Motu Kaikoura rat management programme
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Motu Kaikoura rat management programme
progress report (No.4) December 2017
Michael Lee Trustee
Summary
The Motu Kaikoura Trust has been undertaking a manual rat control operation
(baiting and trapping) on Motu Kaikoura (564 ha) since March 2014, with the
objective of managing rat numbers on the island at or below 5% index level.
Satisfactory progress is indicated by the index of rat abundance which in mid
November 2017 was 2%. This data is supported by the continuing decline of bait
consumption and in the number of rats trapped, bait-take (December) is presently
below 1% (of bait blocks available) and rats trapped in 4% of traps.
This compares to relative rat abundance on nearby Great Barrier Island of 60% or
higher - similar to levels recorded on Motu Kaikoura prior to the start of the manual
operation in 2014.
Fig.1
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Introduction Motu Kaikoura or Kaikoura Island (36°10’ 41” S, 175°19' 28"E), off Port Fitzroy, Great Barrier Island (Aotea). It was purchased in 2004 by the N.Z. government after a campaign to secure it in public ownership. The purchase was funded by contributions from the Nature Heritage Fund (NZ Government), the former Auckland Regional Council, former Auckland territorial local authorities and the ASB Community Trust funding (now Foundation North). The island is a scenic reserve administered by the Motu Kaikoura Trust which was established by the Minister of Conservation in 2004 under section 29 of the Reserves Act (1977) and by an agreement with the Minister which was signed in 2008. The scenic reserve island was opened to the public by the Prime Minister, the Rt. Hon. Helen Clark and Minister of Conservation Hon. Chris Carter in May 2005. The island is of rugged topography with much of the coastline in steep cliffs. The highest point of the island Mt Overlook is 502m asl. Several-hundred-years of human activities, repeated burn-offs and grazing resulted in the island’s natural values becoming severely degraded. In 2004 when the Motu Kaikoura Trust assumed management, the island was beginning to regenerate with manuka and kanuka successional shrublands linking areas of mature coastal forest mainly around the coast and emergent stands of pines located mainly at the southeastern part of the island. However the understory was heavily browsed by fallow deer (Dama dama), feral pigs (Sus scrofa), and rabbits (Oryctolagus cuniculus cuniculus). (Goats (Capra hircus) had been removed in 1993). Like neighbouring Great Barrier Island, Motu Kaikoura was also infested with feral cats (Felis catus), ship rats (Rattus rattus) and kiore (Rattus exulans). Fallow deer, pigs, cats and rabbits were eradicated in the period up to and including 2008. The removal of fallow deer in particular notably accelerated the process of native forest regeneration. In August 2008 an attempt was made to eradicate rats from the island with two aerial applications of ‘Pestoff 20R’ (brodifacoum based). The operation was at first believed to have been successful but after seven months, both rat species R. rattus and R. exulans were detected on the island. The adjacent mainland is only 80 metres away across the Man of War Passage at its closest point, Bunker Point on the Stellin peninsula. This is well within the swimming range of ship rats that periodically reach the island from this area (Bagasra 2013). However the presence of kiore, confirmed by DNA identification (Fewster et al. 2011), provided strong evidence that the aerial operation failed as kiore are known to be poor swimmers and are unlikely to have reached the island unassisted. After seeking expert advice (not all of it consistent) and undertaking a period of review and analysis the Motu Kaikoura Trust formally resolved to manage the island as a ‘mainland island’, treating the water barrier as a ‘fence’ and relying on manual methods to control rats on a permanent basis. This honours a key condition of the Agreement between the Minister of Conservation and the Motu Kaikoura Trust: 2
(e) keep and maintain the Reserve free of litter and work towards the eradication of all
plants and animals (including the eradication of fallow deer, pigs and rats) identified
in the operative Auckland Regional Pest Management Strategy and the Biosecurity
Act 1993;
This is reflected in a key objective of the Motu Kaikoura Biodiversity Management
Plan (2012):
Animal pest control recommendations. Method of control. Target all rodent species (ship rats,
kiore) and aim to containtraps have been trialled in a number of settings, on island and on the nearby
mainland.
