A cry for kelp - the spiky problem of urchin barrens

Saturday, 25 October 2025

Urchin barrens can form when too many predators such as crayfish and snapper are removed from the food web, allowing urchin numbers to explode and over-graze the kelp forests that provide food and shelter for other marine life.

New research shows reefs barren of life-giving kelp are widespread throughout New Zealand. While scientists hope measures to curb kina over-grazing will help, a new, spinier threat is on the march. By Nikki Macdonald.

It’s hard to explain how grim it is to float over a reef and see only sea urchins studding bare rock, knowing it should be a kelp forest alive with flitting fins, says marine scientist Kelsey Miller.

“It’s like walking through somebody’s lawn that they’ve forgotten to water, and it’s brown and lifeless, versus walking through some beautiful bush and seeing tons of fish swimming through like you would birds.”

Miller - a research fellow at the University of Auckland - has spent the past five years investigating the urchin barrens that form when marine food webs are sent out of whack by over-fishing.

Take out the spiny lobsters, blue cod and snapper big enough to crack open kina for a feed, and the urchins take over, grazing kelp forests down to bare reef.

“Kelpers“ brave the mid-winter cold to check the health of Wellington's seaweed forests.

The first nationwide urchin barren stocktake in 25 years just “confirmed their widespread occurrence throughout New Zealand”. In the north-east, more than a third of coastline with rocky reef was an urchin desert.

But there’s finally help on the horizon, after two lawsuits forced Fisheries New Zealand to consider banning crayfishing in the worst affected areas of Northland.

And studies around the country are showing kelp regrows quickly when you remove the grazing kina.

Kelsey Miller’s research project found kelp regenerated quickly after kina were removed from kina barrens, but without other action to increase kina predators, they gradually moved back in.

But an emerging threat is denting environmentalists’ optimism. There’s a new urchin rising on climate change’s tide of warmer waters, and it’s spikier in every respect.

What’s an urchin barren?

When New Zealand’s first marine reserve was established at Leigh, north of Auckland, in 1975, scientists thought its bare reef patches smothered in kina were a natural part of the ecosystem.

Over the next two decades, as snapper and crayfish bounced back and started chomping down kina numbers, those bare spots sprouted kelp and they realised it wasn’t natural at all.

That’s a problem not just for divers wanting to see pretty fish.

“Kelp forests are one of the most biodiverse ecosystem types on the planet, and they are disappearing faster than coral reefs and rainforests. So this isn’t a small issue,” points out Auckland University marine scientist Arie Spyksma.

He knows better than most the scale of the urchin barren problem, having just estimated their national spread.

Spiny rock lobster eat urchins, so when they’re in short supply, urchin populations can get out of control.

“Expansive urchin barrens currently exist, or have been documented to occur, throughout parts of northeastern New Zealand, the Marlborough Sounds, Tasman Bay, Fiordland and Stewart Island,” his paper concludes.

That’s probably an undercount, but not everywhere is vulnerable to urchin barrens. They don’t like very sheltered, heavily sedimented areas, or very exposed coasts, Spyksma says.

Kelsey Miller’s research found that, within 21 months of kina removal, kelp almost half-covered the formerly barren reef.

In a closer analysis of Cape Reinga to East Cape, satellite imagery showed more than a third (36%) of coastline with rocky reefs had barrens within 200m of shore. In the Waikato, that was as high as 43%.

And recent research found kina barrens covered almost half of all reefs in Marlborough’s Queen Charlotte Sound.

While some barrens probably date back to the late 60s or early 70s, it’s only in the past 5-10 years that policy makers have woken up to their importance, and listened to scientists’ cry for kelp.

The great cull

If the answer to stoats eating our native birds is to remove them, then why not just get the kina out of there, so the kelp can recover?

That’s what Miller’s been investigating since 2020.

Using a special research permit, her team removed 400,000 kina from 7ha across four sites in the Hauraki Gulf, where barrens stretched from 3m to 12m in depth.

It’s a laborious process. Because kina in barrens are so starved of food that they’re generally not great to eat, they’re mostly fed to the fishes rather than collected. But even then, reducing numbers to one per square metre takes about 50 diver hours per hectare, Miller says.

The good news is it works. Within two years, the barrens went from 5% canopy cover to an average of 43%, with 3.5 million plants growing back.

