Closing just over 300ha of forest and shrub land, Orokonui Ecosanctuary above Blueskin Bay is the largest haven for indigenous wildlife on the South Island where predators are no longer an immediate threat.
Nonetheless, predators hammer away at the 2m-high, 9km perimeter fence. In November last year, staff and volunteers at the ecosanctuary began a programme of trapping outside the fence.
There are 263 traps. They target rats and mice in the fence hood and three species of mustelids: ferrets, stoats and weasels. The purpose is threefold: first of all, to reduce pressure on the fence as no fence is 100% pest proof; second, to create a relatively safe buffer zone for birds that fly over the fence; and third, to eventually make things safer for all native biota across the Waitati area.
So far, we have caught over 250 pests: mice, rats, rabbits, hedgehogs, ferrets, stoats and weasels. In New Zealand, the weasel population is thinly spread, with trapping generally yielding a ratio of stoats to weasels of 20 to one. But there are weasel hotspots. The ecosanctuary boundary has turned out to be one of these hotspots, with a very high number of weasels being caught relative to the two other mustelid species - 36 weasels compared with nine ferrets and five stoats.
Why are there so many weasels?
The density of a predator species is determined by the density of prey; as prey becomes abundant, the number of predators will increase. In New Zealand, the house mouse (gone feral) is the weasel's main prey, and it is easy to imagine that in seasons when the mouse population expands, there could be a lot of weasels.
A 1200m section on the western boundary of the ecosanctuary is bounded by Cedar Creek Rd, and beyond that there is the Dons Creek subdivision. Much of this area has a wide band of long, rank grass that provides a perfect environment for mice and consequently for weasels too.
They can both scurry about unseen in what are effectively tunnels under the matted grass.
This stretch of fence comprises just 14% of the ecosanctuary boundary but has yielded 43% of the weasel catch; it was weasel heaven before the trappers came along.
Second, the standard Doc 200 trap is set off by a static pressure of around 100-120g, but our traps have been modified so that they go off at around half this level.
Since a female weasel weighs 50-60g (they are really tiny) and a male twice this, it is very likely we have been catching far more weasels than is the case with other trapping operations.
There is evidence to support this view. In a Doc trap, an animal has to cross a sprung plate to get to the bait, and the bigger animals tend to be killed before they are very far on to the sprung plate; the killing bar hits them on the neck.
Weasels (and rats) often make it much further across the plate before the killing bar is activated, crushing them in three or four places. Waffled weasel, anyone?
Published articles report that most weasels caught have been males - animals that would have weighed over 100g, but being neither a trapping expert nor a mustelid expert I did not appreciate how unusual our catch was.
Consequently, no record of sex or size was kept, but I have a very strong impression that many of the weasels we caught were small, either females or juveniles, and would have weighed less than 60g. They were so small that at first glance the trappers often thought that there was nothing in the trap.
It seems that we have a very unusual situation here. While it is gratifying to have identified and reduced the weasel threat, we now have to get on top of the mice so that the weasels will not return. More than a hundred boxed mouse-traps (boxed so we do not catch birds) have been set out and we may need many more.
• Michael Fay, who lives at Waitati, is the co-ordinator of the perimeter trapping programme. He has been an Orokonui Ecosanctuary volunteer since the project's inception. The Wild Ways column appears in the Magazine section on the first Saturday of the month.