The number of snap-traps deployed on the island have been progressively increased
to 792.
Off Island protective management zones
Stellin Peninsula (80m from Motu Kaikoura)
A protective management area on the nearby mainland Bunker Point (Stellin’s
Peninsula) comprising of 10 x bait stations, and 10 x tracking tunnels operated for
several years provided useful comparative data and controlled rat numbers in an
identified invasion area. This was temporarily suspended in 2017 however by
arrangement with the owner Sven Stellin it has now been recommissioned with 12 x
bait stations. Its performance will be reported in the 2018 report.
Nelson Island (100m from Motu Kaikoura)
By agreement with the owner Bryan King since December 2016 have extended our
coverage to Nelson Island (12 ha), with 2 x motels each loaded with 16 bait blocks.
Motu Kaikoura trap and bait station inventory
Coastal Line 123 motels 492 bait blocks 246 traps
East Track 96 motels 384 bait blocks 192 traps
West Track 62 motels 248 bait blocks 80 traps
Parihakoakoa 26 motels 104 bait blocks 52 traps
28 Philproofs 112 bait blocks
Road Line 31 motels 124 bait blocks 60 traps
Badlands 27 Philproofs 108 bait blocks
Slip Track 10 motels 40 bait blocks 20 traps
16 Philproofs 64 bait blocks
16 trap tunnels 16 traps
Pahangahou 14 motels 64 bait blocks 28 traps
15 Philproofs 60 bait blocks
House Track 12 motels 48 bait blocks 24 traps
Midline Track 12 Philproofs 48 bait blocks
Airfield Line 12 motels 48 bait blocks 12 traps
Bradshaws 10 Philproofs 40 bait blocks
Wharf Line 10 motels 40 bait blocks 20 traps
West Link Track 9 Philproofs 36 bait blocks
Nature Line 9 Philproofs* 36 bait blocks
Towers Track 8 motels 32 bait blocks 16 traps
Fence Line Track 8 motels 32 bait blocks 16 traps
Barn Track 5 Philproofs 20 bait blocks 10 traps
End of Airstrip 5 Philproofs 20 bait blocks
* ganged with 9 x A24 traps
Nelson Island 2 motels 32 bait blocks
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Total number of rat ‘motels’ 412 (2 on Nelson Island) Total number of Philproof hoppers 136 (12 on the mainland) Total number of trap tunnels 16 (paired with hoppers) Total number of bait stations 548 Total number of bait blocks available (4 per station) 2192 Total number of snap-traps 792 Total length of formed operational tracks and roads: 29.8 km Total length of coastline network: 16.5 km Bait rotation To minimise bait avoidance (and resistance) the Trust has a policy of rotating use of bait on a more or less annual basis. We have used ‘Pestoff’ (brodificoum 20 ppm), ‘Brigand’ (brodificoum 50 ppm), ‘Storm secure’ (flocoumafen 50 ppm). In 2017 we successfully used ‘Generation Soft Bait’ (difethialone 45 ppm). The active ingredients of all of the above rodenticides are second-generation anti-coagulants. To prolong freshness, baits in motels are placed in sealed plastic bags. Currently we are using ‘Pestoff.’ We will next consider using a non-anticoagulant like cholecaliferol. Expanding the grid In 2017 we expanded the network with another line of 9 bait stations (ganged with A24 traps) and boosted the number of snap traps on the Fenceline and Barn lines. 548 bait stations for 564 ha of habitat does not yet form the ideal 100m x 100m grid. And there are still some areas outside the recommended 100m range of stations which despite the challenges of difficult terrain we intend to cover. Results Total number of rats trapped in 2017: 925 Total number of bait blocks consumed in 2017: 1495. As there is no standard formula for a ratio of bait blocks to rat morbidity a conservative figure is proposed 2 x bait blocks per morbid rat. Therefore an estimated 748 rats killed by bait. Total rats killed in 2017: 1673. After nearly four years of operations rat numbers on Motu Kaikoura continue to track down, year on year, season by season. 5
Fig.2
Fig. 3
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Motu Kaikoura - bait take trend 2014 -2017
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Motu Kaikoura - bait take trend 2014 -2017
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Satisfactory progress is highlighted by the comparison with the situation on Motu
Kaikoura in 2014 and with the ongoing situation on mainland Great Barrier Island.
