Motu Kaikoura Rat Management Program

Progress Report as at July 2015,  by Mike Lee, Trustee


Motu Kaikoura, an island of 535 ha, is a crown-owned scenic reserve purchased in 2004 after a public campaign to secure it in public ownership. The purchase was funded by combined contributions by the government (Nature Heritage Fund), the former Auckland Regional Council, former Auckland territorial local authorities and also ASB Community Trust funding (now Fund North).  The island is administered by the Motu Kaikoura Trust which was established by the Minister of Conservation in 2005.  The island, covered in regenerating native forest and some stands of pine, was infested with exotic mammals: fallow deer (Dama dama); ship rat (Rattus rattus) and kiore (Rattus exulans), with low numbers of feral pigs (Sus scrofa); cats (Felis catus) and rabbits (Oryctolagus cuniculus cuniculus).  Fallow deer, pigs, cats and rabbits were eradicated from the island in or before 2008 but an attempt to eradicate rats in the same year by two aerial applications of brodifacoum baiting 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 100 m grid of bait stations.

Figure 1


A plan to achieve this objective was drawn up and presented to the trustees in January 2014 and approved in February 2014.  Work got underway soon after.   
The Bait Station Network as at 30 June 2015 consists of 540 bait stations (plus 10 stations dedicated to the off-island control line on the Stellin Peninsula).





The total number of managed stations in the field (rat motels and Philproofs) including the off-island control line  =   550 units

Total length of formed tracks and roads  =   28.1 km

Total length of coastline  =   16.5 km


There are also 50 tracking tunnels, grouped in five lines of ten, strategically placed across the island for periodic index monitoring.


Results of baiting-trapping programme  –  March 2014 to June 2015.


Station-by-station data has been recorded by our on-island workers andreported on a fortnightly or monthly basis.   The data has been inputted to Excel™ spreadsheets and presented as histograms to illustrate the progress achieved.


The following histograms indicate percentages of the bait taken
from stations along the main tracks.




Figure 2


Figure 3


Figure 4


Figure 5


Figure 6


Figure 7


Figure 8


Figure 9


Figure 10


Figure 11







Index Monitoring results 2010 – 2015


It is assumed that the aerial bait operation in the winter of 2008 destroyed less than 100% of the target rats.  After the aerial drop and the eradication of the last fallow deer and other pests, and 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 nearby mainland, 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 and 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 tracking 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 Biosecurity and were recorded at 28%.

 In December 2012 rat monitoring by Unitec students recorded a relative density of 43%.

 In July 2013 monitoring by trustees gave a relative density of 77.6%.  Two stations (West Track and Overlook) indicated 100%. 

 In December 2013 monitoring indicated density levels were just over 59%.  While this was a reduction of 16.6% on the figures of July 2013 – it 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 perhaps around the island’s natural rat carrying capacity.


Active rat management began in March 2014.

 Index monitoring by trustees in November 2014 result (2/40)  =  5%. 

 Monitoring by trustees in February 2015 result (4/50)  =  8%

 Monitoring by Auckland Council Biosecurity in April 2015 result (22/50)  =  44%

 Monitoring by Glenfern Sanctuary personnel in May 2015 (11/45)  =  25%

 Monitoring by Island ops manager Clint Stannard in July 2015 (3/5)  =  6%





Figure 12





As indicated in our last report (Dec 2014), over a six-months-period (up until late February 2015), a trend of declining bait take and low index numbers (5% in November 2014) was followed by a very sharp upsurge in rat numbers.  Relative density monitored in the tracking tunnels increased from 8% in mid-February to 44% in mid-April. This correlated with the rat post-breeding season, range expansion and incursions by rats swimming from the nearby mainland, first detected in a study by Auckland University (Aiman Bagasra 2013).  Glenfern Sanctuary also experienced a similar sharp but short-lived spike with rat (kiore) numbers peaking in April. Compounding this natural phenomenon was a disruption to the work program on some lines when two workers resigned. The whole island grid is now the responsibility of our island operations manager Clint Stannard.


Another factor, of academic interest at this stage, is an apparent relative increase in the kiore population, evidenced by DNA samples we collected in February.  (This was also apparent on the neighbouring Glenfern Sanctuary, Scott Sambell pers.comm.).  This sudden spike in rat numbers was picked up in monitoring by Auckland Council (Mitchell et al.) in April.


