Eradication of Pacific rats (Rattus exulans) from , ,

C. R. Veitch Department of Conservation, Private Bag 68-908, Newton, , New Zealand. Present address: 48 Manse Road, Papakura, New Zealand. E-mail: [email protected]

Abstract Tiritiri Matangi Island (220 ha) is 25 km north of in the Hauraki Gulf. Pacific rats, or kiore, (Rattus exulans) were probably introduced to this island between 1100 and 1800 A.D. The impacts of Pacific rats on this ecosystem are not known, but from studies at other locations they were presumed to be detrimental to natural processes. Until 1984 the natural ecosystem was also degraded by removal of forest cover and pastoral farming. Restoration of the natural environment on this island began in 1984. Some 300,000 native trees have been planted and nine species of native bird translocated to the island. Pacific rats were removed in September 1993 with use of an aerial application of Talon 20P rodent bait. Specific actions were taken during this operation to protect identified potential non-target species where the population was considered at risk. Monitoring methods following the operation are de- scribed. There has been no rat sign found since the aerial operation.

Keywords Eradication; brodifacoum; Pacific rat, Rattus exulans; non-target impacts.

INTRODUCTION grassland vegetation from 6% to 60% of the island’s area (Galbraith and Hayson 1995), but planting has now ceased. Tiritiri Matangi is a low-lying island of 220 ha lying 4 km off the and 25 km north of Auck- Between 1973 and 1998 nine species of native bird were land City in the Hauraki Gulf, New Zealand. It is a Scien- introduced to this island. These introductions have been tific Reserve under the Reserves Act 1977 and is open to for two purposes: restoration of the island ecosystem (spe- public visitation. In the year ended June 1994, more than cies marked R ) and providing a refuge for threatened spe- 16,000 people visited Tiritiri Matangi (B. Walter pers. cies (species marked T ). The species are: red-crowned comm.).

Maori occupied Tiritiri Matangi prior to the arrival of Europeans in New Zealand and, from at least 1841, it was grazed by domestic animals. A lighthouse was established on the south-eastern end of the island in 1865. The Crown withdrew the grazing lease in 1971 and management of the island was then taken up by the Hauraki Gulf Mari- time Park Board. At that time it was proposed that, apart from the Lighthouse Reserve area, native vegetation be allowed to regenerate naturally.

Cats (Felis catus), rabbits (Oryctolagus cuniculus), and goats (Capra hircus) have been reported as being estab- lished feral populations on Tiritiri Matangi which were subsequently removed. Cats were probably never estab- lished as a feral population and the occurence referred to by the Dept. of Lands and Survey (1982), and later quoted by Moller and Craig (1987), related to domestic cats owned by a lighthouse keeper (A. Wright pers. comm.). Rabbits which were at one time plentiful had disappeared by 1908 (Dept. of Lands and Survey 1982). The goat population was small and was removed by the lighthouse keepers. This work was under way in 1961 (A. Wright pers. comm.) and no goats were present in 1971 (R. Walter pers. comm.).

In 1979 a programme of planting to enhance regeneration was proposed, with a plan calling for the planting of most of the island while leaving selected areas to regenerate Fig. 1 Tiritiri Matangi Island showing the naturally (Fig. 1). Since 1984 more than 300,000 native wharf, tracks, buildings and the placement of trees have been planted increasing the proportion of non- permanent bait stations.

Pages 360-364 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. Veitch: Eradication of rats from Tiritiri Matangi parakeet R (kakariki) (Cyanoramphus novaezelandiae) DGPS record of helicopter travel immediately after the 1973; saddleback R (tieke) (Philesturnus bait drop, before the helicopter left the island. carunculatus) 1984; brown teal T (pateke) (Anas aucklandica chlorotis) 1987; R (popokatea) A statistically-robust system to check bait spread was also (Mohoua albicilla) 1989; takahe T (Porphyrio mantelli) not possible, but four checks were made to determine the 1991; North Island robin R (toutouwai) (Petroica australis) density of bait spread. Three of these were 50 x 1 m areas 1992; little spotted T (kiwi pukupuku) (Apteryx owenii) on the gravel road. The fourth check was that all bait that 1993; T (hihi) (Notiomystis cincta) 1995; and fell onto a covered takahe enclosure (20 x 12 m) was col- North Island kokako T (Callaeas cinerea) 1998. lected when the polythene cover was removed.

