The humpback highway
The migration of Oceania’s humpback whales, and their final destination in Antarctica, has remained shrouded in mystery. This year, a team of scientists travelled north to intercept and track the whales travelling south. What they discovered only made the great migration more intriguing.
The afternoon wind rises, not a whale sighted for hours, and off the west coast of Rangitahua/Raoul Island, the hunters are getting tired.
The sea is grey and choppy, and from our five-metre chase boat we watch another squall clear the horizon. Behind us, the headland called Hutchison Bluff towers darkly over the ocean like the stern of an ancient ship turned to stone.
A sudden shout breaks our silence: “Blows at five o’clock!”
Heads turn to the south-west in time to see the misty remains of two columns of water and warm air brushed away by the wind: the twin exhalations of a pair of humpback whales. This is what we came for, and we brace to move in their direction. South-west, into the wind.
We follow the trail of blows for close to a mile, watching for the dark roll of whale flesh breaking the surface. Minutes go by without a sign, until, without warning, the water ahead explodes with whale. Two adult humpbacks launch themselves straight into the air, rotating their bodies outwards about an invisible pivot. The 30-tonne animals pause for a moment at the top of their arc before slamming back into the sea with a boom that shocks the ear. And just like that, in a four-second display of grace and animal strength, our quarry has got away.
For a newcomer to the chase, the sight of two fully grown humpback whales breaching is a moment of elation and reverence. Others in our small boat have different priorities. Having missed the chance of a shot from the bow, cetacean scientist Rochelle Constantine lowers her rifle and peers into the water. “At least they’re sloughing some skin,” she says. “Get the net.”
Mystery surrounds the life habits of the humpback whales of Oceania, some of the largest creatures with which we humans share the Earth. In spring each year, humpbacks migrate as far south as it is possible to swim—from the islands of the tropical South Pacific where they breed, past Raoul Island in the Kermadec archipelago north of New Zealand, to their feeding grounds off Antarctica. There they gorge on krill, fattening before undertaking the return journey to the tropics to mate, fight, give birth and sing the complex songs for which they are famous.
But where in Antarctica do they go? And why are the numbers of this particular population of humpback whales so low? These are questions the gun-toting scientists roaming the waters of the Kermadecs were hoping to answer.
All southern hemisphere humpback populations are recovering from a slaughter—almost to extinction—during the industrial whaling era of the 19th and 20th centuries. But the Oceania humpbacks are recovering far more slowly than the whales of east Australia. Cetacean scientists suspect that the answer may have something to do with the location of their traditional Antarctic feeding grounds, which were unknown until this voyage. Searches in the most likely area, the Balleny Islands in the Ross Sea region directly south of New Zealand, have so far turned up mainly east Australian whales.
“We knew the Oceania whales had to be further east,” says Constantine, a researcher at the University of Auckland. “We just didn’t know where.”
Mounting an expedition to search eastwards of the Ross Sea was out of the question. Such a mission would cost more than $6 million, an impossible sum in contemporary ecological research. What Constantine did have, however, were reports of large concentrations of humpback whales around Raoul Island between September and November every year—Oceania humpbacks, heading south.
Constantine thought it might be possible to place satellite tags on these whales in the clear subtropical waters of the Kermadecs and track them to their destination in the Southern Ocean. Raoul, despite being 1000-odd kilometres north-east of the New Zealand mainland, was within the range and budget of a research voyage. And so, in September 2015, the expedition vessel RV Braveheart steamed north from Tauranga, carrying an international group of researchers to New Zealand’s northernmost point in the cause of Antarctic science.
At dawn, the research team assembles on the stern deck of Braveheart. Tied alongside is a six-metre rigid-hulled research boat, a waist-high steel cage bolted to its bow. This is the chief tagger’s station, occupied for the next two weeks by Simon Childerhouse, a marine scientist from Blue Planet Marine in Nelson. Childerhouse operates the tagging gun, a formidable piece of equipment powered by compressed air from a scuba tank. Perched above the bow, he looks like the modern incarnation of a 19th-century whaler, wielding a harpoon.