“These are reversible, and they were reversible very quickly. When you think about that, versus the monumental task of trying to replant trees and waiting decades for them to regrow, it’s really incredible to see kelp growing back within a few months.”

The bad news is, it doesn’t last. After four years, one site hit about 60% canopy coverage, but at all four sites, the urchins are moving back in.

“I expect all of them to start going down now, just because of those kina. The densities are quite alarmingly high now.”

Nicole Miller has been documenting kelp decline in Wellington Harbour since 2016.

That’s because kina aren’t really the problem, says Miller. “They’re indicative of an ecosystem out of balance.”

In fact, kina are critical reef citizens, their noisy munching drawing in fish. They’re also a taonga to many iwi.

It’s the opposite problem to Predator Free NZ - we need to bring back the predators to restore the balance.

“It’s definitely not a solution on its own,” Miller says. “It is a tool, and it is extremely effective in the short term.”

At the opposite end of the North Island, another Miller has been trialling removing kina from a stripped stretch of reef in Wellington.

But while many of the barrens documented by Spyksma date back decades, Nicole Miller was prompted to act by the changes she’s seen in just the past 10 years.

In 2016, the keen diver and chairperson of the capital’s Friends of Taputeranga Marine Reserve Trust started surveying kelp in Wellington Harbour, to monitor the impacts of changing temperature. Instead, she noticed kina numbers exploding, and seaweed disappearing.

“We saw really rapid kelp forest decline in some of the sites. In one year, March 2021 to March 2022, we lost a 6m band of kelp forest …We could see the acceleration towards a really significant tipping point.”

So she approached Taranaki Whānui to lead a project to control kina.

In three months from December 2022, volunteers collected more than 12,000 kina from about 250m of coastline. They were good enough to eat, so were donated.

Within five months, seaweed was regenerating. By about 17 months, it was covering deeper areas.

“It was showing that the kelp forest can still grow, despite sediment, temperature and other things.”

But like Kelsey, Nicole is now seeing kina return.

“The urchins are reoccupying and starting to encroach on the project because, of course, we haven’t restored the predators. We can’t even keep a 250m long coastline healthy, so what does that mean for the scale of the harbour?”

New Zealand's endemic sea urchin Evechinus chloroticus, left, known as kina, has shorter spines than the subtropical Centrostephanus rodgersii, right, which goes deeper, ventures into low-light areas such as caves, and eats invertebrates as well as algae and kelp.

Nicole is on a personal mission to photograph 70km of Wellington’s coastline “just to wake people up to the fact that we don’t have time to wait any more”.

“The situation in the harbour is really dire, with extensive kina barren along large proportions of the inner harbour reefs. That’s actually quite shocking.

“We can protect on a very small scale, like little safe havens, but we would not be able to manually remove urchins at large scale. Impossible.”

Instead, she wants national action to improve fisheries management and increase marine protection, and drastic local measures, before there’s no kelp left to restore.

Nicole Miller learnt to dive at Poor Knights and has watched the Centro urchin progressively graze it to destruction. “The effect of the Centrostephanos is exponentially higher, because they eat everything, everywhere, to a much deeper depth.”

“What I’d like to see in Wellington is actions earlier, so we are not going down the Hauraki Gulf route, where we have to wait until everything is depleted.”

The new threat

Nicole learnt to dive at Northland’s Poor Knights marine reserve and, even after moving to Wellington 15 years ago, the bright, sponge-studded walls called her back every summer.

But in 2023, she decided the devastated dive sites were no longer worth it.

“The change has been so significant …The only thing that was left from really lush underwater forest with a huge, vibrant understorey and lots of colours on the reefs, was barren rocks with red, flimsy alga. Every now and then you’d find a little bit of seaweed, and the Centrostephanus sitting on it. That was a shocker.”

Everybody knows about kina - the Evechinus chloroticus sea urchin that’s found only in New Zealand. But there’s another urchin on the march.

Centrostephanus rodgersii is native to Aotearoa, with museum records dating back to the late 1800s, says Spyksma, who has been studying it for five years.

Marine scientists have been calling for action to reduce urchin barrens for 20 years.

It’s a subtropical species, so isn’t as widespread as kina but warming waters are increasing its range and numbers.

Several things make it a thornier threat. While kina form barrens in relatively shallow water - typically 3m-12m in depth - Centro grazes down to 40m.

