PROPOSED Rat Eradication Programme for Kaikoura Island and Other Outlying Islands

Presentation by Jo Ritchie, October 2007

Islands proposed for inclusion in the programme

Kaikoura535 haDepartment of Conservation
Motuhaku43 haKing
Nelson13 haKing
Grey Group13 haNgati Rehua
Moturako4 haNgati Rehua
Sugar Loaf Islands3 haNgati Rehua


Pest Species Present

Kaikoura:Other Islands:
Fallow DeerShip Rats
PigsFeral Cats
Feral Cats 
Ship Rats 

Kiore are not known to be present on Kaikoura and mice may be present but in low numbers possibly suppressed by rats.  “absence of evidence is not evidence of absence”

Rodents have directly caused or contributed to the extinction of many species of wildlife including birds, small mammals, reptiles, invertebrates, and plants, especially on islands.

Rats are everywhere, often found in trees and have a preference for high density living.  They are the most frequent cause of losses of eggs, chicks and sitting adults of forest birds in non beech forest on NZ mainland.

Seeds, leaves and fruits are targeted in autumn and winter.  Rats often leave ‘husking stations’ – piles of drilled out seeds at the bottom of trees such as nikau, karaka and rimu and eat flowers out of many species such as kiekie.

Many large ground dwelling flightless invertebrates such as large weevils and giant weta have either gone or are only found on rodent free islands.

Burrowing petrels have no chance!  Kiore predation of Cook’s petrel chicks resulted in a 90% breeding failure rate of Cook’s petrel on Little Barrier Island between 1996-2003 BUT following their eradication from Codfish Island Cook’s petrel achieved 80% breeding success in a 4 year period.

Impacts also appear to be more severe on smaller islands, where rat densities tend to be higher and do not fluctuate. Constant predation pressure results in a reduction in colony size on these islands.  Ship rats invasion of islands in the Grey group have been a major factor in the decline of seabirds & lizards.

Rats have remarkable diet flexibility.  Caterpillars are caviar – decimating species such as puriri moth.

Eradication programme 2006 onwards

Deer – pulse hunting, survey, final hunt
Pigs – hunting
Feral cats – secondary poisoning, trapping
Rabbits – poisoning, trapping, shooting
Rodents – aerial baiting, ground follow-up with monitoring, bait stations and traps
Bait stations and monitoring on nearest Aotea peninsula’s to minimize reinvasion
Ship rats are known to swim up to 500m (closest Aotea point is less than 100m)

Eradication principles

All animals can be put at risk by the eradication technique(s).

Animals must be killed at a rate exceeding their rate of increase at all densities; and immigration must be zero.

Aerial vs Ground Baiting

Allows total area to be covered with equal amounts of bait in one day.
Includes coverage of areas impossible to get too by ground (cliffs, rock stacks, tree canopies (e.g. puriri)
Eradication requires that the worst case scenario be considered. i.e. that mice are present
Mice have very small home territory around 10m2 or less
Ground baiting would require a 10x10m grid on all islands and extensive track cutting to establish and maintain grid

For Ground Baiting

In an eradication every animal must have access to bait so all bait stations would need to be filled at the same time and repeated at least once to deal with submissive animals and any survivors.  Some animals may have home ranges smaller than 10m2 and therefore not come in contact with bait stations.  Aerial baiting at 13kg/ha means a bait every 1.24m.  At 7kg/ha a bait every 1.69m.

Island SizeSizeBait Stations
Grey Group13ha1,300
Total bait stations: 61,300
Cost of bait stations: $14.22 each: $ 871,686

Pestoff® Rodent Bait 20R

2 gram cereal pellets containing 20 parts per million brodifacoum (0.002%). Proven to be the most effective bait for rodent eradication – lowest toxic loading + most palatable.
Brodifacoum is a second generation anti coagulant. No bait shyness because of delayed action.
An average ship rat would need 6 baits to die.
A 20g mouse would need ½ of 1 bait but can take up to 20 days to get a lethal dose.
Lethal doses are conservative amounts but it’s the most resistant 10% that are the key.

Brodifacoum in Soil

Not mobile in soil, no residues ever detected.
Attaches to organic matter in the soil where it is broken down by soil micro-organisms into non-toxic products .
Baits break down by swelling, cracking, then crumbling, depending on temperature & humidity .
Complete breakdown by 90- 110 days.
Break down quicker in open areas than in forest.
Withholding period established 60-120 days depending on rate of breakdown.