See below.
Motu Kaikoura
Bait take on island v mainland
100%
90%
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60%
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Island
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control
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Fig.4. Note: the mainland bait station line was withdrawn for most of 2017 but restored in December
Index Monitoring, supported by baiting and trapping data indicates a clear seasonal
pattern to the level of the rat populations on Motu Kaikoura and on the nearby Great
Barrier mainland (the latter at about 10 times the Kaikoura level). This late Autumn
‘spike’ in rat numbers is presumably from residual populations breeding on the island
augmented by ‘swimmers’ was apparent again in 2017 but monitoring in May
indicated a peak of 12% (compared to 25% at the same time in 2015).
Snap-traps are now our most effective means of killing rats and proving to be an
effective augmentation to bait stations.
The ‘Good Nature’ A24 resetting traps have proved to be ineffective in a number of
field trials both on the island and on the nearby mainland.
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Rat species composition on Motu Kaikoura
Also noteworthy is that with the overall reduction of rat numbers there is an apparent
increase in the proportion of kiore (R.exulans) relative to ship rats (R.rattus).
This was first indicated by DNA samples from tails collected in February 2015 and
has been confirmed by a much more extensive island-wide sampling programme in
2017 and processed by EcoGene,Landcare Research in January 2018. This found
out of 25 tail samples from across the island 17 were kiore R. exulans (68%) and 8
(32%) were ship rats R.rattus. This proportion of kiore to ship rats is a direct reversal
of population ratios normally found when these species coexist eg approximately
70% ship rat, 30% kiore. (Russell et al. 2015). Theoretically it would be possible,
though practically difficult, to eradicate kiore entirely from the island but this is a
proposition the Trust will need to consider.
Index monitoring results: 2010 – 2017
There are 50 tracking tunnels, grouped in five lines of 10 with one control line of
10 stations on the nearby mainland. Standard one night monitoring procedure is undertaken.
The corflute tunnels (‘Black Trakka’) have an inked tracking card (from same manufacturer)
which is baited with peanut butter left overnight and collected the next day.
Monitoring lines on Motu Kaikoura and on nearby Great Barrier
Fig. 5.
Latest monitoring results for Motu Kaikoura
Results from monitoring from 14-15 November 2017 were
as follows:
Lodge (RM1) 1/10; Bradshaws (RM2) 0/12; Mangrove (RM3) 0/10;
Ridge (RM4) 0/10; Overlook (RM5) 0/10.
Total 2%.