Since our last report the island’s bait station grid has been progressively expanded with four more lines cut and 50 new bait stations placed. At the same time spacing of stations along existing lines have been rationalised.  There are now 540 stations for the 535 ha area (on an approx. 100 m x 100 m grid).  This is still minimal for these sorts of operations but the grid will be expanded especially out to coastal headlands and intensified if circumstances require it.


All bait stations and motels on the island were cleaned out prior to winter and now that bait take is again very low, old bait has been replaced and fresh bait placed in plastic bags in the stations to preserve it through the winter.  There are now 2160 bait blocks
(4 blocks per station) available for rodent consumption. 


There are 398 new snap traps deployed.  These are concentrated on the perimeter track (East Track and West Track), the Road Line, Airfield Line and Wharf Line in rat motels.  The new plastic traps (T Rex and Snap E) are much easier and more time-efficient to service than the old Victor traps.  This means traps baited with peanut butter are now a permanent part of the operation.  The new line of ten bait stations from the wharf up to the settlement also has two traps per station. Both Pestoff and Brigand brodifacoum bait have been used and there are five buckets, or 40 kg of Brigand (brodifacoum) bait on hand.


Refer to graphics (figs 9, 10 and 12.).  The latest bait take and snap trap indices are now trending down satisfactorily.  As at the end of June, bait take across island was 7%, rats trapped 8% and monitored relative density (early July) was 6%.  Please compare these figures to the bait take on our mainland control line.  This was 80% in June.




New Lines


Apart from the new Wharf Line, two significant new lines have been cut.  One transiting from the northeast coast at Bradshaws Cove, tracking across the valley to the island’s main ridge.  Another from the end of the Parahakoakoa ridgeline track to the northwestern point of the island opposite Nelson island (see fig. 1).  

At the Auckland Council Biosecurity team’s suggestion index monitoring took place in May and again in July timed with similar exercises at the neighbouring Glenfern Sanctuary and at Windy Hill near Tryphena, Great Barrier Island.



Objectives Achieved

The Motu Kaikoura Trust did achieve (and likely has exceeded) its original objective for Year One lowering rat numbers to 5% relative density.   From late February to May the island subsequently experienced a post-rat breeding season spike, the scale of which frankly took us by surprise.  We can assume this was fuelled by remnant ship rat populations on the island, especially along the southern coast line (where bait take is still high at 24%), an increase in kiore numbers elsewhere and significant incursion from the nearby mainland and from Nelson Island.   Changes in personnel in March enabled the whole operation to be managed by one person.  The situation has stabilised and the operation is back on track with rat numbers declining rapidly as indicated by bait take, trapping and index monitoring.   The new lines extending into virgin habitat will reach rats populations there and hopefully contribute to lowering rat numbers below the target 5% index.  Maintenance checking of lines has now been wound back to once per month,  but fortnightly on the Wharf, Road and parts of the Coastal Lines until bait take falls below 5%.   Our focus for mop up operations over the next few months now turns to the Coastal Line – especially the southern coast (stations 98 to 140 in particular). This will also mean the cutting of new lines from the East and West Tracks down to the shoreline.






First of all I would like to thank our Island operations manager Clint Stannard who has assumed the bulk of the work for the rat control programme over the last four months.  Assisted by his wife Jacinda, Clint has done a brilliant job.  In March we farewelled Moana Kake who worked so hard during the first year of the programme.  Robbie Smith also resigned in March but has returned as caretaker and is now manages the Road and Airfield Lines only.

We also offer warms thanks to our mainland neighbours Glenfern Sanctuary, Scott Sambell manager and Biosecurity manager Michael Kinahan for their assistance, especially in undertaking index monitoring in May.

Thanks also to Brett Butland and his Biosecurity team at Auckland Council, especially Jeremy Warden and Mark Mitchell for their advice and ongoing support.


Finally, we wish to express our thanks to the Department of Conservation and the Conservation Community Partnership Fund for the generous funding support that has made this project possible.


At this stage of the project we can report that after a challenging late summer we now have a more extensive and better managed bait/trap station network. This means a better managed island environment, with rat numbers suppressed and declining in time for the all-important spring and early summer when birds and other taxa will be breeding in a significantly healthier ecosystem.




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