Trout munching on mice risks explored

FURRY FOOD SOURCE: Ken August with a large brown trout fattened on mice after the 2009 beech mast.
FURRY FOOD SOURCE: Ken August with a large brown trout fattened on mice after the 2009 beech mast.

A Nelson study is looking at the possibility of trophy-sized trout caught as a result of this year's beech mast being full of mice poisoned with 1080.

The Department of Conservation is providing the funding, the Cawthron Institute is doing the work, and the New Zealand Federation of Freshwater Anglers and Fish & Game are collaborating.

Beech masts - heavy seed drops that occur every few years - produce an explosion in the back-country mouse population.

To counter the latest mast, DOC is preparing for its biggest ever 1080 drop.

But little is known about how many poisoned mice could be eaten by trout, which gorge themselves on the rodents in mast years and grow rapidly to trophy size, often 5kg or more.

Trout feast on mice that try to swim across streams and rivers as their population grows and they try to disperse.

The federation, whose national president is Nelson's David Haynes, has been pushing for research into the potential effects on trout and people who eat them after the fish have consumed poisoned mice.

Haynes said the federation was "rapt" when DOC got in touch and suggested that they work together on the issue, which also covered eels and other native freshwater fish.

"We have always maintained that the risk is the gap in knowledge in this specific area - no research has been done to date."

Cawthron will carry out desktop simulations and laboratory tests, with in-field monitoring being ruled out as being too complex and costly.

DOC says results will be available to the angling community before the start of the next trout fishing season on October 1.

When Hayes went public with his concerns in February, warning anglers to steer clear of trout from waterways near 1080 drops, a freshwater fish specialist and DOC said it was very unlikely that trout would be affected or that there would be any risk to humans.

DOC spokesman Rory Newsam said then that there was already some Niwa research which showed no significant 1080 impact on freshwater fish.

Fish & Game Nelson-Marlborough regional manager Neil Deans said the study was an acknowledgement that there might be a problem, and more research was needed to assess it.

"It's good that the issue has been raised and that DOC has acknowledged that while the risks are probably very low, we aren't able to quantify them without doing some research."

"We know that in a mast seeding year, mice numbers explode, and we also know that trout in back country areas feed on those mice."

It was known that some trout ate large numbers of mice, he said.

"We're certainly aware of fish with up to 12 mice in them, so it's no surprise that they put on condition very fast and are sought after by anglers."

Fish & Game was not anti-1080 and felt that the risk to people was very small, Deans said.

"We don't think it should be an issue but we want to quantify it to assess how low a risk it is."