Kermadec Islands Marine Reserve may appear to be in the middle of nowhere—but really, it’s in the middle of everywhere. It’s a meeting place, an intersection. Lord Howe and Norfolk islands sit at the same latitude 2250 and 1350 km to the west respectively, New Zealand is the largest neighbour 730 km to the south-west, and the islands of Tonga are a logical continuation a thousand or so kilometres to the north-east.
The Kermadecs are a reflection of both that proximity and distance. Politically they are part of New Zealand. Biogeographically, they are distinct, straddling latitudes 29°14’S to 31°24’S. They sprawl across 250 km of ocean, from the austere L’Esperance Rock in the south, past the small but imposing huddle of sulphur-perfumed Curtis and Cheeseman islands, the brooding wedge of Macauley Island, to the lush northern outpost of Raoul and its cluster of outliers. Tiny specks of land in a vast ocean.
But beneath the surface, a whole new context is revealed. The islands are mere mountain-tops, part of a mighty line of volcanoes which mark another meeting place—one of tectonic plates, where the Pacific Plate is subducted beneath the Australian Plate. As the Pacific Plate dives down, it drags and bends the edge of the Australian, which results in the Tonga Trench, a 10-km-deep rift that is one of the deepest places on Earth. To the west of the trench, the Australian Plate buckles upwards to create the Kermadec Arc, which joins the Tonga Ridge in a 2000-km-long mountain chain. The Pacific Plate is forced to great depth, perhaps 200 km into the Earth’s crust, and melts, generating a great wellspring of hot magma which erupts at various points along the ridge, creating the line of submarine volcanoes defining the southern part of the Pacific Ring of Fire.
There are 40 or so volcanic centres, many active, most submarine, stretching from White Island in the south, past the Rumbles and Brothers, Monowai and Hinepuia, to name but a few. About two-thirds of the volcanoes resemble classic volcanic cones, while the remainder are vast calderas, created by the collapse of the volcano’s summit. Rumble III measures 25 km across at the base, while Macauley Island is just the top five per cent of an enormous submerged volcano with a 10-km-long caldera. Recently, it was discovered that Raoul is the uplifted portion of two giant caldera volcanoes. In the past ten years, underwater exploration involving remote-operated vehicles (ROVs), autonomous underwater vehicles (AUVs), manned submersibles and multibeam mapping of the sea floor has revealed a complex and varied terrain, a hotbed of hydrothermal activity where vents spew hot water into the ocean, creating sulphurous chimneys known as ‘black smokers’.
The legacy of all this volcanism is an unsettled world, constantly in flux. Frequent earthquakes, outpourings of lava, ash clouds and huge quantities of pumice ensure that nothing stays the same for long. Volcanoes have been created and destroyed many times in the five million years that this arc has been active, and islands have come and gone. Even an island as large as Raoul is estimated to have been above water for just one or two hundred thousand years. Yet despite this impermanence of land, life has taken hold above and below water, demonstrating both an incredible resilience and a great ability to disperse.
Everything in the Kermadecs has come from somewhere else, travelling by air or sea. Seeds and spores are blown on the wind, carried on the legs or feathers of birds or perhaps rafted in mats of floating vegetation. The flora, or at least the larger elements, is strongly reminiscent of New Zealand. Step ashore and you are in a forest filled with a selection of trees that may look familiar from warm northern New Zealand: nikau, pohutukawa, tree ferns and shrubs such as Hebe. But there’s a twist—many species have evolved in isolation here long enough to be regarded as endemic; found only here. The smaller plants, such as ferns and grasses, more often come from the tropical Pacific. What are missing are plants in common with its subtropical neighbours, Lord Howe and Norfolk islands—these surprising absences make for a flora that botanists call disharmonic, out of tune with the expected distribution.
The real bounty of the Kermadecs is in the subtropical seas that wash about the archipelago. Most of the coastal fish species are either subtropical or tropical, with the former holding a slim lead in terms of numbers of species but significantly more abundant. Although only 10 per cent or so of the fish are also found in New Zealand, blue maomao and two-spot demoiselles—so familiar to the mainland—are ubiquitous. At times, these fish gather around in such huge schools that divers all but disappear from view.
