Where do young sea creatures spend their first weeks? What’s at the root of oceanic food chains? Kelp forests are to Aotearoa what coral reefs are to other marine ecosystems. Or they used to be.
South of Dunedin, a huge kelp forest stretches for 20 kilometres underwater between Brighton and Taieri Mouth. It’s known to locals simply—reverently—as The Kelp.
Sunshine dances through the forest in shafts of dappled light, illuminating gleaming schools of trumpeter, moki the size of car wheels and schools of greenbone. Golden-brown stems and leaves crawl with small animals—shells, crabs, shrimps and sea stars.
The Kelp is so vast it slows the currents that move up the coast. Like a huge net, it captures fish and crayfish larvae in its tangle of leafy foliage, and they settle to the seafloor. The rocks beneath the kelp canopy are carpeted with thousands of juvenile crayfish, which, when they mature, will migrate hundreds of kilometres, helping to replenish the lucrative fishery that sustains communities up and down the coast.
On calm days, fishers tie their dinghies to kelp fronds and haul in blue cod and tarakihi. With a kick of their fins, divers are swallowed by the forest, weaving their way down to the seafloor, where they spear greenbone and moki, or pluck pāua as big as bricks from the rocks. They emerge from the sea with full catch bags, their blood racing from the cold’s embrace.
The Kelp feeds and nourishes this coast, just as it has done for millennia.
But here’s the thing about The Kelp: it no longer exists.
Only a few older fishers remember it. Gary Homan, who fished out of Taieri Mouth for 65 years, is one of them. The Kelp, he says, used to be so dense it presented a major obstacle to fishing boats—he had to go around it, rather than through it, to reach his cray pots up the coast.
But by about the 1980s, Homan reckons, The Kelp had started to wither. Some years, the forest would almost completely die back, only to return a year or two later, albeit not as substantially as it once had.
After a few years of struggling, he says, it “finally gave up”.
Today, there is hardly a skerrick of kelp along this stretch of coast. Instead, there are patches of bare rock, substrate that in a healthy system would grow a lush underwater forest.
“There’s only one fishing boat left here now,” Homan tells me, “and The Kelp probably hasn’t existed since he’s been working.”
So not only is The Kelp gone, but its departure has been so gradual as to have happened without those most affected even realising—a phenomenon known as shifting baselines. For almost all those who today call the South Otago coast home, The Kelp was never even there in the first place.
“A kelp forest is absolutely one of the miracles of life,” says David Schiel, a marine ecologist and biologist at the University of Canterbury. “All you need is a tiny seed, a bit of nutrients and some photons of light, and you get these massive plants.”
Kelp forests, Schiel tells me, are the most diverse biological habitat in our country.
“In any kelp forest you would probably have hundreds of species. You’ll see bryozoans, you’ll see a million little red algae. There’ll be amphipods, isopods, copepods, who knows how many gastropods, all tucked in there.”
Kelp forests do more than just house and feed marine species. Like any plants and algae, they draw in carbon dioxide and may therefore play an important role in combating ocean acidification and climate change. They also buffer the coast from wave action, helping to prevent erosion.
Kelp forests in New Zealand are created by two types of algae. Rimurimu (Macrocystis pyrifera), otherwise known as giant kelp or bladder kelp, is one of the fastest-growing organisms on Earth, capable of adding an astonishing 60 centimetres a day and reaching lengths of 45 metres. It’s named for the air-filled bladders in its stems, which draw them up to the surface.
Rimurapa, or bull kelp, refers to any of New Zealand’s four species of Durvillaea, the thick, leathery stuff that swirls and surges on rocky shores like mermaid’s hair. Technically, rimurapa are not kelps, but fucoid brown algae. However, they create a similar habitat to rimurimu and so play the same ecosystem-engineering role. Together, these species power entire food chains.
“Kelp doesn’t have roots,” explains Chris Hepburn, a marine biologist at the University of Otago. “The water is a kelp forest’s soil. We think it’s actually modifying the water as it moves through. It adds carbon to the water and that drives bacterial productivity. The whole food web thrives off that.”
About 40 per cent of a typical coastal fish can be traced back to kelp forest, according to research led by Steve Wing, also at the University of Otago. Wing and his colleagues determined this by studying stable isotopes—atoms in seawater that become part of the kelp as it grows, then part of the small creatures that eat the kelp. The isotopes are then passed up the food chain to bigger animals that eat them, allowing scientists to track the way organic matter travels through the food chain.
“[Kelp forests] support a large percentage of the species that are there,” says Wing. “They support fisheries and aquaculture. All the biodiversity that we see along the coast can be attributed to kelp forests.”
In particular, crayfish, the basis of a $300 million fishery, rely on healthy kelp forests to nurture their juveniles. But now, some scientists are terrified we are about to lose all those forests.