The Pacific rat, or kiore, (Rattus exulans) was present on To monitor bait decay, two plots, each containing 30 baits the island at the time of first European records but was spread a few centimetres apart, were established on the removed in the operation during September 1993 described day of the airdrop. The baits were placed a few centime- in this paper. The impacts of Pacific rats on this island tres apart on the ground and covered with a 15 mm wire ecosystem are not known from studies at this location, but mesh cage to prevent disturbance by rats or birds. One other studies (e.g. Whitaker 1973; Atkinson 1978; Craig plot was under the canopy of a ngaio (Myoporum laetum) 1986; Atkinson and Moller 1990; Towns 1991; Holdaway tree and the other was in open grassland well away from 1999) show that they are detrimental to natural processes. trees. Ground searches for bait were made on 2 Novem- ber and 20 November 1993 to assess how much bait re- This eradication was instigated and funded by The Sup- mained as potential food for any remaining rats and whether porters of Tiritiri Matangi Inc. with financial support from the captive non-target species could be released. the Royal Forest & Bird Protection Society. The opera- tion was managed by Department of Conservation staff To check for the possible presence of rats after the poison from the Auckland Conservancy office and Auckland Area drop, bait stations were set out from 17 to 30 November office with technical oversight from the Department of 1993. Each station contained a Rentokil wax block ro- Conservation Island Advisory Group. dent bait and a piece of toilet soap secured to a board un- der a tin cover. In total 350 such stations were used at METHODS approximately 30 m intervals on the edges of all major tracks on the island. Poisoning operation Reduction of non-target effects Bait stations and aerial bait spread were both considered for this operation. Aerial bait spread was chosen as it was Five bird species were considered to be at risk from pri- believed it would require the purchase of less bait, have a mary poisoning as they were known or suspected to take greater probability of success on steep coastal areas, bait. The potential impacts on the pukeko (Porphyrio achieve eradication over a shorter time period, and cost melanotus), North Island robin (Petroica longipes) and less overall. (Philesturnus rufusater) were not considered likely to be detrimental at the population level An aerial application of 2.2 tonnes of 2 gram Talon 20P although some mortality of individual birds was antici- pellets was spread at a nominal rate of 10 kg/ha across pated. The potential impacts on the brown teal (Anas Tiritiri Matangi Island on 29 September 1993 with the in- aucklandica) and takahe (Porphyrio mantelli) populations tention of eradicating the rats. This pollard bait was manu- were considered to be detrimental at the population level factured to contain brodifacoum at 20ppm but no samples and action was taken to protect these two species during were tested to check the toxin level. the poisoning operation.

The bait was distributed from a helicopter fitted with a Brown teal had been liberated on the island in 1987 and Differential Global Positioning System (DGPS) which was 1990 but the number present in 1993 was unknown. Be- set to show the pilot a line spacing of 100 metres. The tween 15 and 21 September 1993 nine brown teal were boundary of the island was flown so that this would be captured – four by hand netting and five by hunting with a recorded in the navigation system. The bait was spread trained dog. These birds were kept in an aviary on the from an underslung dedicated bait bucket set to spread island until 11 November. This aviary was covered with a bait at 8 kg/ha to a swath width of 120 metres. Thus, when polythene sheet during the bait drop. overlaps between flight lines were allowed for, an overall sowing rate of 10 kg/ha was expected. Four takahe were present on the island in 1993: a pair with a two-egg nest and an attendant juvenile from the previ- No ground checks were carried out to check for gaps in ous year; and a single male. A 16 x 20 metre enclosure bait distribution, as the dense vegetation on the island made with a one metre high fence was built around the area con- it unlikely that this would be successful. As soon as all taining the nest to keep the three takahe within this area. bait had been spread, a careful check was made of the The vegetation in this area varied from open grassland to three metre high trees. Before the poison drop a poly-