Behind him is Claire Garrigue, a French whale scientist from New Caledonia, carrying a crossbow fitted with razor-tipped darts for collecting biopsy samples. In a second, smaller vessel, Constantine shoulders a rifle modified for the same purpose. The researchers are aware of the disturbance their presence will cause to migrating whales today, and in the long grey minutes before sunrise, the entire team is tense.
Early morning at Raoul has a quality of brooding calm, with squally interludes as the wind wraps around the island from every direction. On a good day the blows and pectoral slaps of humpback whales can be observed close below the island’s cliffs; more often the whales are spotted further out to sea, blurred by the spray of ocean swell.
Whales here are most often seen in small groups: mother-calf pairs, or one or more males escorting a female. Some individuals linger at the island for up to a week, migrating south as new pulses of whales arrive from the north.
Locating and attempting to approach a group of whales on the water is a cautious, day-long process. At Raoul, the humpbacks are warier and more elusive than those the scientists have observed elsewhere, perhaps due to an absence of more pressing urges that occupy their attention in the breeding and feeding grounds.
The boat driver approaches a pod from the side, so that the whale can see the vessel but is not startled. A swift, carefully timed acceleration brings the animal within range of the tagging gun. At this point the crew are only two or three metres from one of the largest creatures of the sea. Childerhouse aims, and in a spray of foam the tag strikes, and the whale is gone.
For the tagger, it’s a moment of anxiety, Childerhouse explains afterwards. “The excitement is to find out what’s going to happen to these whales when we get tags on them,” he says, “and the apprehension is about making sure you don’t send a $5000 tag to the bottom of the ocean while everybody’s watching.
“I become very myopic,” he says. “I only look at the whale. I’m not aware of anyone else or anything else that’s going on. I’m focused on the spot that I’m aiming for.”
That spot is high on the whale’s back, just below and forward of the dorsal fin. When Childerhouse mounts the tag in the gun it looks startlingly large—two centimetres in diameter and the length of a human forearm. But compared to the bulk of an adult humpback it is inconsequential. A well-placed tag will insert fully through the skin and into the blubber of the whale, with only a short, flexible antenna protruding.
The reaction of whales to this procedure is puzzling. Sometimes a vigorous slap of a tail against the water surface, suggesting displeasure or surprise, drenches the boat’s crew. At other times, tagged whales don’t respond at all. Eventually the whale’s skin will expel the tag in the same way the human body expels a thorn.
Usually Childerhouse hits his mark, but on two occasions he misses and the tag disappears beneath the waves. Braveheart’s chef gamely dons scuba gear and manages to retrieve one of these tags, but the other is gone for good.
Sometimes days on the water go by without an opportunity to take a shot. Often whales dive before even a photograph can be taken, the barest raising of tail flukes signalling their departure. When a pod is too difficult to approach closely for tagging, Constantine and Garrigue use the greater range of their rifle and crossbow to collect biopsy samples: tiny cylinders of white blubber and blue-black skin for genetic analysis. Breaching whales also tend to shed small fragments of skin in the violence of their landings. It’s my job to scoop these up with a fine-meshed net. Every data point is precious.
Aboard each research boat, a photographer attempts to capture images of the underside of the tail flukes. Scarred by long rake marks from the teeth of attacking killer whales or round bites from the jaws of cookie-cutter sharks, the flukes, with their stark black-and-white markings, are unique to each whale and can be used to match the whales at Raoul to other sightings throughout the Pacific.
On a choppy, wind-blown sea, capturing these photographs requires precise positioning of the boat behind a whale as it raises its flukes to dive. On calmer days without whale sightings, we lower a hydrophone on a 20-metre cord into the water. The chirps, groans and other vocalisations of the humpbacks can suggest the breeding ground from which a whale has travelled.
The humpback calves are larger and more independent at this stage in their migration than when they departed their breeding grounds. Scientists observe a number of calves playing with one another, and on one occasion even with a pod of bottlenose dolphins. One afternoon, the Braveheart crew spot the body of a dead calf floating in the water, being devoured in chunks by a great white shark. The calf and its mother had been recognised and biopsied several days earlier. Satellite tracks later reveal that the mother of the dead calf lingered in the Kermadecs for 21 days, a longer stay than any other whale tagged on the voyage.
In two weeks and 100 hours on the water, the two sampling boats deploy 25 satellite tags, collect 85 tissue samples, take identifying photographs of 128 whale flukes and make 10 separate recordings of whale vocalisations.
