Motu Kaikoura rat management programme –
Michael Lee Trustee. December 2014.
Motu Kaikoura (535ha) is a crown-owned scenic reserve purchased in 2004 after a public campaign, by a combination of government (Nature Heritage Fund), ARC, city councils and ASB Community Trust funding. It is administered by the Motu Kaikoura Trust, which was established by the Minister of Conservation in 2005. The regenerating island was infested with exotic mammals, fallow deer (Dama dama), ship rat (Rattus rattus) and kiore (Rattus exulans). Fallow deer were eradicated from the island in 2008 but an attempt to eradicate rats in the same year by two aerial applications of brodifacoum failed
The Motu Kaikoura Biodiversity Management Plan (March 2012) states:
‘Animal pest control recommendations. Method of control.
Target all rodent species (ship rats, kiore) and aim to contain ≤5% relative abundance (as measured by rodent monitoring…) by establishing a 100 x 100m grid of bait stations.’
A plan to achieve this objective was drawn up and presented to the trustees in January 2014. The operational work was to be largely carried out by Trust contractor Clint (and Jacinda) Stannard and by the island caretakers Robbie Smith and Moana Kake. The plan was approved by the Trust at its February 2014 meeting.
Additional to the 200 or so Philproof bait stations and rat ‘motels’ already on the island, 120 additional rat motels were built by trust volunteers in mid-2013 and installed along the new 15 km perimeter track. Operations began in late February, with brodifacoum bait (Pestoff 20 ppm), 4 blocks placed per station, along the perimeter track, divided between East and West Track components; as well as the Road Track, between Bradshaws Cove and the island settlement, (the former Lodge site); Pahangahou Track, from the airfield to Overlook; and Parihakoakoa Track which extends from the Road Track by the Airfield northwest towards Nelson Point.
The network was subsequently extended and expanded with 6 tracks, listed below in June, then another two tracks, the Badlands Track, 18 stations, blazed in mid-November, and the House Track, 12 stations, opened in late November (7 km in all). A Coastal Line of bait stations around the island’s 16.5 km coastline was progressively extended and completed in October. All bait stations have been checked and re-baited on a fortnightly basis. Brodifacoum bait, Pestoff (20 ppm) was replaced in July with Brigand (50 ppm). Then in November Pestoff was phased back in.
The island network presently comprises 502 stations (not counting the off-island control line). This is still somewhat less than the 535 needed to make up an optimal 100 x 100 grid, but bait stations have also been augmented by approximately 50 bait bags stapled to the bases of tree stems (eg in the Bradshaw Valley, Overlook and around the old farm house and old sheds). In December 2014, bait blocks were hand-cast down steep faces, impossible to access by other means.
The present list of bait station tracks and lines is as follows:
East Track 96 stations
Coastal Line 140 stations
Slip Track 26 stations
Barn Track 5 stations
Vodafone Track 8 stations
End of Airstrip Track 5 stations
West Link Track 4 stations
Fence Line Track 8 stations
Badlands Track 18 stations
House Track 12 stations
Stellin (control) Track on nearby mainland 10 stations
Total (managed by Clint & Jacinda Stannard) 332 stations
West Track 62 stations
Road Track 44 stations
Airfield Track 7 stations
Parihakoakoa 42 stations
Pahangahou 25 stations
Total (managed by Robbie Smith & Moana Kake) 180 stations
Total number of stations in the field (motels and Philproofs): 512 (including Stellin)
Total length of formed tracks and roads: 24.5km
Total length of coast line: 16.5 km
Results of baiting programme – February to December 2014.
Station by station data has been recorded by our on-island workers and reported on a fortnightly basis. Data inputted to excel sheets expressed in histograms illustrate the progress achieved.
The following histograms indicate actual bait taken, rather than percentages.
Here is a sample of the main tracks.
East Track (96 stations, 384 bait blocks)
West Track (62 stations, 248 bait blocks)
Parihakoakoa Track (42 stations, 168 bait blocks)
Road Track and Airfield Track (51 stations, 204 bait blocks)
Pahangahou Track (25 stations, 100 bait blocks)
Slip Track (26 stations, 104 bait blocks)
Coastal Line (140 stations, 560 bait blocks).
Index Monitoring results 2010 – 2014
It is assumed that the aerial bait operation in the winter of 2008 removed less than 100% of the target rats. After the aerial drop and the eradication of the last fallow deer, after a hiatus of 3 to 6 months, rats, identified as kiore and ship rats, assumed to be survivors, in a significantly improved habitat, boosted by rats swimming from the mainland (confirmed by DNA testing), and in the absence of natural predators, underwent what appears to have been a population irruption.
