The long-spined sea urchin (Centrostephanus rodgersii) is on the verge of a population explosion that will see it cause lifeless ‘barrens’ in the biodiverse reef habitats across large areas of Tasmania’s east coast.
Large rock lobsters are the only effective natural predator of sea urchins, but over-fishing has left too few of them to be an effective control in many areas. Critically, lobsters need to be substantially larger than the legal size before they can flip the urchins on their back and open up their soft underside. The legal catch size is 110 mm for males and 105 mm for females but lobsters need to be at least 140 mm to kill a mature urchin.
If we are to slow down the spread of this sea urchin and save our east coast reefs the state government must take decisive action urgently.
The Institute of Marine and Antarctic Studies (IMAS) released a report in December 2018 that shows that the long-spined sea urchin has exploded in numbers and that an average of 15% of reef habitat has been lost in the 4-40 m depth range on the east coast from Tasman Island to Eddystone Point. The report shows that, in some parts of the north east coast, 50% of reefs have already been destroyed. In 2018 IMAS produced modelling that predicts that 32% of east coast reefs will be destroyed by 2021 and that with no action an average of 50% of east coast reefs may be lost to urchin barrens in the longer term.
The sea urchins eat the critical kelp and sea weeds, the equivalent of eucalypts on land, leaving ‘barrens’ which are largely devoid of other species.
The creation of barrens causes a permanent loss of marine biodiversity as well as removing habitat for rock lobster and abalone which are prized by commercial and recreational fishers. The sea urchins can survive by eating micro algae and maintain the lifeless barren. Any regrowing kelp or seaweed are quickly eaten by the urchins.
In the 1970s this sea urchin was very rare on Tasmania’s east coast but warming waters in recent decades has made it possible for it to breed more successfully. In the presence of sufficient numbers of large rock lobsters the urchin is held in check. This problem has arisen as a result of there being insufficient rock lobster of sufficient size to predate on the sea urchin.
The rock lobster fishery has been severely mismanaged. Fortunately the government has begun to take steps to develop a strategic plan to halt the spread of the sea urchin but it is acting too slowly and cautiously. The only reason the government has done anything is thanks to the work of Jon Bryan who is the TCT’s longest serving employee and has been working tirelessly with industry and government on this issue over recent years.
On 14 December 2018 DPIPWE held the ‘Centrostephanus Forum 2018’ at Blundstone Arena and Jon Bryan and Peter McGlone attended, being the only non-government conservationists present. Our biggest complaint with the forum was that it focused too much on controlling urchins to repair damaged areas — through commercial harvesting, culling, use of underwater robots (drones) to help with culls and liming of urchins – and too little on preventing the problem, most importantly by having sufficient density of large rock lobsters.
There was no speaker talking specifically about the importance of large rock lobsters, although several speakers addressed this issue. In contrast, there were six speakers addressing different ways of controlling sea urchins once they establish. It seemed to us that DPIPWE were determined to focus on a human technological response to controlling sea urchins while down-playing the vital role that lobsters play, as a biological control, in preventing urchins getting out of control.
Lobsters are the best strategy to prevent areas developing into urchin barrens and once there are enough large lobsters there is no need for human intervention. The other strategies are mostly limited to responding once urchins have begun to take over a reef and need to be repeated. The TCT does support efforts to treat barrens when they are in their early stages as well as preventing barrens.
DPIPWE have probably taken this stance because they did not want to give much attention to the impact fishing has had.
To enable enough lobsters to get large enough to be an effective control on the sea urchins will require additional restrictions on fishing as most are currently caught as soon as they get to legal size. This is why the translocation of lobsters from the west coast to the east coast is ineffective as most get caught before they get large enough.
The forum heard speakers talking about culling trials. Culling is where volunteers or paid people attempt to remove all urchins from a given area. A trial at Wineglass Bay succeeded but cost $1.6 million and will eventually need to be repeated. There was a report on the success of commercial harvesting trials which is building to a scale that could be very useful but it is limited to shallow waters, is hard to target divers to areas most important to control urchins and is dependent on government subsidies.
Robots are yet to be trialled in Tasmania, so it is hard to comment on this, other than to note that they can obviously go into deeper water for longer than human divers. If effective they will still need very expensive human support and maintenance systems and each robot will cost several hundred thousand dollars.
A Norwegian scientist spoke about their trials using lime to kill urchins. The lime is piped down to the sea floor and video monitoring is used direct the lime to get total coverage. This gets close to 100% kill rate and can address urchin barrens in deep water. The big problem raised by many people was the potential to kill many other species that we want to protect. Young abalone were considered to be at risk. Liming might be best restricted to areas where there are only urchins but too risky in areas where urchins are just establishing.
DPIPWE has released a forum proceedings document that includes a series of very disappointing recommendations for addressing this problem. The proceedings imply that DPIPWE alone will be drafting a Centrostephanus management strategy and broad stakeholder input may not be requested until the end of this year. After we questioned DPIPWE about this they informed us that they have contracted a leading CSIRO scientist, Dr Tony Westcott, to work with all stakeholders to develop a strategy. While this is a positive move and Dr Westcott has a very good reputation in developing strategies for these types of problems, DPIPWE is moving too slowly with a final strategy not expected until the end of 2019.
We are planning a series of public events and media publicity to build pressure on the state government. Look out for Facebook posts about these events and please come along if you are available.