Introduction

Ninety-six rats have now been caught on Ulva Island since a rat population was detected just after Christmas last year. An aerial spread of bait containing rat poison (brodifacoum) has the highest chance of succeeding.

Ninety-six rats have now been caught on Ulva Island since a rat population was detected just after Christmas last year.

Ulva Island (263ha) is located in Paterson Inlet, Stewart Island and is a key eco-tourism destination due to its pristine forest and abundant wildlife. Rats were initially removed from Ulva Island by 1995 and the island is now home to many endangered birds, including yellowhead/mohua and saddleback/tīeke. Ulva Island is one of a few ‘open sanctuary’ islands where the public are able to visit without a permit.

After considering the advice of experts and the feedback from the local community, the Department has made a decision on which method it will pursue in its attempt to eradicate rats off Ulva Island. It has become apparent in the last few weeks that the only method that gives the best chance of achieving eradication is an aerial spread of bait containing rat poison (brodifacoum). The decision was made as an aerial spread has the highest chance of succeeding and will be the quickest to achieve the result in a situation where time is of the essence.
 
“It is the best tool for the job and we would be complicit if we attempted anything else,” said DOC Area Manager Andy Roberts. “This was reinforced in recent days when we read about the Fregate Island (in the Seychelles Islands) rat eradication failure being blamed on the use of bait stations.”

The Department will be lodging a resource consent application with Environment Southland in the next few weeks. Environment Southland will notify this consent allowing anyone to make a submission. 

“The resource consent process is sound and is an independent evaluation of our proposed methods and controls. We encourage people to make a submission (in support or expressing your concerns), so that Environment Southland can fully evaluate this application.” said Mr Roberts.

Background information Q & A

Why are there so many rats here this time?
How are you going to get rid of the rats?
Can you keep the rats off in the future?
What impact will the rats have on the birds?
Shouldn’t you transfer some birds off to protect them?
How did the rats get there?
What can we do to help?
Could the cost of ground-based operations be reduced via volunteers?
What is the effect in the coastal marine area? Will you poison people who fish in the area?
Why an aerial drop? Surely, the ground-based approach is the safer option.
Has this been done before?
Will this work? What are the chances of success?
When would an aerial drop occur and how long would it take?
Will the island be closed? When? How long until Ulva could be used as normal again?

Why are there so many rats here this time?

At some point, at least one female rat evaded our trap and bait station network and started to breed. Rats are able to produce large numbers of offspring and these can reach sexual maturity within a few months. With large amounts of food on Ulva Island, maximum breeding output is expected. Unchecked, they can go from one individual to over 30 thousand within 18 months. We are already 9 months down that track.

How are you going to get rid of the rats?

The Department has considered advice from the world experts in eradicating rats. We have also taken on board the concerns of the community. We have decided that an aerial spread of rat bait is the best option to achieve success. It has the highest chance of success and it will get the job done in the quickest timeframe.

The method has been extensively used so we have a lot of information about its effect. It is the method that has resulted in most of New Zealand key wildlife sanctuaries (e.g. islands such as Codfish / Whenua Hou, Kapiti, Little Barrier Island, Campbell Island, Taukihepa, and mainland fenced sanctuaries such as Karori, Maungatautiri, and Orokonui).

Can you keep the rats off in the future?

The biosecurity that we had in place managed to stop rats establishing on Ulva for 15 years. But, this time it failed. We will review our biosecurity procedures and develop a new plan that will further reduce the chances of this happening again.

A community meeting is being planned to get everyone’s ideas and thoughts about improvements that could be made – biosecurity needs to be everyone’s responsibility otherwise it will fail. Ulva will continue to be an ‘open’ island as it is important that people get to see these special places. 

What impact will the rats have on the birds?

Ulva Island is home to some of our most endangered birds, such as the mohua and saddleback. These species are particularly vulnerable to rat predation. Rats may be predating individual birds but with the abundance of other food available on the island and the birds coming to the end of their breeding season, then this impact is likely to be minimised.

We have a small window of opportunity to eradicate the rats this winter before the birds start breeding again. If rats are not removed from Ulva, they will wipe out populations of these birds and severely limit other species like robin and riflemen.

Shouldn’t you transfer some birds off to protect them?

The species on Ulva Island are all valuable. We believe that we have a window of opportunity to get these rats before they cause a substantial decline in any of the bird populations.

Transfers take a lot of planning and are labour intensive. We would rather be putting this effort into planning the eradication of rats. If we fail with the eradication, then there may well be a need to transfer some of the species off.

The impact of the baiting operation on most of the birds will be negligible. Some species such as saddleback and robins may lose some individuals, but this will be more than made up for by one years worth of breeding after the rats are removed.

The only species that will suffer substantial decline is weka. But, weka have proven capable of rapidly recovering in numbers after a baiting operation and hence we don’t believe a transfer is warranted.

How did the rats get there?

