24: 187-197. without bitrex. Holdaway, R. 1989. New Zealand’s pre-human avifauna In May 1996 baits with and without bitrex were tested on and its vulnerability. In Rudge, M. R. (ed.). Moas, mam- wild caught Pacific rats (Rattus exulans) on Little Barrier mals and climate in the ecological history of New Zea- Island as part of a larger test of bait flavours. land. New Zealand Journal of Ecology 12. McFadden, I and Greene, T. 1994. Using brodifacoum to Bitrex (denatonium benzoate) may be added to eradicate kiore (Rattus exulans) from Burgess Island brodifacoum at the point of manufacture. The brodifacoum and the Knights Group of the Mokohinau Islands. Wel- in the samples compared came from two different sources; lington, Department of Conservation, Science and Re- one contained bitrex and the other did not. The quantity search Series 70: 18pp. of bitrex in the toxin is not known. In all other respects the baits were the same, being Wanganui No. 7 formula- Wright, A. E. 1980a. Auckland University Field Club Sci- tion dyed green and pressed into 2 gram pellets. The entific Trip to the Mokohinau Islands, May 1979. In- brodifacoum was loaded at 20 ppm. The bait with bitrex troduction and Acknowledgements. Tane 26: 1-6. is now the registered formulation Talon 7-20 and the bait Wright, A.