Tuesday, 21 June 2011


by Michael Fay. Volunteer perimeter trapping co-ordinator

In November 2010 staff and volunteers at the Orokonui Ecosanctuary
began trapping around the outside of the perimeter fence. There are
263 traps -- 88 each for rats and mice in the fence hood, and 87 DoC
200 traps aimed primarily at the three species of mustelids: ferrets,
stoats and weasels. The purpose of this trapping is threefold: to
reduce pressure on the fence, to create a relatively safe buffer zone
for those birds that insist on going beyond the fence, and as the
first step towards creating a safer environment for all of our endemic
biota across the entire 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
considered to be relatively thinly spread with trapping yielding a
ratio of stoats to weasels of 20:1, but it is recorded that there can
be weasel hotspots. The ecosanctuary boundary has turned out to be
one of these, with a very high number of weasels being caught relative
to the two other mustelids -- 35 weasels compared with nine ferrets
and four stoats. If the national ratio had been maintained we would
have caught 700 stoats, a host of Pied Piper proportions which
thankfully we don't have.

Why so many weasels?

a) The density of a predator species is determined by the density of
prey; as prey becomes abundant the number of predators will increase.
For some species this increase will be due to more predators moving
into the area but the weasel is a short-lived species with 75% to 90%
of trapped individuals being less than 12 months old. Consequently,
infant production and survival is the most important mechanism for a
population explosion: bigger and multiple litters, a higher proportion
surviving to weening, and precocious mating by members of the first
litter of that season. In New Zealand the house mouse (gone feral) is
the weasel's main prey, and it is easy to appreciate that one or two
seasons when the mouse population expands will lead to a lot of

b) The standard DoC 200 trap is set off by a static pressure of around
100-120 grams but our traps have been modified so that they go off at
around half this level. Since a female weasel weighs between 50-60
grams (they are really tiny) and a male twice this, it is very likely
that we are catching far more of any weasels that are around than is
the case with other trapping operations. There are a couple of bits
of evidence to support this view. Firstly, 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?
Secondly it is reported that the majority of weasels caught are males,
that is animals that would have weighed in excess of 100 grams. Being
neither a trapping expert nor a mustelid expert I did not appreciate
how unusual our catch was and consequently did not record sex or size
but I have a very strong impression that many of the weasels we caught
were small and would have weighed less than 60 grams. They were so
small that a first glance the trappers often thought that there was
nothing in the trap.

c) The ecosanctuary boundary can be divided up on the basis of the
type of vegetation, e.g. cattle pasture, bush, scrub. There is a 1200
metre section along the western boundary where the ecosanctuary is
bounded by Cedar Creek Road (a 'paper' road under the power line), and
beyond that the Dons Creek sub-division. Much of this area has a
wide band of long and rank grass that provides a perfect environment
for mice and weasels; they can both scurry about unseen in what are
effectively tunnels under the matted grass. This stretch of fence
comprises 14% of the total ecosanctuary boundary but has yielded 43%
of the weasel catch and 37% of the mouse catch. It was weasel heaven
before we came along.
My guess is that a set of circumstances have combined to create a very
unusual situation. Now we have to get on top of the mice.

AND FINALLY - if you have access to dead rabbits or possums we would
really appreciate having them for bait.

Ā ā Ē ē Ī ī Ō ō Ū ū
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