Recent Stoat Incursions in the Hauraki Gulf

ANDREW VEALE (Manaaki Whenua/Landcare Research, Auckland)

When you arrive on Aotea, the first difference that many visitors note are the kākā.  Their abundant shrieks and whistles are one of the many reasons the island is so special.  But why are kākā so abundant on Aotea but rare or absent in apparently identical forests in Northland and the Coromandel?  The answer is that Aotea does not have New Zealand’s most devious predator – the stoat.  There have been multiple studies showing that in mainland forests with no stoat control, kākā fledging are almost non-existent, and many females die trying to defend the nest.  This results in an extreme male bias in these populations whereby old kākā males live out their long life as bachelors.  Kākā chicks and their adult sex ratio are actually the greatest indicator for long-term stoat abundance, because most other forest birds are also affected by rats or cats, but for kākā, stoats are the only animal capable of climbing up to their nests high in trees, while also being tenacious enough to take on a large angry mother kākā.  

Introduction

There are occasional reports of stoat incursions on Aotea, which DOC and Auckland Council take very seriously, but no invaders have ever been confirmed.  Most of these incidents occur over summer, and are fleeting glimpses by visitors to the island.  It is my opinion that probably most or all of the reported incidents on Aotea were mistaken identities.  Banded rails have a long thin body, could have similar colouring to a stoat in the right light, and their darting through the undergrowth could easily look like a stoat.  Also, banded rails are particularly common on Aotea, and uncommon on the mainland (again probably because of stoats), so visitors to the island would not be familiar with them.  Nevertheless, if you see a stoat on Aotea report it immediately!

Stoats have been recorded on almost all islands in Aotearoa within a distance of 5 km from the mainland.  They have not, however, been confirmed on islands further offshore, even those with regular boat traffic such as Aotea and Rakiura.  This indicates that stoats get to islands by swimming, not by hitching a ride on boats.  Lab-based tests that I helped perform in collaboration with Professor Kim King showed that stoats can swim several kilometres with relative ease, and there are numerous observations of them swimming long distances in the wild. It is not impossible that they might hitch a ride to Aotea, particularly if there was a load of hay that was ferried across, but hopefully biosecurity would properly check such loads.  

Most other islands in the Hauraki Gulf are not so lucky as they are closer to shore. Over the last two years there have been three pest-free nature sanctuaries in the Hauraki Gulf invaded by stoats. These have resulted in long protracted incursion responses, some of which remain ongoing. The thin silver lining for all of this is that we are improving our incursion responses, adding new tools to the toolbox to catch these cunning animals.

Invasion 1: Motukorea 

The first incursion was on Mokukorea (Brown’s Island) in April 2020.  A biosecurity team from DOC and Auckland Council with dog handlers and three stoat dogs visited Motukorea and detected the presence of a stoat or stoats.  They found stoat scat, cached predated pigeons, and stoat footprints on the beach.  The scat and dead pigeons were sent to Ecogene® for DNA testing, and stoat DNA was confirmed – proving that the dogs knew what they were talking about.  Over the next few months multiple trips were undertaken by the incursion response team: they put out traps and cameras and used detector dogs to find scat.  The stoat was living in the cliffs on the north of the island feasting on pigeons, and it had little interest in going into the traps.  Auckland Council put out a meat bait with a novel ‘humane’ toxin (PAPP) in it that has been specifically developed for stoats; some was taken.  By mid-May there was no further sign of the stoat on the island, with the inference that it had either died, or swum away…

Invasion 2: Motutapu

Shortly after the stoat sign ceased on Motukorea, stoat footprints were recorded on Motutapu.  Had the stoat swum north?  Motukorea is 2.6 km from Rangitoto/Motutapu and is the closest point from which it could have swum.  DOC immediately put out traps in the vicinity of the footprints and started their incursion response.  Multiple cameras were put out, capturing images of the stoat, and the dog handlers even saw it, but were unable to catch it. Months went by without a catch. An adult male stoat, in good condition, was finally caught in September, near the Rangitoto Wharf.

