Pickling pork: control of feral pigs with sodium nitrate

Killing animals is certainly not what comes to mind first when you think about nature conservation. Sometimes, however, killing a certain species is necessary to conserve another. One of the species which is killed for the sake of conservation is the feral pig, Sus scrofa. Feral pigs are omnivorous which means they basically eat what they can find: not only roots, fruit and grasses but also animals like worms, reptiles, amphibians, birds and small mammals.

Wild pigs at feeding trough (with mortality collars). Image from Lee Shapiro.

In the case of New Zealand, many of the ground-breeding and -dwelling native birds, their eggs as well as their offspring, may end up on the feral pigs’ menu. An example for this is the endangered Hutton’s shearwater: the only seabird in the world which breeds in a subalpine environment. Eight breeding colonies of this threatened bird have already been eradicated by feral pigs. Pigs also prey on New Zealand’s giant land snail species, many of which are seriously threatened. The feral pigs’ fondness for digging also has serious effects on native ecosystems like wetlands and tussock habitats.  Additionally they are a cause of grief for farmers as they raid pastures and crops and kill livestock.

More harmful to the environment than they look - feral pigs.
Young feral pigs – more harmful to the environment tha they look. (Kent Kanouse / creative commons / flickr.com)

When native ecosystems are threatened by invasive species, like the feral pig, conservationists can’t just sit back and let it happen. Something has to be done. This means getting your hands a bit dirty. Bloody even. The options that first come to mind are hunting or trapping the troublemakers. Sometimes this can be difficult as animals learn to avoid traps and hunters. In those cases a more insidious method is needed. A method which is usually linked to the cowards and the traitors in tales since the days of old. Poison.

Poison is widely used to kill invasive species. The most common poison for pest control in New Zealand is sodium fluoroacetate which is far better known as 1080. The problem is that 1080 and other commonly used poisonous substances are deemed inhumane for feral pig control as it takes a long time for them to die. Of course the question can be raised: when is the killing of an animal ever humane? But if lethal control is necessary the top priority should be to use a method that causes as little suffering as possible.

Pickling - the ususal use for sodium nitrate (NaNO₂).
Pickling – the usual use for sodium nitrate NaNO₂ (Dan Ox / creative commons / flickr.com)

This is the reason why scientists are constantly on the lookout for new substances that do the job faster and with less suffering for the animal. In 1985 Australian researchers discovered something interesting. It turns out that pickling animal meat to make it more durable is not the only thing you can do with sodium nitrate (NaNO₂). In the right dose you can also achieve what has to be done before one can think of pickling. In a high enough dose sodium nitrate causes methaemoglobinaemia. This is not only a really difficult word in a crossword puzzle but it also leads to rapid death by a lack of oxygen in the heart, brain and other vital organs.

A team of researchers including Lee Shapiro and Charles Eason of Lincoln University have recently published a study in which they tested the effectiveness of sodium nitrate as a poison for feral pigs. The main problem with sodium nitrate is its taste. It’s incredibly bitter. So what do you do to trick the pigs into eating the poison? You just put the poison into something nice and tasty. And that’s exactly what the researchers did.


Snow White has been tempted (LittleSeaSparrow / creative commons / deviantart.com)

Think of the witch in the fairy tale of Snow White. Would Snow White have just drunk a random potion that some hag offered her on the doorstep? Most likely she would have been a tad more suspicious. So what did the cunning witch do? She put the poison into an apple. A shiny, red and tasty apple. And so Snow White was fooled.

In the case of the feral pigs the apple comes in the shape of a delicious paste ball made of peanut butter, corn and wheat meal, margarine and sugar. Inside this paste ball the is hidden the deadly secret:  25 g of encapsulated sodium nitrate to ‘put the pigs to sleep’. Only for the feral pigs there will be no prince to wake them up with a kiss. The researchers wanted the pigs’ sleep to come quick and last for eternity.

The pigs liked what was offered to them. They were tricked and their fate sealed. Now comes the part where doing science can get a bit nasty. It’s not a nice thing to see an animal die. Many of us have seen a pet die or maybe witnessed the slaughtering of livestock. It gets even worse when you are responsible for the death. I can imagine it was not the easiest task to watch the pigs eat the poisoned bait and then record their symptoms until death. The average time it took for the pigs to finally fall into eternal sleep was about one hour.

An hour sounds quite a long time but it is a big improvement to other poisonous substances.  The paste bait is now produced and sold by the New Zealand company Connovation Ltd. Further research was done to test the use of the substance for the control of another important pest in New Zealand: the brushtail possum. The results seem to be just as promising as for the feral pigs. The outcome of the research by Shapiro, Eason and their collegues led to the official registration of sodium nitrate as a pest control tool for feral pigs and brushtail possums in New Zealand.

So unlike a traditional fairy tale this story has no happy ending for the protagonists, the feral pigs. But maybe it is a step towards one for lots of endangered native species like the Hutton’s shearwater or New Zealand’s giant land snails.

The author Andreas Wiedenmann is a postgraduate student in the Master of International Nature Conservation taught jointly at Lincoln University and University of Göttingen. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.

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