The Restoration of Ahuahu-Great Mercury Island

KATE WATERHOUSE with PETER CORSON

Last November, five years on from the removal of the last rat from the last island in the Mercury group, Peter Corson shared his experiences in an interview with Kate Waterhouse.

Pete Corson is a modest man. He’s not saying it in so many words, but he’s enormously proud of the results they’re seeing on Great Mercury five years down the track. Three species of seabirds, kākāriki out of nowhere, kākā, kererū, pāteke, regeneration of kohekohe and puriri through a pine forest that will never be cut. The results of the five-year post-eradication monitoring are due in 2020 but it’s looking very good indeed.

A Passion for the Mercury Islands

Pete has been a member of the Department of Conservation’s Island Eradication Advisory Group and has worked for the department in Taranaki, Tauranga/Waikato, Rotorua, then nationally in Species and Ecosystem Optimisation – finding the best mix of spending, resource use and representation across the country. It was in 1994 that his passion developed for the Mercury Islands, working on projects to unearth the biodiversity treasures they held, like the tusked wētā (Motuweta isolata) and Whitaker’s skink (Oligosoma whitakeri).

Pete knew the islands were really important and met one of Great Mercury’s owners, Michael Fay, doing earlier work, but it wasn’t until 2009 that the process began. A feasibility study said eradication was possible - only three mammalian pests (ship rats, kiore, and feral cats) and no hedgehogs, stoats, rabbits, or wild pigs. In 2010, ecological surveys were completed and in 2012 Pete started full-time as Project Manager.

Together with the owners, iwi and community members, a team led by the Department of Conservation successfully eradicated ship rats, kiore and feral cats from Ahuahu Great Mercury during the winter and spring of 2014. The department declared it successful in May 2016, completing the last piece of the puzzle – the whole Mercury Group was pest-free once more. But, Pete says, it was the right conversations that led to the right outcome.

The Mercury Islands - A Conservation Treasure

The Mercury Islands consist of seven islands, Great Mercury Island Ahuahu, Red Mercury Island Whakau, and five smaller islands - Korapuki Green Island, Atiu Middle Island, Kawhitu Stanley Island and Moturehu Double Island. They are conservation treasures, partly because two, Green and Middle islands, have never had mammalian pests or predators. Small, at just 3 Ha and 13 Ha respectively, they have provided a window into the pre-human ecology of New Zealand. 

Threatened Species

Many species found on the Mercury Islands are threatened or ‘conservation dependant’ including Whitaker’s skink, robust skink, Pacific gecko and Duvaucel’s gecko, and tīeke (saddleback). A high diversity of invertebrates is present– a common feature of rat-free islands. Fifty species of land snail are found in the group. Middle island has 10 species of lizards plus tuatara. 

Moturehu Double Island and Whakau Red Mercury are home to the critically endangered Mercury Islands tusked wētā. Discovered in 1970 by Tony Whitaker, and only known on one island, in 1989 the population was down to just two males and one male. Captive breeding and subsequent translocations mean that there are wētā on five islands, including four of the Mercury group. 

The smaller islands in the group are identified as an Important Bird Area (IBA) by BirdLife International, as they provide nesting sites for up to 10,000 breeding pairs of Pycroft's petrel (Pterodroma pycrofti). Petrel burrows bring nutrients deep into the island’s soils and provide a labyrinth of habitat for other creatures. 

Pioneering Pest Eradications

Pest eradication work in 1986/1987 saw kiore (Rattus exulans) removed from 18 Ha Korapuki Green Island. At the time, it was the largest New Zealand island to have rats removed. By the early 1990s, three other islands also had pests removed - Double Island at 31 ha in 1989, Stanley Island (99 Ha) in 1991 and Red Mercury Island (225 Ha) in 1992. These six pest-free islands have since been managed by the Department of Conservation on behalf of the Crown as Nature Reserves - the most highly protected reserves under New Zealand legislation, with access by permit only. 

About Ahuahu Great Mercury

The island is located in the Mercury group off the eastern Coromandel coast, within the Hauraki Gulf Marine Park (Figure 1). Only Great Mercury is inhabited today— owned by businessmen Michael Fay and David Richwhite and open to the public.

A Volcanic Past

Great Mercury Island is what remains of an andesite volcano in the north and a Pliocene rhyolitic volcano in the south, connected by an isthmus of sand, called a tombolo. As recently as 8,000 years ago the islands in the archipelago were all still connected. By 6,000 years ago, the islands had approximately their current coastline.

Human occupation

Ngāti Hei are the mana whenua for Ahuahu with a 100-year unbroken relationship with the three subsequent owners of the island. Other iwi have interest in the Mercury Islands group due to the islands’ historic significance for Māori, including Ngāti Whanaunga, Ngati Porou ki Hauraki, Ngāti Maru, Ngāti Tama Te Ra, and the Hauraki Collective. Oral history tells that Ahuahu was the first landing point for Polynesian ocean-going waka in Aotearoa. The island has the highest density of intact archaeological sites in New Zealand.