Rats get to Ulva Island either by hitching a ride on people’s boats or swimming. Ulva is also within the known swimming range of Norway rats, being 730m from the nearest mainland point (with one rock between the island and the mainland), but the frequency of this occurring is very low; otherwise they would arrive more often than they do.

Rats have been observed jumping off a boat and swimming to Ulva in the past, so we do know that boats play a contribution to rats getting to Ulva. Regardless, we need to deal with the current situation and work with the community to find ways of making biosecurity tighter.

What can we do to help?

At the moment, we require funding to pay for the eradication. Donations can be made to the Ulva Island Trust. Check the Ulva Island Trust website for details on how to make a donation.

Could the cost of ground-based operations be reduced via volunteers?

Volunteers are a valued part of DOC's operation. Many of these volunteers are very skilled and it would certainly reduce the cost of a ground-based operation. But, it will add to the complexity and thus increase the risk of failure.

As very few people could commit to volunteer full time for at least a year, we would be increasing the number of people involved in bait station checks – in all likelihood by a large amount. This will put pressure on someone to organise and co-ordinate that volunteer effort.

Both of these factors increase the complexity of the operation and the subsequent risk of something going wrong. Regardless, the decision to use aerial was not based on price, but on what is the best tool for the job. Aerial has more chance of success and it will achieve a rat-free state quicker than a ground-based operation.

What is the effect in the coastal marine area? Will you poison people who fish in the area?

In reality, the risk of this is very low. Some bait will indeed enter the marine environment. This will be in the order two pellets per metre of coast, or less than half a kilogram of bait in a 100m stretch. The well-monitored Kaikoura bait spill dropped 18 tonne in a 100m stretch and effects were limited to that 100m stretch.

While the marine reserve is obviously not the local food basket, we will be discussing concerns about bait entering the marine environment with the Mataitai committee.

One option that has been raised has been for a Rahui to be placed around Ulva Island to completely eliminate any possibility of eating fish that may have had contact with bait. Exploring potential solutions such as these may pave a way forward. We are also seeking advice from national marine and fisheries experts.

Why an aerial drop? Surely, the ground-based approach is the safer option.

We have now completed many island eradications using aerial spread of bait that has resulted in some of our most unique and valuable pest free sites, including Codfish Island / Whenua Hou and Campbell Island. Ulva Island is at threat from rats. We only have a limited range of tools to deal with this. We need to use the best tool for the job that will save Ulva from rat-induced devastation.

The Department’s preference is for an aerial operation as this has a greater chance of success than a bait station operation and can be completed much more quickly – effectively restoring Ulva Island to a rat-free state before too much damage is done by the rats on the wildlife or the businesses that rely on this wildlife.

Unfortunately, a ground-based eradication is far from a “safe option”, with bait available in the environment for a whole year and a much higher chance of failure. It is also a much more expensive option. With an aerial operation, bait will be taken to where the rats are so that they don’t have to move any distance to find some bait to eat. A bait station operation would require rats to travel some distance, through areas of high natural food abundance, to obtain a meal of rat bait – increasing the risk that they simply won’t encounter bait.

Has this been done before?

While there have been many eradications worldwide, almost all of these have been with established rat populations that have reached peak numbers and exhausted the food supply.

The situation on Ulva is very different where rats are newly invaded and have unlimited food supplies. This situation has only been encountered once before, on Fregate Island in the Seychelles where rats invaded in 1995 and they attempted to eradicate those using bait stations. This failed and the recommendations from this attempt were that the rats weren’t using the bait stations and if the situation was encountered again then bait should be presented to the rats in a way that didn’t use bait stations (e.g. via an aerial application). Rats were eventually eradicated from the island 5 years later by an aerial baiting operation after the rat population had peaked.

Will this work? What are the chances of success?

This is new ground and, even though we are using the best tool in the box, we may still fail to eradicate these rats. The rats have unlimited food available and may not have any interest in eating the bait.

Even though there is a very real risk of failure, there is also a good chance of success and this, coupled with the current value of Ulva Island, make us think that an attempt is worth it. The alternative is to wait until rats have reached peak numbers and exhausted the food supply.

When would an aerial drop occur and how long would it take?

An aerial drop would occur sometime between June and September. The exact timing depends on obtaining a resource consent and then having a four day fine weather forecast. Four days of fine weather will keep the bait in good condition for enough time for rats to find and eat it. The best practice for eradications is for two bait drops to occur about a week apart. The bait drops will only take a few hours on each of the two days.

Will the island be closed? When? How long until Ulva could be used as normal again?

It is likely that we would close Ulva Island on each of the days of the two bait drops. Bait will be cleared from tracks and beaches early the following morning and the island will again be able to be visited.

Signs warning that bait may be present will be present and visible until we are sure there is no further risk of bait being present. As long as people don’t let children go unsupervised and avoid eating or handling any baits they encounter, then the island could be used as normal from the day immediately following each bait drop.

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