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Motu Kaikoura Rat Index Monitoring – 2008 - 2017
Island Island control Island control
Dec 2008 5%* Feb 2015 8% Aug 2016 12% 60%
Dec 2009 10%* Apr 2015 44% 50% Nov 2016Summary The Motu Kaikoura Trust using manual methods has significantly reduced rat numbers on Motu kaikoura and has kept them at 5% or below especially during the ecologically important Spring season for the last four years. Despite rat numbers in the temperate coastal forests of Great Barrier Island being comparatively high all year round, we have confirmed a distinct pattern of seasonal increases or ‘spikes’ in population levels, beginning in February and lasting up until June. Judging by control data this seasonal pattern is apparent both on Motu Kaikoura and on Great Barrier Island. However on Motu Kaikoura, as noted, it is encouraging that in response to ongoing management the spike in rat numbers was lower in 2017 than 2016 and has been progressively lower each year going back to 2014 when we began the control programme. Also as a result of the programme, a reversal has occurred in the relative proportions of the remnant rat populations on the island with kiore present in greater proportion than ship rats. This is almost an exact reversal of what would normally be expected in unmanaged environments where these two species cohabit. In 2016 by arrangement with the owner, Bryan King, the Trust has extended the rat management programme to Nelson Island (12 ha) which is only 100m from the western point of Motu Kaikoura. Using only two bait stations we have significantly reduced the population of rats on Nelson. In 2018 we will attempt achieving zero density on Nelson and by arrangement with the owner Mr King and Auckland Council Biodiversity intensify management on adjacent Motu Haku (43 ha). On Motu Kaikoura ‘habitat repair’ due to natural forest regeneration and reducing the environmental burden of rats is resulting in improving ecological health, evidenced by increased forest regeneration and diversity of understory. While this remains to be measured objectively, monitoring indicates a greater presence of small forest birds in particular and noticeably more bird song – indicators of recovering ecological processes. The success and consistency of our rat management regime means Motu Kaikoura is now an attractive option for the release of bird species which would not be expected to establish on the island naturally. Sustainable progress in managing rats on the island now enables the Trust to focus on achieving its wider conservation restorations goals as set in the Motu Kaikoura Trust Biodiversity Management Plan. Acknowledgement and Thanks I would like to thank Motu Kaikoura Island ranger Clint Stannard for his hard work and dedication in making this operation a conservation success story. Assisted by his wife Jacinda, Clint has done a superb job. I would also like to thank former resident volunteer Robbie Smith left the island towards the end of 2017 for his contribution to the programme. Thanks are also due for the ongoing support of Brett Butland of Auckland Council’s Predator-free Auckland and Jonathan Miles and the Biosecurity team at Auckland Council both in terms of providing advice and material support. 10
References
Bagasra, A. (2013). Ship rat re-invasion behaviour from Great Barrier Island to Motu
Kaikoura. University of Auckland, School of Biological Sciences (unpublished
report).
Doig, H. 2016. Motukaikoura Trust – Annual Report.
Fewster, R.M., S.D. Miller and J. Ritchie. 2011. DNA profiling - a management tool
for rat eradication. pp 426-431 In: Veitch, C. R., M.N. Clout and D.R. Towns
(eds.) Island invasives: eradication and management. IUCN, Gland.
Hamilton, O. 2011. Distribution and density of ship rat (Rattus rattus) on Motu
Kaikoura, Hauraki Gulf, following reinvasion after eradication. University of
Auckland, School of Biological Sciences (unpublished report).
Lee, M. 2014. Motu Kaikoura rat management programme – progress report.
Lee, M. 2015. Motu Kaikoura rat management programme – progress report.
Mitchell, M. 2010. Report on Motu Kaikoura rat monitoring. Auckland Council.
Unpublished report to Motu Kaikoura Trust.
Mitchell, M. Warden, J. Shields, B. 2015. Motu Kaikoura Rodent Monitor April 2015.
Unpublished report to Motu Kaikoura Trust.
Mitchell, M. 2015. Warden, J. Shields. B. Report on Motu Kaikoura rat monitoring
November 2015. Auckland Council. Unpublished report to Motu Kaikoura
Trust.
Motu Kaikoura Trust. 2005. Motu Kaikoura Management Plan, Draft 24. Motu
Kaikoura Trust, Auckland.
Motu Kaikoura Trust. 2012. Motu Kaikoura Biodiversity management plan, Motu
Kaikoura Trust, Auckland.
Motu Kaikoura Trust. 2018. Draft Motu Kaikoura Scenic Reserve Management Plan.
Motu Kaikoura Trust, Auckland.
Russell, J.C., Caut, S. Anderson, S.H. & Lee, M. 2015. Invasive rat interactions and
over-invasion on a coral atoll Biological Conservation 185 59–65
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