Habitat Restoration

Great Mercury is 1,872 Ha – half the size of Te Paparahi, or about six times the size of Rakitu. Corson estimates 600 Ha is in farmland, 675 Ha in pine forest, and 600 Ha is native ecosystems in various states of recovery. The owners had been undertaking restoration initiatives including goat eradication in 1981 and the planting of more than 600,000 native trees.

Many different habitats with surprisingly good ecological values were found during a survey undertaken in 2010, despite a history of burning. A pine forest was planted after a fire in 1981 that burned 675 Ha of bush. Even the pine forest had good levels of native vegetation in the understorey and the island was found to be large enough for self-sustaining populations of many species. But there were almost no birds. After 700 years of kiore and at least 100 years of ship rats, they simply couldn’t withstand the plague proportions that by now prevailed on Great Mercury. Pete takes up the story.

Can you describe why ship rats, kiore and feral cats were eradicated from Great Mercury?

Two reasons, plagues of rats and human reasons. We knew we were losing things, and that we would gain things in the absence of cats and rats. Like the kākā that were trying to nest, we were finding raided kākā nests or skeletons of seabirds eaten by cats and rats.

But the human reasons were things like not having rats coming into bedrooms at night. Michael Fay said it was war when the rats chewed the collars of his dinner jackets, after they had chewed the generator cables, after the farm manager’s wife was almost electrocuted because a rat chewed the electrical cable behind the shower - which also started a fire. Rats were coming out to people’s boats at night. Some diseases are carried by rats – Leptospirosis, Yersinia is like the plague; and food and gardens were affected. There was the cost of people having to deal with rat invasions in the 45% unoccupied houses – there was huge potential for damage.

With six high-value conservation islands just to the south, rat-infested Ahuahu Great Mercury presented a huge risk and a certain beneficiary, from source populations of endangered species, right next door. It was also an island people could land - and they did.

While just 18 people live permanently on the island, there are many frequent visitors. Ahuahu Great Mercury is a focal point for the boating community, with thousands visiting each year, including hundreds every summer weekend from Coromandel towns as well as large cruising vessels from elsewhere. Many spend time on the island - some regular, some not, and some semi-permanent. At least 400 people are to be found in the main anchorages at New Year.

Making it happen: the human factor

The owners approached the Department of Conservation. They had had enough of the rats. After 700 years it was time to give the wildlife a chance to fight back. Pete continues:

We started in 2010, talking about the values and threats and what was needed. There were no expectations but we tried to understand what we had, what was there that was of value. Collectively, what did we think - DOC technical people and owners. But we hadn’t had to manage people before, more the species and vegetation.

He highlights how important it is that iwi feel they have had a say and can protect their interests. We always recognised these interests and gave them information. We always said: “Come and talk to us - we can have a joint conversation about this”. There was also a requirement under the Hauraki Gulf Marine Park Act to work collectively.

Engaging Stakeholders

The project had a database of more than 100 stakeholders, including the Department of Conservation, owners, residents, the Whitianga/Mercury Bay community and businesses. Whitianga’s school connected often to the island and projects looked at “wicked” (complex) problems - involving iwi, the department and Ministry for the Environment, on issues such as ecology, heritage, and coastal erosion. Champions were identified - cray fishers, long liners, builders, the Commodore of the boating club and more.

The team had conversations with those who spent a lot of time at on the island. Before the feasibility study, they visited people to gauge thoughts and opinions, always asking who, how and why should we engage?

What were people’s requirements and expectations? Examples of issues discussed included:

• If you catch kiore can they go to the museum so we don’t lose that genetic information?

• Can you tell us about the impact on ruru? and

• I want to close off the water tanks in my houseboat during the operation.

There was discussion about the rodent toxin being used and information about broadifacoum was provided. Pete recalls:

Someone said that the shellfish at Kaikoura had brodifacoum in them - I said well yes they had 16 tonnes go into the sea in one spot (from a truck crash) - we are going to have 8 kg per hectare and 1 kg per hectare in the marine environment, most of which will float to shore and break up rapidly. We showed them some videos of process and the GPS flight lines showing the boundaries – and we put a Go-pro on a bucket to show what happens and gave them still images.

During the cull of pūkeko ahead of the operation (a risk as they take baits) - Hauraki Collective took feathers for cloak making. There were karakia for the entire operation well before the drops – protocol. Assets of fishers and fishing areas were allowed for, and cray fishers pulled back from the shore prior to the operation.

Support from other agencies was also there, including the Harbour Master and Coast Guard who played information every weather forecast. Key messages were consistent: Great Mercury and other islands are pest free: Please check your boats and gear for pests.

Making it Happen: The Project Details

By end of 2012, outcomes were settled and a feasibility study completed. Fay and Richwhite stumped up $750,000, matching the Department of Conservation’s contribution. The objectives of the project, which began in 2014, were to reduce the threat of incursions to other pest-free islands in the group from Ahuahu Great Mercury, assist ecosystems to function, revive species populations and showcase conservation.