At last the island-hopping stoat had been caught – or had it?  There was some suspicion that perhaps this wasn’t the only stoat on the island.  Stoat sign had recently been recorded near the original site 8 kms away.  Why would the wily stoat that avoided traps for months, run across the island in a few days and go into a standard trap baited with boring old bait? These suspicions turned out to be well founded; a month or so later predation events on various critically endangered birds were observed. Again, DNA confirmed the presence of stoats for these predation events.  More trapping and work with detector dogs followed, and eventually another stoat was caught near the spot where the original footprints were recorded in January 2021.  This was another male.  Unfortunately, more stoat sign continued to turn up over the subsequent months, and more predation events occurred.  There was a stoat at large that was too clever to go into normal traps.  Genetics from scat indicated that this stoat was also a male. Finally, a third stoat was caught in November 2021 near where dogs had found sign. The DOC trapper had created an effective natural-looking trap.  The surrounds were made from driftwood, and it was baited with an old infertile penguin egg, some meat, stoat bedding, and a speaker playing baby stoat sounds. Stoat females are fertile from birth, and generally they are mated over September to November before they leave the nest.  I had access to a litter of stoats that were research animals and I recorded their very distinctive high-pitched cries and sent these recordings to DOC.  A female stoat den with fertile female kits calling proved irresistible to the male stoat. There is ongoing research at Manaaki Whenua and with Cacophony to investigate the usefulness of sound lures to attract a range of species.

While it is hoped that this was the last stoat on the island, more work is required to confirm there are no more.  Genetically this last stoat caught was tied to the scat found, so at least there is some hope.  Amazingly, current genetic work indicates that probably all three stoats were unrelated.  They independently swam to the island, rather than being born there.

Invasion 3: Shakespear Regional Park 

Meanwhile, a third stoat invasion was occurring in Shakespear Regional Park.  A female stoat had been recorded in the park in 2020 and avoided all attempts to catch her.  Then in January 2021, two juvenile stoats, a male and a female, were caught in traps inside the park.  Clearly, the adult female had birthed a litter inside the park, and there were more animals present.  Auckland Council threw everything that they could at the response to save the the little spotted kiwi, which are vulnerable to stoat at every stage of their life, tīeke and the recently translocated hihi population they had reintroduced to the park. This included Brad Windust and his stoat detection dog Wero, who were critical in finding stoat dens/caches sites, trail cameras, thermal cameras and DNA techniques. Their rangers monitored these in real time at times at night, trying to respond immediately to any detections. In May, finally a female stoat was caught in a live trap using an old-school Edgar trap from the 1970s. Within a day a second male was live trapped in the same trap, followed a few days later by another. Cameras had shown that the other stoats were coming near the trap when they were inside. Clearly the stoats were interested in the trap because of the sounds of their captured siblings, and the scent indicating they had been there. Another two stoats were caught over July in live traps, and a further one was caught in a kill trap.  All of the live-trapped stoats were sent to the animal facilities at Manaaki Whenua Landcare Research where they are the subject of ongoing behavioural research. This research is led by Dr Patrick Garvey and aims to find out what the behavioural differences are for the clever animals that live in sanctuaries and avoid traps. 

Genetic tests were conducted at Ecogene® on a selection of fresh scat recovered from the park (with the assistance of detection dogs), identifying which individual they came from and the sex of individuals.  This helped refine where traps were placed – knowing the home ranges of each animal, and confirming the number of animals present. It was found that there were two males identified from the scat that had not been caught; one has since been caught. No further detections have been made for months so the last male probably either died of natural causes, or left the park. Again, vigilant surveillance continues, but hopefully this incursion has been successfully controlled.

Conclusions

There are many lessons from all of this.  Teams across the country are working on developing far better tools to detect and trap stoats, including thermal cameras, DNA based tools, detector dogs, and sound and scent lures. Many of these are already being deployed or discussed in Te Korowai o Waiheke’s ambitious project to remove stoats from Waiheke Island.  Similar work is also happening for other pest species relevant to Aotea. The work put in by Auckland Council and DOC has been exemplary, and the costs of these incursion responses has been very significant, but that is what it takes to capture or kill those animals smart enough to avoid traps. The people involved in these responses have worked hard to catch these clever animals, and talking to many of them is like talking to Wile E. Coyote, always scheming on how to do it better next time.  It’s nice I could play a part but the real work was done by the incursion response teams at Auckland Council and DOC, and the scientists at EcoGene®.