Over the summer of 2017 and 2018, Mads Thomsen, an ecologist with the University of Canterbury, headed out on his monthly trip to survey and collect kelp samples from rimurapa forests along the coast. Where he would usually find a swirling mass of kelp fronds, he found only bare rock and open water.
“At first I thought I was on the wrong part of the reef,” he tells me. “But you could see the remnants of the kelp. They have this really massive structure that keeps them attached to the rock called a holdfast. You could see those sticking up. All around the Christchurch area, I could see all these remnants.”
That summer, a marine heatwave caused sea surface temperatures in New Zealand to climb to record levels. “That event, over just a few months, wiped out all the bull kelp around Christchurch and Lyttelton Harbour,” he says.
Four years later, the kelp still hasn’t recovered in these places. Bladder kelp also died back—its beds along the Canterbury coast shrank in the wake of the marine heatwave, a NIWA study led by Leigh Tait found. But the bladder kelp beds quickly recovered when sea temperatures returned to a more normal state. With its spectacular rate of growth, rimurimu seems to be more resilient than rimurapa to short-term spikes in temperature.
When that temperature change is prolonged, however, devastation can occur. And if the change in ocean temperature lasts long enough, invasive species seize the opportunity to move in. In Tasmania, ocean warming has caused more than 90 per cent of that coast’s rimurimu forests to vanish, utterly changing marine ecosystems. Long-spined sea urchins, which thrive in warmer waters, have moved south to invade the denuded coast. Now, their grazing prevents kelp re-establishing. In California, it’s the same story: almost 95 per cent of kelp forests disappeared between 2009 and 2019, and invasive sea urchins have taken over.
Could New Zealand’s kelp forests be next? As our seas warm up, studies have tracked an eastward retreat of rimurimu in the Marlborough Sounds to cool-water refuges in Queen Charlotte Sound. Meanwhile, anecdotal reports suggest dwindling forests in Fiordland and the disappearance of rimurimu from its former northernmost reach—Castlepoint, on the Wairarapa coast.
Stable isotopes are also revealing disturbing patterns in our oceans. Many New Zealand fishers might be surprised to know that hāpuku and ling, or hokarai—both regarded as deep-water fish species today—were once common inhabitants of near-shore kelp forests.
Steve Wing and his team gathered bones of these and other fish from museum collections and midden sites in Otago and, by analysing stable isotopes in these remains, have found that fish that were once heavily reliant on kelp forests have now become much more dependent on deep-water food sources.
Similarly, New Zealand sea lions, which once got all the food they needed from kelp forests, are now hunting further out, according to research led by the University of Otago’s Lucy Wing. That, of course, means swimming further, diving deeper and burning far more energy. The coastal marine ecosystem, it seems, is being forced out to sea for its survival.
The future of our kelp is also linked to the way we treat the land. Once, the forests in New Zealand’s ocean were mirrored by lush forests on shore. When those trees were felled, soils began eroding. Since World War II, the intensification of farming—and its expansion into more marginal areas of hill country—has increased the amount of sediment running into the ocean. Today, when commercial pine forests are harvested, they leave behind expanses of bare, unconsolidated soil which, without proper management, also ends up heading for the sea, darkening the coastal waters.
“Kelps need certain qualities of light to reproduce,” says Schiel. “As you dump all this stuff into the coastal zone, the light quality and spectral qualities diminish quite considerably.”
And, as Hepburn has found, this sediment acts like a smothering blanket. “It covers seaweeds so they can’t photosynthesise as well. It also produces a film over the reef, so spores can’t attach and grow.”
In concert with warming oceans, sedimentation is thought to be a huge driver of kelp loss around the country. Research has shown that ocean warming affects kelp most in areas where water clarity is already reduced. “There’s a real interaction between sedimentation and warming,” says Hepburn.
As kelp forests are pushed back by dirty, warm water, unwelcome newcomers move in. In New Zealand, it’s the invasive kelp Undaria pinnatifida, native to the northwest Pacific, which has been well established in New Zealand for decades, and grows rapidly, completely changing ecosystems. Its soft, ribbon-like fronds don’t provide the same rigid structure as rimurimu and rimurapa for other species to live amongst. It’s also thought be inedible to many native species that depend on kelp as a food source.
And, once Undaria takes hold—as it already has around much of the country—it’s almost impossible to get rid of.
Chris Hepburn doesn’t hold back in his assessment of the future of our native kelp beds. “The thing that defines, for me, our coast,” he says, “is the kelp. It’s our coral reef. There’s nothing else like it. We could lose it in the next 20 years. Eventually, we might have to go to the Auckland Islands to see this stuff.”
For months, I’ve been busy filming wildlife for NHNZ Worldwide, making the next series of Our Big Blue Backyard. Throughout summer, I pay regular visits to the Aramoana Mole, the century-old breakwater at the entrance to Otago Harbour.
