A team comprised of staff from Te Papa and the Department of Conservation (DOC) recently spent a week surveying islands in northern Fiordland. In this second blog based on the trip, vertebrates curator Colin Miskelly describes some of the sites visited and discoveries made.
There are a lot of islands in coastal Fiordland. A quick look at a map of the region shows 50 named islands and another 22 named clusters. There are 18 islands in the appropriately named ‘Many Islands’ in Dusky Sound alone. And then there are all the unnamed islands, islets, and rock stacks.
Homes for seabirds
Any island large enough to have rock crevices or soil (for burrowing) could provide a breeding site for petrels. Petrels are a diverse group of seabirds, with 37 species breeding in the New Zealand region, nearly all of which nest in burrows excavated in soil.
The three most abundant and widespread species in Fiordland are sooty shearwater (tītī), broad-billed prion (pararā) and mottled petrel (korure). Finding their colonies has been the focus of a series of expeditions by Te Papa and DOC staff since 2016.
The seabird surveys started in Dusky Sound (which is rumoured to have an island for every day of the year), then moved to the nearby island-rich fiords of Chalky Inlet, Preservation Inlet, and Breaksea Sound. By early 2020 we had completed surveys of 175 islands in southern Fiordland, and so shifted our gaze further north.
Big mountains and few islands
The remaining section of the Fiordland coast has about 125 km of coastline, from Dagg Sound north to Milford Sound. A feature of Fiordland is that as you go north, the mountains get higher, the fiord sides get steeper, and there are fewer islands. The northernmost fiord of Milford Sound is famed for its spectacular scenery. The only island in the outer fiord (Post Office Rock) is an infinitesimal speck among this grandeur, yet we found 73 sooty shearwater burrows there.
We started the survey at Deep Cove in Doubtful Sound, and immediately headed north to Milford Sound, to make the most of a few days of settled weather. As we worked our way southward, one of the highlights was landing on an unnamed island in Poison Bay, just south of Milford Sound.
Not only was it an important tawaki/Fiordland crested penguin breeding site, but we found broad-billed prions breeding there, more than 50 km north of where they were known to occur in Fiordland.
A few days later we discovered another range extension, with about 50 mottled petrel burrows (and two corpses) on Seymour Island in Doubtful Sound – the first report of this species breeding north of Breaksea Sound in recent times.
A nasty surprise
Most of the islands in northern Fiordland do not receive any pest control, and are close to the mainland, and so it was not a surprise to find evidence of rat presence. This included footprints, droppings, and gnawed seabird corpses.
However, we were very surprised to find rats to be present on Nee Island, off the south-west corner of 8140 ha Secretary Island, at the entrance to Doubtful Sound.
Secretary Island is the largest island near mainland New Zealand that has never had rats or mice establish. It is also the site of a major island restoration project by DOC, with red deer eradicated, and an extensive network of stoat traps maintained 3-4 times a year. Nee Island lies only a few hundred metres offshore, which is far too close for comfort.
Within 12 days of our visit, DOC staff had returned to Nee Island, had used traps and trail cameras to confirm the presence of Norway rats, and had hand-spread brodifacoum bait to eradicate the rats.
One of my personal highlights was landing on western Shelter Island, at the entrance to Doubtful Sound. I last visited the island (as a DOC scientist) in July 1993, as part of a weka genetic sampling project.
The weka on the Shelter Islands are mainly the black colour morph (black is the predominant colour for weka in coastal Fiordland). However, a few birds in 1993 had a mixture of black and white feathers, presumably due to a genetic mutation. My 1993 notes describe one bird caught on this island as having “about half the head and underparts white”.
I photographed an almost identical bird on the same island in November 2020, indicating the persistence of this genetic mutation in the population for at least 27 years.
The last island
We spent the last night of the survey at the head of Hall Arm, close to the Southern Winds’ home port of Deep Cove. There was only one island left to complete the survey – Rolla Island (0.8 ha) at the entrance to Hall Arm.
It was a great feeling to step ashore on this beautiful little island, knowing that we had managed to land on every island on our target list for the entire Fiordland coast over the course of the four surveys.
Before we started the surveys in 2016, there were only about a dozen Fiordland islands where petrels were known to breed, and no population estimates for any site.
After landing on 217 islands, we now know that there are at least 165 petrel colonies in Fiordland, and probably more than 66,000 breeding pairs. This is far more than anyone expected, and provides a great springboard for these populations to recover as rats and stoats are cleared from more and larger islands, and from the adjacent mainland.