The scientific team is exhausted by sunset, but the whales continue to offer valuable data—in pings from their attendant satellite tags, and in song. On still nights, I lie awake with the slow throb of whale song reverberating through the hull of the ship, like a heartbeat.
The excitement of the field starts to fade as scientists process their data. In laboratories from Auckland to Oregon, photographs of tail flukes must be edited and digitally compared against thousands of other images from across the Pacific, and genetic analysis done on tissue samples. This is the grinding, monotonous work that is the backbone of cetacean research, but information has already emerged that is changing scientists’ understanding of the Oceania humpbacks.
It now seems that whales gather at Raoul Island not from a single tropical breeding ground, but from a 3600-kilometre stretch of the Pacific encompassing New Caledonia, Tonga, American Samoa, Niue and the Cook Islands.
Analysis of the satellite data shows that this sweep of tropical ocean mirrors the vast tract of Antarctic waters to which the whales disperse. On leaving Raoul, most of the whales meander south through the rest of the Kermadec archipelago. Travelling individually or in small groups, they then move directly south-east, into the empty quarter of the southern Pacific unbroken by islands for thousands of nautical miles.
As a group of whales moved south-east, one mother and calf diverted from the main migration path to move slowly around the East Cape of the North Island, as if giving greeting to the coast of Paikea before continuing south. (A thousand years ago, voyaging Polynesians followed the springtime route of whales from the tropics, stopping at Raoul before moving on to these same North Island shores.)
As they approached Antarctica, whale after whale broke from the main migration route, each taking its own course. Several settled to feed just east of the Ross Sea, as scientists had predicted. Some swam even further east, to the Amundsen Sea, directly south of the central Pacific. Most astonishingly, for the watching researchers, several whales swam without stopping to the remote ice-edge of the Bellingshausen Sea flanking the Antarctic Peninsula, some 3500 kilometres from the Ross Sea.
In nine weeks of constant swimming, one whale travelled more than 7000 kilometres from Raoul before stopping to feed in the Bellingshausen: a total distance of 8500 kilometres from its breeding ground.
The migration path of Oceania humpbacks appears shaped like a contorted hourglass: two wide distributions to the north and south, with the Kermadecs the narrow neck at the centre. Quite why animals that have not eaten for six months would add hundreds of kilometres to their migration in order to spend several days at Raoul remains a mystery. Perhaps it is a place of socialisation, where cultural interaction occurs and songs are shared from whale to whale. Analysis of the many variants of song recorded at Raoul may shed new light on this singing behaviour.
What seems clear is that the migration path is longer by far than that taken by the east Australian humpbacks. The Australian whales migrate yearly between the Great Barrier Reef and Antarctic waters south of New Zealand and Tasmania, then back to the tropics along the same path, with some individuals passing by mainland New Zealand. This is still an immense distance, but nowhere near that travelled by the Oceania whales, which return northwards to their breeding grounds by an unknown route that bypasses the Kermadecs. Scientists are interested to know whether the physiological challenge posed by this longer migration may reduce the overall reproductive success of the Oceania population.
Despite researchers having located the Antarctic destinations of these whales, Constantine stresses how little they know about this region. The sea ice is melting faster in the Bellingshausen than in any other part of the Antarctic, and the plankton is less abundant than in the Ross Sea, suggesting that perhaps a lack of food could be another reason for the Oceania humpbacks’ persistently slow recovery.
Despite the great distances the whales travel and the comparatively meagre rewards when they arrive, the scientists were impressed by the health of the humpbacks encountered at Raoul. They were in extraordinary physical condition for animals that had gone six months without feeding, as well as swimming from polar waters to the tropics, mating or giving birth, and swimming part of the way back.
Meeting these whales, which have been part of the story of Aotearoa from the beginning, brings us face to face with their ongoing role in our history.
“From the northernmost to the southernmost parts of New Zealand, from the Kermadecs to Antarctica, one thing that we have in common is our humpback whales,” says Constantine. “We need to be mindful that their recovery is still very slow, and they remain vulnerable to human impacts. Now that they’re coming back, our interaction with them is only going to intensify. We need to prepare ourselves for that.”