Trout to be fed poison-laced pellets to assess risks from 1080 drop

The risk of trout eating poisoned mice is being assessed. Photo / Zane Moss

Pellets laced with 1080 poison will be fed to trout to assess potential risks from the planned massive dump of the pesticide in native forests.

Nelson-based scientists have started the work after an anglers' group challenged claims by the Department of Conservation about potential for the poison to end up in trout. The department intends to spread hundreds of tonnes of bait with trace amounts of 1080 over forests to protect threatened native birds from a surge in pest populations.

The department initially accused the Federation of Freshwater Anglers of making "alarming statements" but has since agreed to fund a study to determine whether back-country trout enthusiasts faced any risk from 1080 accumulating in fish.

The federation warned anglers potentially were at risk from eating big 'trophy' trout that had consumed mice exposed to 1080 - a claim the department rejected.

But it has changed its position and is investing about $70,000 supporting research by the Cawthron Institute that involves an assessment of the risk along with laboratory trials to measure how much 1080 remains in trout after they have eaten poisoned mice.

Cawthron ecotoxicologist Dr Louis Tremblay said computer modelling and a literature review would help estimate the amount of 1080 likely to accumulate in trout tissue after fish consumed poisoned mice.

"In the second stage we will measure levels of 1080 residue in the tissue of trout that have been orally exposed to 1080. The amount of 1080 used will represent a likely maximum dose a trout could ingest through eating contaminated mice," Dr Tremblay told the Weekend Herald.

Thirty-five brown trout from a Fish & Game hatchery will be used for the research, which still awaits approval from an external animal ethics committee. He said the research should determine the risk, if any, to anglers from eating trout that have eaten mice contaminated with 1080 following aerial operations.

Dr Tremblay said little was known about the toxicity of 1080 to trout, although studies with eels fed 1080 indicated the risk to humans from eating affected meat was extremely low.

"What is unknown is how much 1080, if any, will end up in trout tissue as a result of trout eating food potentially contaminated with 1080," he said.

Results need to be available before the fishing season starts in October, which is when the large-scale 1080 programme will be in full swing. Cawthron expects study outcomes will be available by the end of June.

The 1080 drop, one of the largest undertaken anywhere in the world, is the department's response to a forecast explosion in rat and stoat numbers arising from bumper seed production in beech trees. Rat and mice populations are expected to grow rapidly as abundant seed drops to the forest floor. Stoats, which feed on mice and rats, are predicted to become plentiful, which in turn poses a significant threat to native birds. The 1080 drop, planned for later this year, is designed to disrupt the sequence, known as the predator-plague cycle. Latest forest surveys indicate that moderate to "very heavy" amounts of seed is forming in beech forests, which one DoC scientist likens to a 10- to 20-year event.

DoC rangers have been shooting canopy branches out of beech trees in forests to get a fix on seed production. DoC scientist Graeme Elliott said results confirmed the beech "mast" - the big crop of seed following a heavy spring flowering - was proving to be a widespread event. "The key issue is that this is widespread seeding and looks set to fuel an explosion in forest predators like rats and stoats in a large number of places."

Forests in Kahurangi, Abel Tasman, Arthurs Pass, Westland, Mt Aspiring and Fiordland national parks, and conservation land in Marlborough, Canterbury, the West Coast and Otago will be targeted in the 1080 blitz. As much as 650 tonnes of bait is expected to be dropped.