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Ferrets, stoats and weasels belong to a group of animals known as mustelids.
These are the largest of the three species and are about 48-56cm long, including the tail. Their colours vary but they are usually dark brown or blackish with a creamy under fur, but they can appear almost white. The tail is uniformly dark. The face is pale with a dark mask over the eyes. Adult males are generally larger than the females.
Stoats are the most common of the three mustelids and grow to 34-40cm long, including the tail. Stoats are very thin and about half the size of a rabbit. They have a chestnut-brown coat, which turns white in winter, a light-coloured belly, and a bushy, black-tipped tail.
Weasels are the smallest and least common of the three mustelids in New Zealand. They have a thin, muscular body and a small head. Their colouring is very like the stoat, but with a more red-brown coat, and a shorter tail. They grow to 20-25cm long and will attack prey that is much larger than themselves.
Ferrets are generally absent or in low numbers in areas of high rainfall, where there are few rabbits, or deep within forested areas. It was originally thought that ferrets were limited to open country like pasture, scrubland, coastal areas, and in the fringes of forests. However, recent research has found ferrets within some Northland forests, placing added pressure on already threatened kiwi populations.
Stoats will live in any habitat where they can find prey. In New Zealand they can be found at any altitude, in any kind of forest - exotic or native - in scrub, dunes, and pasture. They are even known to occur near human settlements. In open country they are less common than ferrets, but in the forest they are much more common.
Weasels are usually found where there are plenty of mice, in gardens and near buildings, rather than in open paddocks.
Mustelids can be devastating to native bird life and other fauna. They have very good hearing and a strong sense of smell.
Ferrets mainly hunt at night. Their main prey are rabbits and hares but they also feed on native birds, especially ground-nesting birds, and lizards, frogs and large native invertebrates (for example, weta). They are good climbers and can steal eggs and chicks from nests in trees. Ferrets are one of the few predators able to kill an adult kiwi. They will also kill little blue penguins, possums, lizards, eels, hedgehogs and other small mammals.
Stoats are extremely fierce and will kill more prey than they need for food if they have the opportunity. They will also attack prey much larger than themselves. It is estimated that 60% of North Island brown kiwi chicks born each year are killed by stoats. Stoats hunt during the day or at night and can cover large distances. The main prey of stoats are rodents, birds, rabbits, hares, possums and invertebrates (particularly weta). Lizards, freshwater crayfish, carrion, birds, eggs, hedgehogs and fish are also taken.
Weasels are not as common in New Zealand as other mustelids, but they also have an impact on native birds and lizards, especially skinks. They kill most of their prey underground, and are usually found where there are plenty of mice, in gardens and near buildings.
Ferrets are successful breeders, producing between four and eight kittens per litter and one or two litters each year. Within three months of being born, the young ferret is capable of moving out into its own territory. There is high mortality in the first year, and an average lifespan in the wild may be 4-5 years.
Stoats produce a single litter of up to 12 kits each year. Female stoats have the unusual ability to carry fertilised eggs inside their bodies from mating in summer until the following spring. Young stoats are adults at two months, and female kittens can be mated while still in the nest. They can travel large distances, even crossing water. Most stoats live less than one year but adult mortality is lower and they may reach 6-8 years of age.
Weasels produce 1-2 litters each year, each containing 4-6 young. The young of the first litter grow very fast and are weaned at four weeks, at which time their eyes open. They are able to hunt and kill at 5-8 weeks.
The easiest way to determine whether or not mustelids are present in an area include:
The 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 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 being injured (150mm for kiwi).
It is 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 – will not 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 are 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 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.
The best bait… Salted rabbit