We can hindcast that from a very low population base after the aerial drop, rat levels increased to between 5 -10 % relative density by December 2009.
In December 2010 rat levels were monitored across the island by Auckland Council Biosecurity (former ARC) officers, who established the existing 5 x 10 tunnel monitoring lines and found the relative density to be 16%.
In February 2012 (just over one year later) rat levels again were monitored by the Council and were recorded at 28%.
In December 2012 rat monitoring by the Trust and Unitec (Mel Galbraith’s Natural Sciences student team) recorded a relative density of 43%.
In July 2013 monitoring by the Trust indicated a relative density of 77.6%. Two stations (West Track and Overlook) indicating 100%.
In December 2013 the monitored density levels were just over 59%. While this was a reduction of 16.6% on the figures of July 2013 – this was significantly higher (16%) than the figure for the previous December. The only positive to be taken from this was the information on just how high rat numbers could reach, without competitors and natural predators. Given the fall-off in numbers (with only cursory trapping and poisoning) from July to December, the December 2013 relative density level of 59% is likely to be at around the island’s natural rat carrying capacity
This year, a combination of circumstances and bad weather over a prolonged winter, limited opportunities for light plane visits and prevented planned winter monitoring. Nevertheless the baiting work proceeded methodically.
To calibrate significantly declining bait-take evident from August onward and to confirm this was not due to bait avoidance, an index monitoring exercise was thought useful, rather than waiting for December. The first practical opportunity for this was on 3 November when trustees flew to the island and activated four of the five monitoring lines (Bradshaws (10 stations), Lodge (4 stations), Overlook (East Track) 10 stations and Mangrove Valley (East Track), 6 stations. This produced a remarkably good result of only one set of rat prints detected out of 30 cards, therefore 3.3%. On the 12 November the writer revisited the island, during which time the West ridge monitoring line was activated and produced. This one set of footprints in one of the 10 tunnels. Combined the results, this works out at 5%. However it must be pointed out that this is not an ‘official’ result – the exercise was carried out with the principal purpose of confirming bait station trends.
We have not publicised these results until now because the Trust’s official monitoring exercise (and annual bird count) was scheduled to take place in mid-December. Unfortunately this expedition (by the Unitec Natural Sciences team) was cancelled due to severe weather and will now take place in mid-January. Also the November monitoring took place on two separate occasions, (albeit separated only by eight days) – which is not ideal. These results in the meantime provide a useful indicator and confirm that the recorded bait-take decline reflects a real decline in the rat population, and that bait avoidance is not a significant problem. These results are enormously encouraging – especially compared to the situation one year ago.
Index monitoring results past over the past 18 months
July 2013 Dec 2013 Nov 2014
Lodge (RM1) 3/7 (43%) 2/7 (29%) 0/4 (0%)
Bradshaws (RM2) 7/10 (70%) 7/9 (78%) 1/10 (10%)
Mangrove Valley (RM3) 9/12 (75%) 3/9 (33%) 0/6 (0%)
West Ridge (RM4) 9/9 (100%) 10/10 (100%) 1/10 (10%)
Overlook (RM5) 10/10 (100%) 4/9 (45%) 0/10 (0%)
79% 59% 5%
Motu Kaikoura. Rat relative densities December 2008 – November 2014
A steady trend of declining bait take, confirmed by the tracking tunnel monitoring on 3-4 November and 11-12 November would indicate hundreds of hectares of the island – especially the interior – are now clear of rats. This data is supported by natural cues, such as noticeably higher numbers of native birds visible across the island – especially native pigeons – and numerous weta prints in tracking tunnels. Also notable is the sight of untouched puriri (Vitex lucens) fruit and untouched windfall rewarewa (Knightia excelsa) flowers, lying on the East Track.
Workers have now been asked to activate the snap traps placed in the rat motels to pick up any possible bait-avoiding or poison-resistant rats. To broaden bait station attraction, Brigand bait has been augmented in each station with the more recently purchased and differently flavoured Pestoff.
While much of the interior of the island appears to have been cleared, an area of infestation still remains along the southwestern coast, especially from coastal station 120 to 138 (please see fig.1 and fig.8). This coincides with that section of the West Track where rats have been trapped. That is to be expected as the Coastal Line stations are relatively new – the line was only completed in October. However the trend of bait-take on the Coastal Line, indicates the same decline as the other lines. Attention now focuses on trapping. Low numbers of rats (bait avoiders?) have been trapped. Trapping results will be collated and be made available along with the official index monitoring results in January.