Pete again:

We looked at a small governance group to work through a project plan to achieve the objectives. This comprised DOC, an owner, and an eradication expert. We kept it very small to make sure it made decisions. We said we want this to be open access and a showcase for conservation. We had dedicated time and resourcing to develop and plan the (communication) work. Of the project management hours about 3,000 (60%) were about dealing with people or issues people had. So after a while we brought in an extra role 12 months ahead of the eradication to do the technical planning work.

Rodent baiting was done using helicopters due to the rugged terrain and size of the island, with hand baiting around sensitive and difficult areas. Baiting was followed up with cat control. A conservation dog and handler took five months to find the last three cats.

On 13 May 2016, the Department of Conservation announced that Ahuahu Great Mercury was free of mammalian pests.

Pete believes having a consistent face to deal with over the eight to nine years of the project was really important to people. Someone to have a conversation with. Conservation, he says, is all about having cups of tea.

After Five Years, What Have the Results Been?

Pete is excited:

We’ve found 60 pāteke when we didn’t see any between 2010 and 2016. (Australasian) bittern are turning up, and the New Zealand dotterel population hit 58. The 675 Ha of pine forest now will not be logged ever, it’s a nursery forest. This has never been done before on this scale. There’s great understory of pūriri and kohekohe. We are repeating the forest bird survey soon … kākā are going ballistic. There were one or two records of kākāriki before the eradication - now they are heard multiple times every day. Kererū are more apparent now, so we now have seed dispensers, seed rain and carpets of seedlings.

In the Mercury group, natural seabird recolonisation potential was high(1). Six species of seabird are believed to have recolonised naturally (Figure 2), including grey-faced petrel (Pterodroma macoptera), fluttering shearwater (Puffinus gavia) and diving petrel (Pelecanoides urinatrix), which usually reappear quickly after eradication of rats, and three more exciting arrivals – flesh-footed shearwaters, Pycroft’s petrel and little shearwater. Suppression of seabirds and other species by rats has had an as yet unquantifiable knock on effect on soil health, lizards, insects, snails and vegetation.

What Else Have You Seen Change?

Socially, the boat ramp surveys show 95% of people are aware that the islands are pest free, up from 50%. Before the eradication, 20 or more islands were pest-free but people hadn’t known, so there are benefits are for all the islands not just Great Mercury... there’s a halo of boats going from Great Mercury to Aotea and on to the Poor Knights. We’ve had only one incursion, a ship rat which was immediately detected in the surveillance network.

Pete explains that the islands are being managed as an archipelago—ecologically, for biosecurity, weeds, fire risk, Argentine ants, restoration planning and translocations – across ownership, working in partnership. Incursion response is critical and volunteer response is a benefit of high levels of community ownership.

What Advice Would You Give on How to Protect and Restore Aotea?

Start with the conversations - before any feasibility study, visit people to gauge thoughts and opinions. Start with why we are starting to think about this, and the ‘why’ comes from values. Acknowledge the values of the place (Aotea). It’s 27,000 ha, has potential for conservation recovery, is a large, globally significant biodiversity site. You’ve got Chevron skink, endemic kanuka forest, kauri forest, it’s a merging point for northern and southern species. There are ecosystems and species to warrant scientific and government interest and investment.

Pete suggests that on top of these conservation values come iwi and social values. Connecting with nature, community efforts and marine ecosystems will yield additional values. Start with no expectations, understand what you have, what is of value? And stop any new pests arriving.

If you decide, “Let’s do something”, ask, what should we do? Who, and how and why should we involve different groups? Be clear about that. What would the objectives be? For example, celebrate the possum and stoat-free status, let’s not allow any more pests here, let’s do some island eradications. Rule 1 here is: Tools that are acceptable to the community.

And you need a clear story. With Rakitu, DOC didn’t create a clear enough story, so what is it? On the Orkneys and Lord Howe Island ecologists fronted (the story) but were seen as pushing their own barrow, be aware of that. You need to be really open in the early phases, setting values in the group that are common, the things (you) agree on. For example: Don’t let more pests get to the island. What is the 100 year view? With iwi, DOC, landowners, what might the outcomes be? You need a collective understanding of the vision for the place. How will we observe that? What will we see?

Inspired now by the Ahuahu Great Mercury story, I wonder about this. Do people want to see functioning ecosystems on Aotea, without rats and cats, and with healthy abundant native species? Just as they are starting to see on Ahuahu Great Mercury now, five years on. Cuppa anyone?

Notes and Sources:

1. Towns, D.R., Borrelle, S.B., Thoresen, J., Buxton, R.T., & Annette Evans, A. 2016. Mercury Islands and their role in understanding seabird island restoration. NZ Journal of Ecology, January 2016.

2. Presentation to Gulf Islands Trusts Forum, Peter Corson, October 2019.

3. New Zealand Geographic Issue 137: Treasure Island https://www.nzgeo.com/stories/treasure-island/