Tackling the long spined sea urchin
The incursion of the long spined sea urchin
Centrostephanus rodgersii into Tasmanian waters in the past 20 years represents a significant range extension for this species. Unchecked, the urchin’s presence represents a risk to the ecological balance of the
important Tasmanian rocky reef ecosystem on the East Coast.
Video courtesy of Institute for Marine and Antarctic Studies
Options such as eradication are simply not achievable. Rather, a multi-pronged approach to mitigating the urchin’s impact is more likely to be effective. However, there is no 'silver bullet' identified for this problem.
How did it get here?
The long spined urchin has undergone range extension southwards from its native habitat in New South Wales waters over the past 20 years. It has increasingly become established in Tasmanian waters due to warming waters off the East Coast and changes to the East Australian Current.
(There is also a native species of sea urchin in Tasmania -
how to tell the difference.)
Why is it a problem?
Fisheries that rely on healthy rocky reef habitat are very important to the Tasmanian community from an economic and social perspective. The community has an expectation of pristine reef systems.
However, once established on a reef, the long spined urchins graze on kelp and other marine plants, forming bare patches called ‘incipient’ or emerging barrens.
Once an incipient barren is created, if urchin numbers continue to increase the level of overgrazing on kelp may completely denude the reef of all seaweed, forming an ‘extensive’ or established barren.
Barrens significantly impact on the biodiversity of rocky reef habitat and important recreational and commercial fisheries particularly abalone, rock lobster and some scalefish species.
Large rock lobsters are one of the few natural predators of the long spined urchin. However, lobster stocks have declined on the East Coast due to periods of low recruitment and fishing activity.
A combination of climate change and depleted lobster stocks are important factors contributing to the expansion of long spined urchin barrens.
What is known about urchin barrens?
A recent report on the
IMAS long term study of long spined sea urchins on reefs along Tasmania's East Coast by the Institute for Marine and Antarctic Studies (IMAS) has measured increases in the urchin population and the barren areas they create by overgrazing kelp beds.
Urchin barrens can extend over hectares of the seabed and in depths up to 60 plus metres. Peak barren formation occurs in a depth range of 20 - 30m.
There is some baseline data of the abundance and distribution of long spined sea urchin barrens. In particular, systematic surveys were conducted in 2001/02 to map distribution and abundance and some of these areas were then re-surveyed in 2008/09.
Now, this work has been repeated to monitor changes and trends. The abundance of long spined sea urchins and the extent of their impact on kelp beds in eastern Tasmania was re-surveyed by IMAS divers and underwater towed-video in 2016/17 - and assessed relative to the baselines established in 2001/02.
Is the problem getting worse?
The latest IMAS survey estimates that the biomass of long spined urchins in 0 - 18m depth is around 3,276 tonnes with major barren formations in the vicinity of St Helens, Freycinet, Schouten Island and Maria Island.
Smaller barrens are currently forming in the Eddystone Point, Four Mile Creek, Bicheno and the Tasman Peninsula regions.
Data from a towed video survey in 18 - 30m depth range suggest a similar biomass level to inshore or a total long spined urchin biomass of around 6500 tonnes.
What management action is being taken?
A multi-pronged approach to tackling the long spined sea urchin problem is in place with a range of future options also being considered. Current and past research data is available to underpin any future management actions that will be undertaken and this information base is expanding rapidly.
Reducing the risk of the ongoing destruction of healthy kelp beds and the rehabilitation of established urchin barrens back to healthy kelp beds are fundamentally different problems requiring different solutions.
Total eradication of the long spined urchin is not considered possible. However, without active intervention, urchin barrens are predicted to continue to increase.
There are a number of different activities or programs in place now and a range of options that could be considered in the future. The IMAS report also provides some advice on possible actions that might be considered. Encouragingly, the IMAS report indicates that actions can be taken to further tackle the problem. The report notes that continued proactive management of urchin overgrazing is required and that the ‘…observed annual increase in tonnage of urchins has been of a scale that control, such as by rebuilding of predators and upscaling of culling and/ or harvesting would appear plausible’.
The multi pronged sea urchin strategy has three key objectives:
- Stop growth of existing barrens
- Prevent establishment of new barrens
- Promote recovery of full barrens
Strategies to achieve the objectives
The severity of the threat posed by long spined sea urchins to Tasmania's rocky reef ecosystems and consequently to the fisheries, tourism operations and local communities who depend on the natural balance being maintained requires the urchin to be tackled with an array of weapons.
- a biological approach such as the re-building and translocation of rock lobster stocks;
- market based strategies such as subsidies and the development of new domestic and overseas markets; and
- a physical approach which includes divers smashing and harvesting the pest.