Bait Breakdown Pattern

Brodifacoum has very low solubility in water and is slightly soluble in water at pH 9.2 (freshwater is 7) or above but solubility reduces exponentially with decreasing pH.  Leaching from soil into water is therefore unlikely to occur.  Flow and volume of a waterway will also affect distribution of any bait- or soil-bound brodifacoum entering it.
Brodifacoum in water will bind to sediments.

No residues of brodifacoum have been detected in water bodies.

Bait breakdown after placement in pasture at;
A = 0 days
B = Day 30
C = Day 60
D = Day 90

Photo: Paul Craddock

Hauturu (Little Barrier Island): 8 water samples were taken directly down stream from baits lying in stream beds within 24 hours of the aerial drop. Brodifacoum was not detected in any of the samples taken Samples tested from bore water on the island did not detect any brodifacoum.
Maungatautari Ecological Island (Waikato): 183 stream water samples were taken from 4 streams flowing out of the area that was baited 1hr, 2hrs, 3hrs, 6hrs, 9hrs, 12 hrs, 24hrs, 48hrs, 72hrs, 2 weeks, 3 months post drops. No brodifacoum detected.

Brodifacoum in Marine Environment

Marine environment is not intentionally baited.
Same breakdown process as for freshwater.
Wave action and grinding with sand particles accelerates breakdown and dilution.
Bait dropped into the sea off Kapiti Island disintegrated in 15 minutes.
Hauturu: Paua and scallops were tested after the aerial bait drops. No brodifacoum was detected.

Flying conditions:

Flying the coast at the calmest time of the day.
Applying the bait with the bucket spinner turned off so that bait falls directly down and does not fly outwards.
Ensuring that any turns that are done out to sea near the mussel farms are not with a full bucket (so bait does not fall out of the top).
Turns are with the spinner turned off and utilize slow wide turns (tight, fast turns can make bait come out of the bucket).

Targeting the Rats

Overseas travel is common for rats.  A swim across 500m of calm sea is not a problem although they prefer a warm boat or a mooring rope to shore.

Beating them at their own game;

Techniques to be used to prevent/minimize reinvasion

Biosecurity plan (will include checking stores before they come onto the island, bait stations in aircraft, around airfield etc).
Island chain eradication
Bait stations along coast – Kaikoura and GBI
Tracking tunnels
Signage and info
Community partnership
Gear and transport

DNA fingerprinting

(Acknowledgement: Rachel Fewster, Auckland University)

DNA sampling of ship rats from various locations on Great Barrier and offshore islands.
Differences in DNA from location to location.
Great Barrier and Kaikoura rats very similar.
Relative genetic isolation across small channels of water.
(Kaikoura- Nelson 100m, GBI- Broken Islands 200m).

Two key questions are;
Is isolation caused by absence of suitable landing spots as opposed to distance over water?
What makes a rat swim to a new place?

Value to Kaikoura Project

Differences in DNA potentially allow distinction between invaders and survivors.  Therefore we can distinguish easily between a survivor from Kaikoura and a reinvader from Nelson or Motuhaku, or by boat from the Broken Islands.

Additional samples from Kaikoura prior to eradication will determine whether there is enough variation to distinguish these from Great Barrier

Can potentially allow targeted effort in key areas as opposed to wider area.

On and On

Track cutting and installation of bait stations and tracking tunnels almost complete
Ordering and delivery of bait
Code of Practice operational planning systems in place by early 2008
Aerial baiting contract with Skywork Helicopters and baiting logistics
2 breaks of Fine weather requested between July and October

Benefits of rat free islands

Smaller islands in Kaikoura chain previously held large breeding populations of shearwaters, prions and petrels – likely to recover
Lizard populations have recovered and increased exponentially following rodent eradications
Increased fruiting and flowering will over time accelerate natural regeneration processes, encourage natural return of some lost species (e.g. bellbird) and increase numbers of species such as kereru
Reintroductions of rodent sensitive native species

Smaller islands could provide unique habitats for rare lizard species such as chevron skink and tuatara

Will Scarlett, Kaikoura Hunt, Rod Miller, Harry Doig, Judy Gilbert, Ngati Rehua, mussel farmers and local landowners, Rachel Fewster, Phil Thomson, Roger Stevenson, George Wilson, Chris Wild, Rolien Elliot, Alan Gardiner, Mike Lee, Jack Craw