It eats organisms like sponges as well as kelp and algae, and hangs out in low-light areas which kina don’t like, such as archways and caves, Spyksma says.

“It’s got the potential, as the population increases, to form barrens and over-graze in a much greater range of areas than kina does.”

It also has longer spines, making it harder for snapper to crack into. Which leaves rock lobster as its primary predator. And we don’t have many of those, even in marine reserves like Poor Knights, where Centro urchins increased more than nine-fold from 1999-2022.

It also feeds differently, which means 2-2½ Centro per square metre can create an urchin barren, whereas you need 6-15 kina to have the same effect.

“I think it’s shaping up as more of an issue than kina barren,” Spyksma says.

So what’s the plan?

Auckland University associate professor of marine science, Nick Shears, has been monitoring reefs in northern New Zealand for 25 years, and did the last comprehensive survey of urchin barrens in 1999.

“We’ve been pushing for marine protection, better management of fisheries, for a long time.

“The science has been there for 20 years, but it’s really only been the last five or 10 years that there’s been recognition of the impact of fishing that leads to the over-abundance of kina.”

That came after two court cases brought by Northland iwi and the Environmental Law Initiative, which found Fisheries New Zealand must consider the impact on the rest of the marine environment when setting rock lobster catch limits.

Fisheries New Zealand cut commercial quotas, and last July introduced a new special permit for kina barren urchin removal. Just two permits have been granted so far, for Matheson Bay north of Auckland, and Kotukutuku Point in the Marlborough Sounds.

A three-year rock lobster fishing ban was implemented in the inner Hauraki Gulf in April, and submissions have just closed on a proposal to close rock lobster fishing in part of Northland.

Urchins aren’t the only threat to seaweed forests. Giant kelp is vulnerable to marine heatwaves.

Fisheries Minister Shane Jones says he’s awaiting advice from officials before making a decision on the closure.

Sport Fishing Council president Scott Macindoe favours the ban plan.

“Clearly, the ecosystem is out of balance. We have killed too many of the big predators … It's not just the predator-prey conversation, it's all the other animals that have vanished, that lived in that kelp forest. That's the tragedy.

“We need to back off and give this fragile near-shore environment a rest … Pretending the problem is just going to fix itself is not good enough. As a generation, we’ve overdrawn.”

But Shears worries it could be too little, too late. Warming seas mean cool water rock lobster in northern New Zealand are already “pretty precarious”.

“We don’t know if they’re going to recover, or not.”

And to control Centro urchins, you don’t just need more lobster, you need more lobster big enough to get their arms around the urchin’s long spines and roll it over to eat it, Shears says.

“Our populations that are capable of doing that are really low, and really they are the only hope, in terms of keeping Centrostephanus numbers down.”

The Rock Lobster Industry Council, however, thinks lobster numbers are being unfairly scapegoated as the source of urchin barrens. Kelp loss can also be caused by other stressors, such as temperature and sedimentation, its submission says.

While it has supported a commercial crayfishing closure for Northland’s east coast since April, it argues barrens would be better solved by removing or culling the urchins, importing predators, or focusing catch controls just on barren areas.

That would reduce the impact on the 2850-tonne, $350 million industry, its submission says.

Long-time Marlborough Sounds resident Pete Beech, who founded Guardians of the Sounds, also questions whether kina are really the problem.

“As a young man, there were millions of kinas in the sound, and the kelp beds were perfectly healthy. I don’t think it’s kina that are destroying the kelp beds.”

Instead he suspects pollution, and sedimentation.

It’s true that the picture can be more complex than simply reducing predators causes urchin barrens, Shears says.

In his recent Marlborough Sounds research, he found that while kelp loss was closely linked to kina overpopulation, other factors are also important, such as water flow and wave exposure. Giant kelp is also vulnerable to marine heatwaves.

“With warmer temperatures, you’ve got a tougher environment for the kelp, plus kina - it’s just not a good combination.”

However, as elsewhere, when they trialled removing kina, the kelp bounced back.

Like all the marine scientists, Shears is trying to be optimistic that belated action on marine protection will slowly nurse our kelp forests back to health.

“It’s really positive that action is being taken. If we had taken action sooner, we would have been able to build greater resilience. So we’re at a point now where we desperately need to do something.

“The urchins are portrayed as being the problem, whereas it’s actually humans that are the problem.”