The Science

Sea urchins & biodiversity

Go snorkelling or diving off most of the coast of south east Australia and you’ll hopefully find a world of undulating seaweed beds rich in marine life.  Rock lobsters, sea stars, sea cucumbers and thousands of fish species live in these biodiversity hotspots. 

Or you may find yourself floating over an ‘urchin barren’ – a starkly different place where the kelp and seaweed beds have been removed and a rocky platform covered in spiny sea urchins remains.

“The equivalent is taking a bulldozer into a rainforest and razing it back to bare earth,” says Professor Craig Johnson, marine scientist with University of Tasmania and one of the principal research scientists involved with ‘Explore the Seafloor’

“Primary production drops by about 100-fold, and the habitat changes massively with a crash in biodiversity and a crash in local fisheries production, so it’s a very significant issue.”

The culprit is a sea urchin called Centrostephanus rodgersii –  otherwise known as the long-spine sea urchin.  It’s a native species of NSW that also occurs in New Zealand but over the last few decades its range has extended southwards into the waters around Tasmania.

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Centrostephanus rodgersii are advancing on areas around Tasmania, sometimes with devastating impacts. (Credit: Craig Johnson)

Marine herbivores

Sea urchins are typically just one of the species living in a seaweed bed.  They’re herbivores that use their five massive teeth to scrape away at kelp and other types of seaweed.

However, not all sea urchins have the same appetite.  The invasive Centrostephanus are big – with their spine canopy they can get up to around 30 cm diameter – and their appetite is also large.

Centrostephanus can eat seaweed like it’s going through a threshing machine,” says Johnson.

“In NSW where Centrostephanus occurs naturally, about 50% of the shallow reefs are Centrostephanus barrens.  We’d be devastated if the east coast of Tasmania ended up with 50% urchin barrens.”
 
While both Centrostephanus and the native urchin -  Heliocidaris -  have the capacity to overgraze seaweed beds,  Heliocidaris tends to  form smaller barren areas in sheltered waters, while Centrostephanus can form  extensive urchin barrens on rocky reefs in very exposed locations.

What makes extensive urchin barrens even more of a problem is that they’re almost impossible to return back to seaweed beds. 

Sea urchins have the astonishing ability to modify their diet.  After they’ve cleared out the kelp and other seaweeds they’ll scrape microalgae off the rocks or tuck into any drift seaweed they can find. They can survive on very little and actually thrive on the urchin barrens, making them very hard to shift.

“Once you get a big urchin barren you get to keep it for a very long time,” says Johnson

The East Australian Current and rock lobsters

The Centrostephanus urchin has become a problem around Tasmania because of two interdependent factors – changes to the East Australian current (the EAC) and commercial fishing of rock lobsters, the natural predators of sea urchins.

The EAC – made famous in ‘Finding Nemo’ – is the western boundary current of the South Pacific Gyre – a great big circulating mass of water that stretches from Australia to the west coast of South America.  As a result of climate change, all the gyres in the Southern Hemisphere are starting to spin faster, which means the currents run harder and faster.

“The influence of the western margin has increased southwards, it’s a bit like turning the hose on a bit harder, the jet forces further out into the swimming pool,” says Johnson.

The main flow of the EAC doesn’t come down to Tasmania, but leaves the Australian coast off southern NSW towards the top end of New Zealand, and off that big ‘corner’ off NSW it sheds eddies which sends EAC water southwards. In recent decades there have been more and larger eddies spinning off towards Tasmania.

“The EAC is a big highway that’s not only transporting heat, but it’s also transporting larvae of urchins and a whole lot of things,” says Johnson.

South east Australia and the east coast of Tasmania are warming at around 4 times the global average, making it, literally, a global hotspot.  Larvae that are transported southwards find it easy to establish new populations in the warmer waters.

“Since about 1980 there have been more than 40 species of fish establish populations on the East coast of Tasmania … populations that are reproducing. They might have previously come down as occasional vagrants but they certainly weren’t established populations, and now they are,” says Johnson.

One of the species on the list of new species in Tasmania is Centrostephanus – the first one was officially observed off St Helen’s in north east Tasmania in 1978.  Now, Centrostephanus has established populations right down the east coast and there are occasional populations along the south coast and even around as far as Port Davey in the west.

The other piece of the urchin barren puzzle is the rock lobsters.

Large rock lobsters are the key predators for the Centrostephanus sea urchin.  If large and plentiful enough, they’re more than capable of keeping a lid on the urchin population so that it doesn’t build up to the point that it forms urchin barrens.

“If the urchins were just in Tasmania and they were just another species in a healthy kelp forest no one would worry,” says Johnson, “The real problem is not that the urchins are here, the real problem is the urchin barrens. “

You can only get a Centrostephanus barren in Tassie through an interaction between climate change and ecological overfishing of large lobsters.

“The lobster populations are not overfished in the sense that there is likely to be a collapse of the fishery but they are overfished in the sense that we’ve lost ecosystem function,” says Johnson.

“If we just had the climate change but there was no overfishing of lobsters, we wouldn’t have a problem because the sea urchin would be here but it would be controlled by big lobsters. If we just had the fishing but not the climate change it wouldn’t be a problem because the urchin would still be restricted to NSW. It’s only when you get those two things together that we’ve got a problem.”

In areas around Tasmania where there are really big rock lobsters, such as in the marine protected areas where fishing isn’t allowed, there are no urchin barrens.

Managing the problem

Assessing the extent of the urchin barrens around Tasmania is an important first step in understanding how to solve the problem – and this is one aspect that Explore the Seafloor is assisting in.

“At the moment we don’t think the barrens go into those deeper waters.  If it turns out they do there is even more urgency in working out some sort of management solution in terms of what to do about the urchins,” says Johnson.

Controlling urchins at low levels in healthy seaweed beds is much easier than controlling them across massive barrens where there are huge numbers of urchins, so it’s also important to pinpoint emerging barrens. 

The only viable management response to the urchin problem on large scales and in deeper water is re-building populations of big lobsters, which means making difficult decisions around the valuable rock lobster fishing industry. 

“So, it’s much better that you make those decisions on the basis of really sound knowledge rather than what may or may not be,” says Johnson.

“We’ve got reasonable data for divable depths but in those deeper depths we don’t have sufficient data.  So analysing photos from deeper water habitat is just part of building the picture so we get greater and greater certainty.”

While placing restrictions on rock lobster fishing may sound like an extreme management response, the flipside is that once large urchin barrens are established biodiversity drops dramatically, and this may have a significant impact on other commercial fisheries that rely on seaweed beds in one form or another.

By Kylie Andrews

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