Seabirds wiped out on remote Golden Bay rock stack

Thursday, 7 January 2021

Seabirds once flourished on South Nguroa Island, off Golden Bay's remote western coast, but all have been wiped out.

A handful of rock stacks along the five kilometre long stretch of coast running between Cape Farewell and Nguroa along Golden Bay’s remote western flank have for eons provided breeding havens for upwards of 10,000 burrow-nesting fluttering shearwaters and diving petrels.

In effect, they are Kahurangi arks.

Around half of these breeding seabirds used to live on the largest of these ‘bird islands’, 72m-high South Nguroa Island which lies a150m offshore just north of Nguroa Bay. But no more. They have been wiped out, almost certainly wiped by stoats, who swam out to the stack and climbed its near vertical conglomerate sides which have two arches through its base.

Because of its inaccessibility, no human history of this 1.5 hectare island exists. An impenetrable fortress, so everyone thought.

A diving petrel chick on South Nguroa, photographed during the original January 1997 expedition to the island.

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This unnamed rock stack along the Nguroa-Farewell coast remains unbreached.

That was until last December, when three seabird experts were dropped off by helicopter on the stack. They found all the burrows deserted, the colony presumably wiped out sometime in the last two decades.

This time frame is known, because the only other visit to the top of this impressive stack was in January 1997, when three DOC personnel – Ian Miller of Nelson and Simon Walls and Greg Napp of Takaka were dropped there. It is feasible to presume they were the first ever humans to get atop it.

What they found was that the vegetated and inclined cap of the island was an important breeding area for fluttering and sooty shearwaters along with diving petrel. The men recorded that all the available land was taken up with burrows at a density of around one per two square metres on average.

The estimate from this visit was around 5,000 burrowing seabirds lived on this island.

But when three seabird experts were flown out to the island last December in a helicopter paid for by the Farewell Wharariki Ecosanctuary Trust, their intention to determine a possible relocation source of birds for their ecosanctuary at Cape Farewell, they found that the entire population of shearwaters and petrels had gone, completely eradicated off the rock.

Stoats cop the blame, being strong swimmers and skilful climbers. Twenty five traps on the mainland are now in place, in two overlapping arcs facing the island. Four stoats have already been caught.

As the latest Farewell Wharariki Healthpost Nature Trust newsletter points out, this incursion makes the introduction of seabird species into their ecosanctuary even more vital.

The report of the original 1997 party out to South Nguroa was worth dusting off in light of recent developments.

Leaving from Takaka Aerodrome, the trio were landed for an overnight stay on the northern high tip of the island which slopes off steeply to the north-west, its hill slope strongly following the tilted 14 degree bedding plane of the surrounding coast. The island is Pakawau Group Conglomerate and sandstone of the Cretaceous period.

Their brief was to record vegetation types, collect any interesting invertebrates, observe birdlife and look for reptiles.

Walls recalls being dropped off: “There was only one place we could jump out, everywhere else we just couldn’t tell how far the ground was under the dense canopy. The pilot rested one skid on the slope, we jumped out, then he was off. It did cross our minds that we may be the first humans to come out here.”

What confronted them was a veritable wonderland of coastal nature, superbly adapted to some of the harshest conditions this environment could dish up. The first thing they noticed was that bird burrows were everywhere and extra care had to be taken as they moved around the island.

Despite the infertile parent rock, the plant cover grows profusely here in a relatively thin cap of soil atop the island, the result of the bedrock disintegrating down into silica-rich gritty soil which then gets enriched by loess, and guano from the burrowing birds.

Plant growth makes humus which adds in and rots down to produce a gritty brown loam, what a soil scientist would classify hygrous to subhygrous. Annual rainfall along this section of coast is approximately 1700mm, with this island suffering significant periods of moisture deficiency during long dry periods.

Walls went around to catalogue all of the 65 plant species which have colonised this rock. Taupata predominates but is interspersed with a range of other species, all windshorn to dwarf-like status by the strong winds and salt spray.

Some shrub species he found to be no bigger than 300mm high, allowing the ferns, herbs and grasses to merge with the low canopy. Walls observed that the taupata in places formed pure thickets, while elsewhere other shrubs and vines were mixed with it to form “clumped mosaic patterns”.

In all cases these shrublands had a canopy with a smooth windshorn upper surface which were composed of small, interwoven, shortened and branching twigs with leaves arranged to minimise wind resistance.

The trio discovered the island had two pronounced gullies with sufficient soil depth to support taupata of forest stature. A savage environment for sure, this was plant survival and adaption at its best.

Observed too were significant areas of wharariki or coastal flax, but only one small patch of harakeke. Walls noted that knotted clubrush (isolepis nodosus) grew on the fringes, often confined to cliff edges or on open rocky places which were the transitional zones between low herbfields of pigmyweed (Crassula) and iceplant on rock, with taupata or Hebe elliptica on better soils.

Six scurvy grass plants were found, the first such positive identification of this endemic member of the cress family ever to be made along this section of coast.

At the time of their mid-summer visit, fluttering shearwater chicks were nearly fully feathered, the sooty shearwater chicks were at the half-grown fluffy stage, although diving petrel chicks were scarce. It was important the men stayed the night because they could observe the adult birds returning from sea to rest and feed their young.

As dusk settled, the men made their camp under the stars on some ground near where they had landed. Surprising to them in the fading light was the huge flocks of starlings which returned to roost here. Most of the vegetation is splattered with their droppings, it being surmised that it was these birds who were largely responsible for bringing mainland seeds back to the island.

As a result of all the bird activity, it is not surprising that the ground under the scrubland foliage was mostly bare, apart from sparse clumps of shining spleenwort (Asplenium oblongifolium) and hounds tongue fern (phymatosorus diversifolius).

The men also observed other bird residents, such as the black-backed gulls which were nesting on the bare outcrops at the top of the island. Two chicks at nest abandoning stage stood on one nest site, while adults and juveniles all flew nearby.

A pair of variable oystercatchers had a nest on the cliff edge some 60m above the sea, while a handful of white faced herons roosted on ledges around the bluffs. Other birds spotted included resident blackbirds, waxeyes and hedge-sparrows, while red billed gulls were also seen in the vicinity.

No reptiles could be found on the island, nor any evidence they existed there. At the time, the island did not appear to have any mice, rats, possums, cats or mustelids, nor could any sign be had of large invertebrates such as slugs, weta, weevils and snails.

Ian Miller did come up with a list of smaller ones though; five species of flies including two crane fly species, one small solitary wasp, a grasshopper, and six species of beetle.

Not long after their return, Walls petitioned the Minister of Crown Lands to bring all the stacks and islands around Cape Farewell, along with the four Archway Islands at Wharariki, into formal Crown ownership.

They had all simply been left out of the cadastral system. They were formally gazetted in 2002, not only acknowledging the unique habitats these islands provide for their wealth of inhabitants, but also providing a model for restoration projects along this coast, such as the Cape Farewell Wharariki Ecosanctuary.

Well so we thought, until the stoats swam across.