Tawharanui Regional Park
Pellet breakdown and soil residues were measured at Tawharanui Regional
Park using the same baits as proposed for the Kaikoura eradication and
showed that there was no evidence that it contaminated the soil. In this
study, baits placed in wire cages (to stop rodents etc getting them)
were found to completely disintegrate after a maximum of 110 days in all
habitat types. Soil samples taken from directly below the baits (i.e.
the worst case scenario) had residues on or below the minimum level of
detection (0.02µg/g) after 110 days. (µg/g is micrograms/gm) A microgram
is 1 millionth of a gram: one of the smallest units of weight or mass
Accidental spill into the sea off the Kaikoura coast (South Island)
Another well known example is the accident in which a truck carrying 18
tonnes of brodifacoum baits fell into the sea off the Kaikoura coast in
2001. A large amount of bait yes, but a very small amount of actual
toxin — equivalent to 0.360 kg or 360 grams: about a cupful of
brodifacoum entering the sea. Monitoring was undertaken by Environment
Canterbury and the Cawthron Institute. Brodifacoum was detected above
0.02 ug/l in samples from mussels, limpets and paua collected from the
intertidal zone near the crash site. These levels were above both the
Australia and New Zealand Food Health Standards limit for consumption of
seafood. Accordingly a precautionary approach was taken in terms of
public health warnings and closures.
The residues were thought to be at least partially due to the continued
exposure of mussels to brodifacoum through filter feeding. Initial high
exposure of mussels to bait fragments suspended in water would have
decreased as the bait particles were dispersed from the immediate spill
site over the first week. However contamination was localised to an
approximately 100-m2 area around the spill site. Public health warnings
against shellfish collecting were lifted by the NZ Food Safety Authority
in May 2004.
It is important to point out that a spill of this magnitude has never
occurred during an aerial baiting operation and is extremely unlikely to
occur given the rigorous procedures employed during the operations and
the fact that the volume of bait that will be over any given point at
one time is limited to that carried by a helicopter. At Kaikoura Island
this will be between 600–800 kg. In addition a number of other studies
including those at Little Barrier and within the Kapiti Island marine
reserve found no observable effects of brodifacoum on marine ecosystems
(water and marine species) after aerial bait drops on those islands.
Safe and careful use is the key
As someone who uses products like brodifacoum regularly, it’s my
responsibility to continue to keep up with information as it comes to
hand, take all practicable steps to minimise harmful effects on the
environment, be sensitive to community concerns about the use of toxins
and provide people with as much information as they need. It’s up to all
of us who use these products to be informed about their use and use them
with care. If you have concerns be up front, let the relevant people
know but be prepared to listen to both sides of the story and only then
make a decision.
A constant learning curve
To date rodents have been removed from over 40 islands in New Zealand
ranging from under a hectare to 11,300 hectares. Many of these
operations have been undertaken by D.o.C using brodifacoum. In addition a
number of community groups have been undertaking rodent eradications
behind pest proof fences on the mainland also using brodifacoum. All
work to stringent environmental standards and from each operation we
gain more information and refine our techniques, both from scientific
studies but also from observations on the ground by skilled personnel.
This is not new science - its exacting, evolving science that is
constantly being improved as we learn more. We are world leaders in this
area of work.
If you would like to learn more
A good summary of brodifacoum is contained in the recently completed
feasibility study to examine whether it’s possible to eradicate rats,
wild cats and possums from Stewart Island. This study is very easy
reading and can be downloaded at no cost from the Stewart Island/Raikura
Community and Environment Trust (SIRCET) website (www.sircet.org.nz).
Summary reports will also be written as the Kaikoura Island operation
proceeds (required under the resource consent rules) and will be
available on request from the Motu Kaikoura Trust or from the ARC or
Auckland City Council.
Definitive Study on Water Solubility at Maungatautari
A very comprehensive study was undertaken by the Maungatautari
Ecological Island Trust during one of its aerial baiting operations.
Maungatautari is a 3500 hectare area. Numerous streams flow out of the
mountain, many of which provide stock and drinking water to surrounding
Two fenced ‘cells’ (35ha and 65ha) each received two bait drops of
Pestoff 20R brodifacoum cereal bait in September and October 2004.
15kg/ha was applied on the first drop and 8kg/ha in the second. The area
(c.8ha) immediately around the inside of both cell fences was hand
spread. A total of 183 stream water samples were taken from 4 streams
flowing out of the area that were baited. In each stream, samples were
taken at the fence boundary and again 800 metres downstream. Sampling
times after each drop were 1hr, 2hrs, 3hrs, 6hrs, 9hrs, 12 hrs, 24hrs,
48hrs, 72hrs, 2 weeks, 3 months. At no stage was more than 0.02 µg/l*
(the minimum level of detection) of brodifacoum found in any of the
Simply put, no brodifacoum was found in any of the samples and a human
or animal would have had to drink more water than was physically
possible to get even a detectable level of the toxin. In fact no
brodifacoum has ever been found in any of the many water samples taken
across a range of aerial baiting operations.
* µg/l is micrograms per litre.