The three-week-long Kermadec Biodiscovery Expedition in May this year added perhaps 16 new records to the 165 previously recorded coastal fish species. They were all tropical or subtropical species, so what determines which fish settled these isolated islands?
For the first weeks or months of their life, many species of coastal fish are at sea, cast adrift to make their own way in the ocean. These small eggs and larvae are swept by currents, aided by wind blowing the surface water. But the larvae aren’t passive passengers, they are active swimmers, quite capable of swimming 10 or even 20 km in a day, using sight, smell and even sound to orient themselves and locate reefs.
A juvenile brown surgeonfish, widespread in the tropical Indopacific, was collected on the last dive of the Kermadec Biodiscovery Expedition—the first time this species has been recorded in New Zealand waters. Acanthurus nigrofuscus begins life as a one millimetre egg fertilised during a mass spawning event on the edge of a coral reef. After a single day at sea, a tiny larva hatches, less than two millimetres long, and for the next two months it is an ocean voyager, developing its swimming skills until it can reach sustained speeds of more than two kilometres an hour—not bad for a creature less than three centimetres long. At two months old, it’s ready to settle down—assuming there is a reef in the vicinity.
Despite this great ability to voyage to new lands, fish larvae are generally stay-at-homes that don’t embark on a great ocean voyage with an uncertain outcome. Between a third and two-thirds of larvae live out the pelagic phase of their youth, but still settle in the area where they were spawned—the marine equivalent of a young Kiwi returning from his or her OE to settle in New Zealand. Which makes good sense, as home is usually the most suitable habitat, filled with favourite foods, water of the right temperature and, more importantly, other fish of the same species.
But this surgeonfish belonged to the proportion of fish larvae given to wanderlust. It left the Great Barrier Reef and was caught up in the East Australian Current, riding down Australia’s east coast and out into the Tasman, where it joined a slower-moving current known as the Tasman Front. Here the current drifts at less than a kilometre an hour, snaking towards North Cape and peeling off in gentle tributaries that wander northwards into the Pacific. The larval surgeonfish was carried by one of these eddies and had the rare good fortune to make it to shore—had it stayed in the main flow, it would have been carried on down the east coast of New Zealand in the warm East Auckland Current, been deflected by East Cape and jettisoned east towards Chile.
These currents are a feature of the gigantic South Pacific subtropical gyre, which circulates counter-clockwise in the South Pacific basin, kept in motion by constant easterly winds along its northern flank and the westerly pattern of the deeper latitudes. The Kermadecs are parked in the middle of this gyre, unaffected by the currents that patrol its boundaries. Transient eddies caused by winds and significant internal tides slosh back and forth in the basin between the Kermadec Ridge and the Colville Ridge to the west, making it relatively easy for marine larvae to move between the islands of the archipelago but much harder—though by no means impossible—for them to arrive from elsewhere or make their way further afield.
The chances of a single larval fish ever reaching any of the Kermadec Islands is pretty slim, and the possibility of arriving to find a fish of the same species yet opposite sex at the right time is witheringly small. Even so, one has only to dip beneath the waves to see that the miraculous has happened, that these islands in the middle of nowhere have attracted life, from everywhere. And not just fish—there are corals, sea urchins, algae, crabs and all manner of other plants and invertebrates.
If getting there is one puzzle, what the fish feed on once they are there is another confounding the biologists. The water is blue and clear as gin, with visibility often greater than 30 m. But unlike the chlorophyll-laden soup washing around the mainland, or the cold rich water flowing through the Tonga Trench just to the east, tropical water is nutrient-poor, the oceanic equivalent of a desert. It’s not just the water that seems barren—the steep underwater flanks of Curtis and Cheeseman islands resemble bare concrete walls, covered only in a smothering layer of coarse sand. Yet at one of the most seemingly barren sites the fish-collecting team recorded an astonishingly array of 31 fish species.
