Stoat (Mustela ermina) Photo: DoC.
Mustela nivalis vulgari
Weasels are the smallest of the three mustelids – at 20cm long – and the one you're less likely to encounter. They are brown with a white underbelly, and they eat mainly mice, small birds, eggs, lizards and insects.
Their numbers can increase dramatically if plenty of food is available as female weasels reach sexual maturity at three to four months of age, and can produce up to 12 young a year. However, if food is in short supply their numbers can decrease just as quickly.
Stoats and weasels have similar colouring but stoats are larger, at 40cm long. They can kill larger prey than a weasel, which gives them a wider variety of food, so they survive in much higher numbers.
Apart from their size, you can also tell the difference between a stoat and a weasel by the stoat's longer tail, which usually has a black tip. Stoats have one or two litters a year, with four to eight young in each litter.
Ferrets farmed for their fur are called fitches. After the collapse of the fur industry in the mid-1980's large numbers of fitches were released and quickly established in the wild. Now New Zealand has the largest known population of feral ferrets in the world. Ferrets produce four to eight young each year. Ferrets are larger and stockier than the two smaller mustelids. They are a yellow-white colour with long black guard hairs and a characteristic black face mask. Albinos – which are pure white and have pink eyes – are common.
Ferrets feed mainly on small animals including rabbits, rodents, possums and birds, plus whatever opportunity offers, including eggs, lizards, frogs and insects. The decline of kiwi numbers in some areas has been linked to the appearance of ferrets. Ferrets can not only kill kiwi chicks, they also kill sub adults – juveniles – and even adult kiwi. It is also possible they may carry Bovine tuberculosis (Tb) and transmit it to cattle.
Ferrets are the largest of the three mustelids - weasel (left), stoat (middle) and ferret (right).
The English Fenn trap is a well known mustelid trap. It is a steel spring trap which kills small animals by breaking their backs. These traps can be ordered from the Northland Regional Council.
Fenn traps come in two sizes: the smaller Mark 4 size is best used for smaller weasels and stoats while the Mark 6 trap will handle the larger ferret, and is also suitable for smaller animals.
The best way to set a Fenn trap is to build a wooden tunnel just large enough to fit the trap, with just enough height for the trap to close. You can also build the tunnel to fit two traps – referred to as a "double set". The traps should be placed far enough back from the tunnel entrance to prevent non-target prey getting injured (150mm for kiwi).
It's best to place the traps on either side of the bait and make sure the tunnel will allow the stoats to be funnelled to the plate of the trap. The entrance of the tunnel should be made just large enough for a mustelid so other species – like birds – won't go in. Plastic tunnels or covers are also available for Fenn traps as an alternative to wooden sets.
If you have only one trap then make the tunnel blind and place the bait at the far end. The aim is for the animal to walk over the trap to reach the bait, regardless of whether you're using one or two traps.
DOC series traps
The Department of Conservation (DOC) series of traps – 150, 200 and 250 – are used nationally for predator control. The DOC150 and 200 are suitable for catching stoats and weasels, while the larger DOC 250 can kill larger ferrets as well as stoats and weasels. These traps are easy to use but you need to be fairly strong to set them. They are designed to fit into a wooden cover.
All DOC series traps are capable of killing rats, hedgehogs, stoats and weasels. You can also buy the DOC series traps from the Northland Regional Council. For more information on their use contact the regional council or visit www.doc.govt.nz
The best lure for mustelids is fresh or salted rabbit. Eggs and fish-flavoured cat food also work well. Traps should be set where mustelids like to go, e.g. stream margins, the edges of habitat, near roads and pathways. A piece of bait or fish can be dragged around on a piece of string to make a scent trail leading to the trap.
You can often come across mustelids – particularly ferrets – while spotlighting rabbits and possums. They can certainly be shot, although shooting alone is not an effective control measure. Ferrets have green eyes, similar to a cat in a spotlight, with the exception of albinos, which have pink eyes.
Pestoff Ferret paste is the only current registered toxin used for the control of ferrets. It contains an anti-coagulant poison mixed with a fish paste. Occasionally mustelids may die from secondary poisoning after eating the carcasses of rabbits and possums killed by poison.
New toxins are continually being developed and trialled. For up-to-date information contact the Northland Regional Council.
What to look for
- Mustelid scats – droppings – are long and thin, often with a characteristic tapering twist at each end. They are filled with fur, feathers and bone fragments and are hard and black when dry.
- Mustelids secrete onto their scats a thick, oily yellow powerful smelling fluid called musk. Scats are often found out in the open – such as in the middle of a track – as a sign to other mustelids in the area.
- Mustelids usually eat the flesh from the neck and head area of their prey.
The best bait… Salted rabbit
- Shot rabbits can be frozen whole until you have enough to salt (a dozen rabbits make a bucketful).
- Thaw rabbits (if you leave them partially frozen there is less splatter).
- Use a meat cleaver and chopping block to chop off the head and feet.
- Skin the rabbit by simply pulling the skin off.
- Gut the rabbit. Split the carcass in half with the cleaver then chop it into bait-size pieces – about 25 per reasonable size rabbit.
- Layer the pieces in a 20 litre bucket with a handful or two of non-iodised agricultural salt per rabbit.
- Mix, cover and leave in a cool place for 24 hours.
- Mix again then drain off the liquid.
- Bag the pieces in plastic bags and freeze. The pieces of bait should freeze free-flow and be ready to use as you need them. Excess bait left after a day's trapping can be re-frozen.
- Put the bait on a peg above the ground in your traps to help it last and to aid scent dispersal. The bait should last around three weeks depending on the weather and the shade on your trap site.
- Dispose of all used bait carefully – either bury or remove from the area. You must not leave used bait in the field where stoats may find it, taste it and be put off going into traps.
Find the brochure: Download the mustelid publication
For more mustelid control information contact Northland Regional Council Biosecurity staff on: 0800 002 004
Or refer to the Regional Pest Management Strategies at:
www.npca.org.nz or www.ermanz.govt.nz