All these strategies are currently being employed and are having an impact. There are also a number of research projects which are showing promise for future control options.
Liming is a real possibility following its successful use in controlling sea urchins overseas. These overseas cases have shown builders lime has a devastating affect on the urchins when only a few grains are applied to the test (shell inside the spines). However there will need to be extensive testing under local conditions before any Tasmanian ecosystems are exposed to the treatment.
The urchins are also being assessed for their effectiveness as a fertiliser which is showing some promising results while yet another project is evaluating urchin waste for pharmaceutical applications including the development of a anti-coagulent. These varied applications all make taking the urchins a more attractive proposition for divers and processors alike.
The mechanical approach has not been forgotten with robots being developed to help mitigate the problem. It is hoped that these robots will be able to go where divers cannot, helping to control the problem by operating continuously at all depths.
Below is a brief description of some of the more prominent tools currently being used to control the incursion.
On extensive barrens, kelp will only be restored by ‘direct human intervention’ such as culling. Predators such as large lobsters are unlikely to reduce urchin numbers to the point where the seaweed can re-establish itself. In other areas including incipient barrens, commercial harvesting can be an important tool.
It was initially thought there were no markets for long spined sea urchin roe as the roe was considered bitter and unpalatable. Market interest in the product has now increased and a processing technique has been developed which shows promise for market development in Asia.
The live weight tonnage of product sold has gradually been increasing, reaching 97 tonnes in 2013/14. With the incentive of a subsidy provided by the abalone industry in 2017/2018 there has been increased interest from processors resulting in a harvest of 190 tonnes in 2017/2018 season. The 2018/2019 season is shaping up to be even more productive for the commercial fishery with over 114 tonnes having already been taken in just the first couple of months of the season.
Long spined sea urchin processing at St Helens
Ongoing commercial harvesting is likely to be an important ongoing activity to reduce urchin numbers. The need for a continued subsidy is an issue that will require further consideration.
East Coast Stock Rebuilding Strategy
The East Coast Stock Rebuilding Strategy commenced in 2013 and aims to rebuild lobster stocks from less than 10% in some areas to greater than 20% of an unfished or virgin fishery by 2023, by limiting the total catch (commercial and recreational) each year from an area between Eddystone Point and Bruny Island
More information about the
East Coast Stock Rebuilding Strategy
What role can large lobsters play?Large lobsters at sufficient density are able to limit urchin populations effectively in healthy kelp beds to help minimize risk of barrens formation. Large lobsters are also able to limit urchin populations on incipient barrens, facilitating recovery of seaweeds and minimizing risk of further expansion of barrens.
Increasing rock lobster biomass alone will not result in the recovery of extensive urchin barrens. While large lobsters do prey on urchins on extensive barrens which has been shown to significantly reduce urchin densities, they are unable to reduce urchin numbers to the point where kelp and other seaweed can re-establish. Even if extensive barrens are closed to lobster fishing indefinitely and populations of large lobsters on barrens are boosted by transplanting at realistically achievable levels, the likelihood of recovery of kelp is negligible in the absence of any other mechanism to reduce urchin density. On extensive barrens, kelp will only be restored by additional intervention, for example by removing urchins using divers, quicklime, or robots.
Once urchins are at sufficiently low densities to allow reef habitat to recover, maintaining populations of large lobsters in regenerated kelp beds is likely to be sufficient to prevent return to urchin barrens.
Rock Lobster Translocation Program
The stock rebuilding strategy is also supported by the East Coast Translocation Program. Under this program, undersize lobsters from slow-growing deep-water areas in the South West are translocated to inshore east coast locations including known incipient barren areas.
Read more about the
Rock Lobster Translocation Program
Culling by smashingCommercial divers and abalone divers have been culling long spined urchins during the conduct of their normal fishing activities on an ad-hoc basis or as part of research projects over the last 10 years. This is another ‘direct human intervention’ tool that can help protect small specific patches of reef from turning into extensive barrens.
Culling of long spined urchins can be undertaken by commercial divers
In terms of other forms of physical intervention, trials have shown that urchins can be culled from discrete areas and that regrowth of kelp may occur in that area. These removals are more successful if at least two episodes of removal are conducted – and may require further ongoing efforts for long term removal.
However, such programs would seem practical in only small or key areas due to the resource intensive effort required. Other forms of voluntary action may also have some impact.
The utility of developing a management framework for recreational dive groups to undertake similar ‘protect a patch’ activities may be considered in the future.
To assess the effectiveness of current and novel mitigation methods, future surveys of urchin density and barren coverage would be required. The effectiveness of fishing/ mitigation measures may also benefit from fine-scale spatial mapping of effort via GPS and depth loggers and/ or mobile web-applications.