Brodifacoum Case Studies by Jo Ritchie

Environmental News Issue 15 Winter 2008

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 used.
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 farms.
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 samples.
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.