361 Turning the tide: the eradication of invasive species thene sheet was drawn over this entire area. For the single By 20 November, 52 days after the drop, the pellets in the male takahe a 20 x 12 m enclosure, containing mostly grass- bait decay monitoring plots were still whole but totally land with a few low shrubs, was constructed. Before the black with mould. A sample of this was found to contain bait drop this takahe was locked in the island’s potting 30.6 ppm brodifacoum. The bait that was spread was pre- shed and a polythene sheet placed, mostly at ground level, sumed to contain 20 ppm but this was not tested so it is not over the enclosure. After the drop the polythene sheets known how much the brodifacoum content of this decayed were removed. All takahe remained in their enclosures bait varied from the probability of the fresh bait carrying until 3 December, 61 days after the bait drop. more than 20 ppm.

To monitor the impact of this poison drop on non-target Some 302 person hours were spent, mostly by volunteers species, many people searched randomly for dead birds at with varying skill levels, searching for dead birds during irregular intervals over the month following the drop. Birds the month following the bait drop. The 42 birds found found in reasonably fresh condition were placed in a freezer and an assessment of their condition and the probability for necropsy and residue analysis. The colour-banded of brodifacoum poisoning are listed in Table 1. populations of robins and saddleback had been monitored since their introduction to the island in 1984 and 1992 respectively. They continued to be monitored throughout Table 1 Birds found dead in November 1993 the operation. Bird count data gathered by members of after the aerial poison drop to eradicate Pacific the Ornithological Society of New Zealand since 1987 have rats from Tiritiri Matangi Island. There was no since been analysed (Graham and Veitch 2002). autopsy of some birds as they were too de- cayed or had been recorded as found and For ongoing protection of the island and monitoring for discarded. Birds which were examined were rodent presence, 51 Rentokil rodent bait stations each con- recorded as poisoned if haemorrhaging con- taining one Rentokil wax block bait containing sistent with brodifacoum poisoning was found. bromadiolone at 50 ppm were placed at potential landing The assay results are µg/g brodifacoum in each places (Fig. 1). These have been inspected monthly since liver sample. 1993 and the bait is refreshed at three to six month inter- vals. Species Date Autopsy Brodifacoum (µg/g) RESULTS Paradise shelduck 14/10 1 poisoned The checks of the DGPS indicated that bait spread over Tadorna variegata the island had been thorough. Brown teal 7/10 1 poisoned Anas aucklandica 8/10 1 poisoned The bait density checks on the road located 14, 17, and 15 12/10 1 no autopsy baits in each 50 x 1 m plot. This is equivalent to about Spotless crake 12/10 1 no sign 0.04±0.01 6 kg/ha, but baits may have bounced off this hard surface, Porzana tabuensis thus reducing the count. On the takahe pen cover 180 Pukeko 5/10-10/10 4 poisoned baits were found, equivalent to about 15 kg/ha, suggesting Porphyrio 6/10-31/10 17 no autopsy a double bait spread over this particular area. melanotus Red-crowned parakeet 23/10 1 see note 1 0.00 An unplanned check of bait abundance in randomly se- Cyanoramphus novaezelandiae lected areas of grassland and forest five days after the bait Skylark 14/10 1 no autopsy drop located two to seven baits per 50m2 searched, or about Alauda arvensis 0.8 to 2.8 kg/ha. This indicates that there was adequate Blackbird 16/10-30/10 4 no autopsy bait for all rats to have had access to bait during the days Turdus merula immediately after the drop. Chaffinch 7/10 1 poisoned Fringilla coelebs 7/10 1 no autopsy In the 24 hours up to 0900 on 30 September, the second House sparrow 13/10 1 no autopsy night after the bait drop, 1.1 mm of rain were recorded on Passer domesticus Tiritiri Matangi Island. The next rainfall was 9.7 mm on Common myna 9/10-29/10 4 no autopsy 13 October. The baits in the bait decay plots remained Acridotheres tristis whole but slowly became blackened with mould. A fur- Saddleback 15/10 1 poisoned 0.46±0.10 ther unplanned bait search on 2 November revealed no Philesturnus 17/10 1 no autopsy bait in grassland areas and in most forest areas but some carunculatus 24/10 1 no autopsy was found at the foot of a steep rock face (J. Henry pers. comm.). This was assayed and found to contain 22.1 ppm Note 1. This parakeet had haemorrhaging under breast skin and brodifacoum. No bait was found during the ground search around the heart which could have been consistent with brodifacoum poisoning and it also had blood on the back of the on 20 November. head consistent with fighting.