Surface waters at the Kermadecs average in the low 20s—too cold for coral reefs, and too warm for the large brown algae that are familiar around mainland New Zealand. So while there is plenty of coral typical of tropical environments—especially around Raoul at the north of the island group—it exists just as small colonies or individual growths. And there is certainly no lack of diversity in the algae, either in species or form, which ranges from delicate fern-like fronds to rough stubble. There is clearly plenty to feed the many herbivorous fish.
Among the most famous inhabitants of the Kermadecs are the giant limpets, which live in the shallow intertidal and subtidal areas. At 12 cm long, they’re among the largest of their kind in the world. Limpets are homebodies; they don’t need to venture far to find fast-growing algae to graze. In fact, the largest are themselves often covered in a lush fur of algae, which provides an ample paddock for young limpets to graze on the backs of adults.
Perhaps the most striking feature of the Kermadec Islands’ coastal seas are the apex predators, such as the stately spotted black grouper. They’re built like blimps, solid of girth and perhaps as old as 50 years. Grouper like these are occasionally seen around New Zealand’s North Island, but apex predators never occur in large numbers and it doesn’t take many successful catches to fish them to extinction.
But groupers are not the only predators here. Nearly 40 species of sharks have been recorded along the northern Kermadec Ridge, and Galapagos sharks are particularly numerous around the islands. The ones that accompanied the divers on the Kermadec Biodiscovery Expedition were all immature, from juvenile 80-cm whippersnappers to feisty 1.5-metre adolescents. Regardless of size, they were bold, curious and fearless around us.
While the study was confined to a narrow coastal strip defined by safe diving depths of just 30 m, that is no barrier for marine life. In many places the sea floor falls away precipitously from the shore, giving way quickly from shallow water to mid and deep water. It’s out in the deeper water that the adult Galapagos sharks range, along with the newly described Kermadec spiny dogfish— sufficiently different from those in New Zealand waters to describe it as a new species, endemic to these islands.
The deep-sea realm of the Kermadecs is less studied even than the coastal margins, yet it is clear that much diversity lies beyond the reach of sunlight, especially around some of the seamounts. Here, cold-water corals such as black coral, bamboo coral and bubblegum octocoral share the title of megafauna with glass sponges and squat lobsters. This is an unknown realm, where each research voyage brings back species new to science, and it is clear that much remains to be discovered.
As well as the residents, there is a steady stream of oceanic visitors to the Kermadecs. Satellite tags have revealed that great white sharks pass along the volcanic chain on what seems to be an annual migration between the cool southern waters of New Zealand and a variety of tropical destinations. In summer, turtles such as green and hawksbill turtles are seen around Raoul Island, feeding and resting for a while before moving into the tropical Pacific. Bottlenose dolphins are seen around the islands at some times of the year, and there are suggestions that these may be slightly different from those found in New Zealand.
In the past few years, it has become clear that the Kermadecs are an important stop-over point for humpback whales heading south, from their winter breeding grounds in the tropics to summer feeding grounds in the Antarctic. On just one October day in 2009, 112 humpbacks were sighted in the waters around Raoul, among them many mothers with their calves, perhaps resting and socialising during their long swim south.
The Kermadec Biodiscovery Expedition had just two weeks in the archipelago, and focused its efforts on recording the biodiversity around the islands. But it is clear that coastal habitat is just a tiny part of what makes these islands unique. These little rock outposts in the middle of a huge ocean are an important way-point for ocean travellers and a rare speck of land for seabirds.
How life gathers around these rocks or passes by, and how it is connected to the temperate water of the mainland and the tropics to the north, are mysteries that continue to intrigue marine scientists.
At 748,000 ha, the Kermadec Marine Reserve is by far the largest in New Zealand, a place that has benefited from two decades of protection from fishing. It is an environment largely unaltered by humans, an enormous laboratory for scientific research, and a reference library for the near-magical symbiosis of an ecosystem in balance, lest we forget.