Every year, a supercharged ocean current due to climate change brings destructive and spiny species south from New South Wales to the rocky reefs of eastern Tasmania.
Long-spined sea urchins are bad news for Tasmanian marine life, as this species can quickly gnaw through swaying kelp forests to bald rocks, forming underwater moonscapes known as urchin urchins.
Seaweed is an important food and habitat for stony reef animals, so when urchins create a barren wasteland, it’s like a vibrant patch of rainforest.
“Because it is a focal species and can alter coral reefs, it is potentially the single largest threat to coral reefs in eastern Tasmania,” says marine ecologist Scott Laing, associate professor at the Institute of Marine and Antarctic Studies.
“The long-spine sea urchin population has been increasing in Tasmania and has been since the early 1990s, reaching an estimated 20 million by 2017,” said Dr Ling.
For 14 years, researchers at the institute have conducted important experiments to slow the effect of sea urchins.
The fishery is trying to find a positive side by making a profit from the restaurant market wanting to buy sea urchin roe.
Yet their numbers are still growing.
The Senate inquiry into sea urchins, initiated by Senator Peter Wish Wilson, will hold its first public hearing in Hobart on Friday, and experts will discuss how to better manage the species at a Tasmanian government workshop this week.
The East Coast is facing a double whammy
The east coast of Tasmania is a climate change hotspot. It is warming at four times the global ocean rate.
This is due to the East Australian Warm Current (EAC) extending from the coast of New South Wales.
“It’s getting stronger because of the warming of the atmosphere over the Pacific,” explains Greta Bickle, director of the Center for Marine Environmental Sociology.
Professor Pecl said: “As the atmosphere warms, it literally rotates with the wind. This physically forces the current more and more onto our coasts.”
“It is the current that drove Nemo’s father down the coast in [animated] film.
“So the East Coast gets a double whammy, it’s getting the underlying warming that most of the rest of the ocean is getting, and it’s getting that change in the existing system.”
Just like Nemo’s dad, sea creatures navigate the current, which means Tasmania is visited by a host of northern species.
This supercharged warming has been linked to the movement of about 100 species that either arrived in Tasmanian waters or expanded their range southward down the east coast of Tasmania. There is even a website where fishermen and divers can record these observations.
Not all northern species are greeted with dread.
“King snapper and yellowtail coming to Tasmania are excellent for recreational fishing opportunities,” said Professor Pecl.
But the effects of long-spined sea urchins, which ride the current as tiny planktonic larvae, “equivalent to invasive species.”
The protective power of lobsters
Mitigating the impact of hedgehogs is difficult, because every year the EAC will drop a new batch of hedgehog larvae.
For more than 14 years, Scott Laing and colleague Craig Johnston have been donning wetsuits and diving in Tasmanian waters – to understand the changing ecology of coral reefs.
Their goal is to see if lobsters — predators that feed on urchins among other things — can help keep the spiny creatures in check.
They set up testing sites along the east coast of Tasmania.
In St Helens in the north, urchins have already greatly affected the seascape, while near the Tasmanian peninsula in the south of the state, small cleared patches of kelp are more like “spot fires”.
Giant lobsters were moved to the trial sites and harvests were restricted.
The results were then compared with nearby ‘control’ sites where no action was taken.
In the south, where fishing continued and lobsters were not introduced, there was an increase in the numbers of hedgehogs and the size of locusts.
But, when lobsters were added and protected, the number of hedgehogs decreased and stopped increasing.
“It really showed that if we initially had large numbers of predatory lobsters on the reef, we could really mitigate the risk of urchins taking over,” said Dr. Ling.
In the waters off St. Helens, where the damage was already extensive, the increase in crab populations had no effect on the size of the pens.
“Fifty crayfish per hectare of kelp-dominated coral reefs could keep urchins from overgrazing,” he said.
This indicates that when it comes to managing hedgehogs, prevention is more effective than cure.
The good thing is that hedgehogs are delicious
Lobsters aren’t the only ones that eat long-spined sea urchins.
Hedgehog fish are also a hit on human menus—so commercial harvesting helps keep the population down.
Sea urchin reproductive structures, often called roe or monoe, are eaten all over the world, including in Tasmania.
Between 400 and 500 tons [of urchins are harvested] “Every year, that’s the equivalent of removing more than a million urchins,” said John Kane, a fisheries expert at the Institute of Marine and Antarctic Studies.
Harvesting is hard work. Divers hand-pick their spiky coral quarry.
It is not commercially viable to harvest in areas with low numbers of hedgehogs, or to harvest young hedgehogs, so commercial harvesting cannot be relied upon to control wild populations.
Government-funded “take all” crops were recently tested and are scheduled to continue next season, which may help prevent hedgehog snails from forming.
The answers are in our hands
No matter how many urchins have been removed by lobsters or by divers, they keep coming back, making a trip on the EAC.
Dr. Kane explains that if the zealous divers removed all the hedgehogs, they risked shutting down the company.
“After 10 years, the hedgehogs and raptors will be back and then we don’t have a major control mechanism of fisheries… We’re back to square one,” he said.
“This is the delicate balance you’re fighting with at the moment.
“The bulldozers are not going away. We just need to think about the acceptable level.”
How exactly lobsters, urchins and kelp are managed is something that will be considered in this year’s Senate investigation.