Every year an ocean current supercharged by climate change brings a destructive, spiky species south from New South Wales to the rocky reefs of eastern Tasmania.
The long-spined sea urchin is bad news for Tasmanian sea life as the species can quickly nibble swaying kelp forests to bald rocks, forming underwater moonscapes known as urchin barrens.
Seaweed is important food and habitat for the animals of a rocky reef, so when urchins create a barren it's akin to clear-felling a vibrant rainforest.
"Given that it's such a pivotal species and can transform the reef, it's probably the single biggest threat for reefs in eastern Tasmania," says marine ecologist Scott Ling, an associate professor at the Institute of Marine and Antarctic Studies.
"The long-spined sea urchin population is exploding in Tasmania and has been since the early 1990s, reaching an estimated 20 million by 2017," Dr Ling said.
For 14 years researchers at the institute have run significant experiments to slow the impact of sea urchins.
A fishery is trying to find an upside by making a buck out of a willing restaurant market for sea urchin roe.
Yet still their numbers grow.
A Senate Inquiry into sea urchins, initiated by Senator Peter Whish-Wilson, will hold its first public hearing in Hobart on Friday, and experts are discussing how to better manage the species at a workshop convened by the Tasmanian government this week.
East coast faces 'double whammy'
The Tasmanian east coast is a climate change hotspot. It is warming at about four times the rate of the global ocean average.
This is because of the warm East Australian Current (EAC) extending from the New South Wales coast.
"It is increasing in strength because of the warming in the atmosphere above the Pacific Ocean," explains Gretta Pecl, director of the Centre for Marine Socioecology.
"As the atmosphere warms it literally spins up the wind. This is physically forcing the current further and stronger down our coast," Professor Pecl said.
"It's the current that brought Nemo's dad down the coast in the [animated] movie.
"So, the east coast gets a double whammy, it gets the underlying warming that most of the rest of the ocean gets, and it gets this change in current system."
Just like Nemo's dad, sea creatures hitch a ride on the current, which means Tasmania is being visited by a host of northern species.
This supercharged warming has been linked to the movement of about 100 species that have either arrived in Tasmanian waters or extended their range south down the Tasmanian east coast. There is even a website were fishers and divers can record these observations.
Not all the northern species are greeted with dismay.
"Snapper and yellow-tailed king fish coming to Tasmania are seen as excellent for recreational fishing opportunities," Professor Pecl said.
But the impacts of long-spined sea urchins, who ride the current when they are tiny planktonic larvae, are "equivalent to an invasive species".
Protective power of lobsters
Mitigating the impact of the urchins is challenging, because every year the EAC will bring a new batch of urchin larvae down.
For more than 14 years, Scott Ling and his colleague Craig Johnston have been donning wetsuits and plunging into Tasmanian waters – to understand the changing ecology of the reefs.
Their aim is to see if lobsters — predators that dine on urchins among other things — could help keep the spiky creatures in check.
They set up experiment sites along Tasmania's east coast.
At St Helens in the north, urchin barrens had already significantly affected the sea-scape, whereas near the Tasmanian peninsula in the state's south, the small patches cleared of kelp were more like "spot fires".
Hefty lobsters were translocated to the experiment sites and harvesting was restricted.
The results were then compared with nearby "control" sites where no action was taken.
In the south, where fishing continued and no lobsters were introduced, there was an increase in urchin numbers and the size of barrens.
But, when lobsters were added and protected, urchin numbers reduced and the barrens stopped increasing.
"It really demonstrated that if we initially had large numbers of predatory lobsters on the reef, we could really mitigate the risk of urchins taking over," Dr Ling said.
In waters off St Helens, where the damage was already extensive, increasing lobster numbers had no effect on the size of the barrens.
"Fifty large lobsters per hectare of kelp-dominated reef can keep a lid on urchin overgrazing," he said.
This suggests that when it comes to managing urchins, prevention is more effective than cure.
Good thing urchins are delicious
Lobsters are not the only ones who eat long-spined sea urchins.
Urchins are also a hit on human menus — so commercial harvesting is helps to keep the population down.
Sea urchin reproductive structures, often called roe or uni, are eaten around the world, including in Tasmania.
"Between 400 and 500 tonnes [of urchins are harvested] per year, which equates to over a million urchins being removed," said John Keane, fisheries expert at the Institute of Marine and Antarctic Studies.
Harvesting is tough work. Divers pick their spiky quarry off the reef by hand.
It is not commercially viable to harvest in areas with low numbers of urchins, or to harvest small urchins, so commercial harvesting cannot be relied on to keep the wild population under control.
Recently government-funded "take all" harvests have been trialled and are set to continue next season, which may help to prevent urchin barrens forming.
The answers are in our hands
No matter how many urchins are removed by lobsters or by divers they keep returning, hitching a ride on the EAC.
If gung-ho divers remove all the urchins, they run the risk of shutting down the business, Dr Keane explains.
"Fast forward 10 years, the urchins and the barrens will come back and then we don't have a major control mechanism being the fisheries … we're back to square one," he said.
"That's the delicate balance that you are battling with at the moment.
"Urchins aren't going away. We just need think of what is an acceptable level."
Exactly how the lobsters, urchins and kelp are managed is something that will be considered at this year's Senate inquiry.