A kelp forest lines one side of the Mole, concealing whole sunken ships. I learned to scuba dive here, and have spent many hours amid these wrecks, drifting between the sponge-encrusted steel and the curtains of rimurimu, where fish and countless invertebrates make their underwater city.
This summer, I observe the forest from above. From this perspective, I see how the kelp forest is a magnet for the harbour’s wildlife. Sea lions weave in and out of it, searching for prey. I watch them bring their catches—skate, octopus and wrasse—to the surface, flinging the carcasses around until they disintegrate in a whirl of spray and gnashing teeth.
Fur seals laze in the canopy, ropes of kelp flowing over their sides in what must feel like a massage from the sea. On the ebb tide, when kelp thickens in the lowering water, red-billed gulls flock to forage, dancing atop a raft of rolling weed. Terns pierce the leafy sea, emerging with the little silver fish they feed to their chicks and use to impress prospective mates.
People, too, come to the kelp for sustenance and recreation. It’s common to see a spearfisher’s fins flapping around on the surface. Families gather at the breakwater to cast their fishing lines into the kelp.
This is my kelp—my local patch, my connection to this part of our coast. I can’t imagine it disappearing. But have I taken it for granted? Deadly waters, warm as a wound, are seeping south.
Last summer, trucks carried 1269 tonnes of dead salmon from fish farms in the Marlborough Sounds to the Blenheim landfill—the result of warming oceans—and New Zealand King Salmon, the country’s biggest salmon producer, closed three of its farms in the Sounds.
The salmon farming industry now has its eyes on the Otago coast, near the small village of Karitāne, where massive offshore farms are planned. It is no coincidence that this coast, washed by the cold Southland current, is also one of the last strongholds of giant kelp in New Zealand. Both salmon and kelp need cool, clear waters to thrive.
I visit Karitāne on a day the local waka group is heading out, so I grab a life jacket and jump aboard. We paddle out past the breakers, into the lee of the Huriawa Peninsula, our fibreglass waka gliding across grasping kelp fronds. In ancient times, a busy pā stood on the headland, and the people who lived there harvested much of their kai from the kelp forest. The record of the sea’s importance to their lives is retained in midden sites along the coast.
The rimurimu stands here in Karitāne are still pretty healthy, although as local tāngata tiaki Brendan Flack tells me, this could be “the last of it” along this part of the Otago coast. A hundred kilometres to the south, rivers, including the Clutha/Mata-Au, ooze out of the motu’s side, bringing with them everything they have gathered on their journey. It’s a soup including runoff from building developments, farming and forestry. Erosion along the coast also drags huge amounts of sludge into the water.
Ocean currents pull it all north, along a coast that is now largely devoid of kelp. Karitāne’s distance from big river mouths, says Flack, protects this area from the worst of it. “We’re fortunate we’ve got a pretty small catchment here,” he tells me, “so we can do something
Flack is part of the East Otago Taiāpure Management Committee—a group of locals doing their best to protect their local kelp forests and fisheries. A big part of that involves planting trees along the banks of the local river, the Waikouaiti, to slow erosion. They’re also working hard to keep Undaria at bay, by training teams of divers to pull it out on a regular basis. It’s an ongoing battle. They’ll most likely never eradicate it from these shores.
In the face of climate change, toiling at a local level like this might seem futile, but as University of Otago researcher Matt Desmond explains, it is worthwhile. “If we can minimise those local stressors,” he tells me, “we’ve got a better chance of having resilience against the global ones.”
Desmond, along with Chris Hepburn and others at Otago, is experimenting with re-seeding denuded parts of the Otago coast with rimurimu.
To do this, they collect the reproductive parts of wild kelp and, in the lab, induce them to release eggs and sperm. The resulting sporophytes are then seeded into spots where kelp has vanished, such as the Brighton-Taieri coast.
To grow kelp, Desmond and Hepburn are drawing heavily on aquaculture techniques developed overseas. “The key thing is being able to produce a lot of sporophytes,” says Hepburn. “But once we establish those populations, we hope that it’ll just self-recruit.”
As part of these efforts, the team is also trialling different strains of kelp from around the country in order to find those best suited to a warmer, dirtier ocean. “What we’re trying to do is look for resilience,” says Desmond. “We know there are some strains that show greater tolerance to high temperatures. We’re trying to isolate those. I guess you could say we’re domesticating them.”
But all of these efforts will be in vain if we can’t halt the amount of sediment and pollution pouring off the land.
“You need the conditions to be correct,” says Hepburn. “You need to clean up before you can restore.”
If we are to have any hope of getting lush forests like The Kelp back, he says, farmers and foresters can and must limit the amount of sediment running off their blocks, through riparian planting and the use of sediment dams and silt traps.
And here, Hepburn finds cause for hope. “It’s an opportunity,” he says, “because we can actually deal with that. The solutions are on the land.”