Of the 502 bait stations in the field, only 24 now show signs of rat activity – (of those, 22 of these are in the Coastal Line) while 488 bait stations are now indicating no activity. Approximately 2560 bait blocks are available in stations, mostly a combination of Pestoff and Brigand baits, and approximately 50 blocks in bait bags, across the island. 180 kg of bait has been used since the programme began in February, with bait take declining to very low figures in recent weeks (see graphs).
It is important to note the programme is ongoing and that we have yet to deploy our full resources. There is 190 kg of brodifacoum bait in hand, (14 x 8 kg containers of Brigand and 8 x 10 containers of Pestoff), 40 x Philproof stations and 22 Albia snap-traps. These resources will be progressively deployed when new lines are opened, and elsewhere as necessary.
Hopefully remaining rat presence along the southwestern coastline will have been mopped-up by the end of January 2015. Checking of bait stations by field workers has now been reduced to once per month, with the exception of the Coastal Line, though traps will be serviced once per fortnight. Several new lines will be cut.
It is now feasible for the Trust to go on to try to achieve a zero rat relative density level across the island. The 17 December meeting of the Motu Kaikoura Trust, after reviewing progress of the programme over the last year, formally resolved as a policy to replace the previous 5% rat relative density objective, with the objective to use best endeavours to achieve and maintain the island at 0% rat relative density.
The results of the January monitoring will be reported to DOC. stakeholders and the public, as soon as it has been completed.
In summary I am pleased to report the Motu Kaikoura rat management programme has made successful headway – thanks in large part to the dedicated efforts of our contractors Clint and Jacinda Stannard and the island caretakers Robbie Smith and Moana Kake, and with the support of DOC and Auckland Council.
Motu Kaikoura at 535ha (Taylor 1989) or 564 ha (certificate of title) is the largest New Zealand island in which a serious attempt to reduce rats to zero-density levels using bait stations has been made. The second biggest island to successfully employ bait station techniques was Ulva Island (270 ha) off Stewart Island, declared rat-free by DOC in 1997. Ulva is approximately half the size of Motu Kaikoura. The largest island in the Hauraki Gulf to successfully eradicate rats using bait stations was Rakino Island (147ha) by the ARC in 2000. In each of these cases the rat species targeted was Norway rat (Rattus norvegicus). On Motu Kaikoura the target rodents have been ship rat and kiore.
The Motu Kaikoura Trust can be reasonably confident that by the end of first 12 months of operation (end of January 2015) our programme will have achieved its original objectives. Next year, hopefully after achieving a confirmed zero rodent relative density we will then modify our operation to a more defensive strategy to safeguard the island’s new status. We will also be extending operations – baiting and 6 monthly follow-up checks to the islands of the Grey Group and where possible working cooperatively with the private owner of Motuhaku and Nelson Islands and Glenfern Sanctuary to clear and keep rats off these islands. It is of course important to remember that as an inshore island, Kaikoura’s close proximity to the Great Barrier mainland will always be problematic for biosecurity. Reinvasion by ‘swimmers’ can be expected, especially in the Autumn, but there is now a comprehensive system in place to ensure any future rat incursion on the island is temporary and ecologically not significant.
The Motu Kaikoura Trust once having confirmed <5% rat relative density and ideally 0% relative density, and therefore ‘sanctuary’ status, can then turn some of its focus from the biosecurity of Motu Kaikoura to the island’s biodiversity and its amenity.
We wish to thank DOC Auckland Partnerships Manager Pieter Tuinder and Funds Adviser Anna-Mareia Hammond for their very helpful assistance and support in securing a funding grant of $18,250 (including GST) in two tranches. This on the basis of $9125.25 contribution from the Motu Kaikoura Trust.
The programme has also received welcome assistance from Auckland Council Biosecurity Manager Brett Butland who arranged a donation of 20 x 8kg containers of Brigand bait.
Assistance has been also given by Todd Bolton at Sealink whose barge delivered the bait stations and there has ben assistance from the DOC vessel Hauturu for delivering bait. Finally warm thanks also to Scott Sambell of Glenfern Sanctuary for inputting our GPS data into his ArcGIS computer programme enabling the production of a detailed map of the island, its tracks, bait stations and monitoring lines.