Frequently Asked Questions

The duration of the baiting is weather dependent, and will vary in relation to the number of helicopters. The plan is to treat the entire island once, wait 10-14 days, and then do it again, to maximise the chances of reaching every mouse. Each bait treatment will take about 6-8 days of good weather, assuming three helicopters can operate at once, so with luck the total operation should be completed in 1-2 months.

Cereal bait pellets laced with brodifacoum poison will be dropped all over the island from bait buckets slung beneath helicopters. The bucket is linked to a GPS log, which records exactly which parts of the island have been treated, so that the entire island can be covered.

The poison is a second-generation anticoagulent, similar to commercial rat poison, but more slow acting, so the mice don’t learn to associate the bait with subsequent deaths. It makes them feel unwell, but they are unlikely to experience any pain or distress; they retreat to their burrows (reducing the risk of being eaten by scavenging birds), go to sleep and don’t wake up.

This depends on the temperature and the amount of rain, but it can last for weeks or even months under certain conditions. Fortunately it does not dissolve in water, and Marion winters are typically very wet, so we expect all the poison to flush out of the system into the ocean (to undetectable concentrations) within a couple of months.

We don’t know exactly – presumably either in materials landed ashore to support sealing operations, or from an early ship-wreck. The first record of mice on Marion Island is from an early sealer’s log written in 1818.

In the last decade, more than 95% of attempts to eradicate mice from islands have been successful. We can’t be 100% certain, because Marion will be the largest island attempted to date, but experts who have been involved in successful operations on Macquarie Island and South Georgia are confident that it should succeed on Marion Island too. The operation is over-engineered to minimise the risk of failure.

Albatrosses and most petrels only breed in habitats lacking terrestrial mammal predators. Because they have never experienced anything like this before, they haven’t evolved appropriate responses to being attacked by fast-moving, nocturnal predators.

The operation will require about 300 tonnes of poison bait to be spread over the island; the bait alone will cost around R30 million, and then there is the cost of the helicopters, specialist pilots, ship costs to transport materials and personnel to the islands, etc.

Marion Island already has strict biosecurity measures in force, limiting the amounts and types of materials that can be taken ashore, all of which are inspected before loading on the supply ship at the home port and again ashore on arrival and unpacking. Only ships that are certified rodent free before leaving port can visit the island, and there is no dock where ships can tie up, so the risk of a rodent sneaking ashore are very small. Even stricter measures will be in force during and will remain in place after the operation.

It is thought that giant petrels are unlikely to scavenge mice but gulls, skuas and sheathbills will likely take any dead mice they can see. However, the risk of secondary poisoning is fairly low because most mice will retreat to their burrows after consuming a poisoned pellet, reducing the risk of being eaten by scavenging birds. We will put plans in place (such as keeping some birds in captivity until the environment is safe, and then releasing them) to minimise the risks to non-target species.

Most mice will die in their burrows, where they will decay within a few months. However, the plan currently includes a small team to scour the coastal areas for carcasses, to minimise risks to birds.

Yes; mice have been eradicated from more than 70 islands worldwide, but Marion Island will be the largest island attempted to date where mice are the only introduced mammal. South Georgia is much larger, but mice only occurred in two rat-free areas with a total area less than that of Marion. Rats are easier to eradicate because they occur at lower densities than mice and have larger home ranges, so require less bait to be delivered less precisely than when targeting mice alone. At Australia’s Macquarie Island, three species (rabbits, rats and mice) were successfully eradicated in 2014, so even though that island is smaller than Marion Island, it was a very complex operation; in this respect, Marion Island should be easier to achieve success than Macquarie Island.

No; Kelp Gulls and Brown Skuas are the two birds that most often prey on mice, but mice make up only a tiny proportion of their diet. And mice have greatly reduced the natural prey of both species (the gulls eat large invertebrates, and many skuas rely on burrowing petrels), so eradicating mice will actually result in more food for these birds, including for the sheathbills.

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