362 Veitch: Eradication of rats from Tiritiri Matangi

The brown teal were released from their aviary on 11 No- vember, six weeks after the bait drop. This species is not very conspicuous so their survival cannot be quantified. One pair, at least, raised young during the following sum- mer of 1993/94.

The four takahe were released from their enclosures on 3 December, nine weeks after the bait drop. They all sur- vived. Other takahe have been introduced to the island since the eradication operation and the takahe population continues to thrive.

Fig. 2 Counts of pukeko on all transects in Bellbirds (Anthornis melanura), tuis (Prosthemadera November of each year (OSNZ data). novaeseelandiae), parakeets (Cyanoramphus novaezelandiae), and saddlebacks all appear to have in- creased in number since the removal of rats from this is- land (Graham and Veitch 2002). The pukeko population declined by more than 90% in the weeks following the operation. Subsequent observations DISCUSSION and counts by the Ornithological Society show that this population increased to higher than pre-poison drop num- bers within two years of the operation (Fig. 2). There was no sign of rodent presence in two years follow- ing completion of this operation. The operation has there- The robin population was estimated to be 40 birds at the fore been declared successful and there has also been no time of the poison drop. About 11% of these died at the sign of rodents on the island since then. There is no evi- time of the poison drop and this mortality was attributed dence to suggest that the methods used have been detri- to the poison. This had no impact on population viability mental to any non-target population in the longer term. (Armstrong and Ewen 2000). Before the operation began we knew it would not be pos- The colour-banded cohort of saddlebacks in Wattle Val- sible to check on the continuity of bait spread, due to the ley has been observed, and new birds have been banded nature of the vegetation on the island, but we did consider since their introduction to the island in 1984. Natural it possible to check the density of the bait spread using the mortality has been low and the population has continued methods described. The results of this work show that to increase. Following the poison bait drop about 21% of using the roads was not an effective way to check bait this colour-banded population was lost (B. Walter pers. spread. The polythene sheet on the takahe pen was an comm.). One bird was found dead and it contained effective way to check bait density but many replicates of brodifacoum (Table 1). The Ornithological Society counts this would be needed to ensure that there were adequate also recorded a reduction in their counts through Wattle samples from areas where two flight lines did not overlap. Valley but subsequent counts show that this has not been detrimental to the saddleback population in the medium No samples of the fresh bait were checked for brodifacoum term (Fig. 3). content. The bait found on 2 November, 35 days after the drop, contained brodifacoum at 22.1 ppm which was within the expected range. The bait in the decay plots was found to contain 30.6 ppm brodifacoum after 53 days of weath- ering. For this trial a portion of the bait used should have been stored for assay alongside the trial baits. As this was not done there is no way of knowing whether the 30.6 ppm was a high toxin load to start with, was caused by subse- quent change in bait composition, or was an error in the assay process. ACKNOWLEDGMENTS

I thank Jim Henry, who managed most of the field work, and Ray and Barbara Walter who have been present throughout this work, have assisted with field work and recorded many of the events. I also thank the numerous Fig. 3 Counts of saddlebacks on the Wattle Department of Conservation staff and volunteers who as- Track transect in November of each year (OSNZ sisted with this work or observations related to it. data).

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