Controlling Crown-of Thorns Starfish
© Great Barrier Reef Marine Park Authority 1995
The application or use of the information contained herein shall be at the sole risk and responsibility of the user. The Great Barrier Reef Marine Park Authority does not provide any warranty or assurance as to the accuracy or suitability of the whole or any part of the information, for any particular purpose or application. Subject only to any contrary non-excludable statutory obligations neither the Great Barrier Reef Marine Park Authority nor its personnel will be responsible to any party using this information, or any other person claiming through that party, for any consequence of its use or application (whether in whole or part).
National Library of Australia Cataloguing-in-Publication data:
ISBN 0 642 17421 0.
1. Crown-of-thorns starfish - Queensland - Great Barrier Reef.
PO Box 1379
Telephone: (07) 4781 8811
When are Controls Necessary?
The crown-of-thorns starfish (Acanthaster planci) is a natural inhabitant of coral reefs throughout the Indo-Pacific Region. 'Normal' (non-outbreaking) densities of starfish range from about 1 per hectare (an area 100 m x 100 m) to 15 per hectare depending on the amount of coral cover (food) available. When in low densities the damage to corals as a result of the starfish's feeding can be sustained with no apparent long-term damage to the reefs. While individual starfish consume between 5 m2 and 13 m2 of coral a year, they prefer the faster growing Acropora coral species. This means that damaged corals are fairly quickly replaced.
According to some theories the feeding starfish may help to maintain high diversity on coral reefs by allowing other species to colonise space created when corals are killed. Because this is a natural process the Great Barrier Reef Marine Park Authority (GBRMPA) does not recommend that starfish be killed when their numbers are low.
When the numbers of A. planci exceed the capacity of the reef to cope with feeding by the starfish, the damage can be dramatic and recovery can take a long time. The 'flip' from non-outbreaking to outbreaking populations is usually very obvious, involving perhaps a 10-fold (or more) increase in starfish numbers. In severe outbreaks the starfish will eat most corals, including massive forms such as brain corals which can take decades to centuries to recover because of their slow growth rates.
The causes of outbreaks are still unknown despite considerable research into the problem. Outbreaks almost certainly occur naturally, but they may be made more frequent or more intense by human activities (most likely through effects on water quality and perhaps overfishing of their natural predators).
GBRMPA has a general policy of not interfering with natural processes. The policy regarding crown-of-thorns starfish is that, unless it can be proven that outbreaks are caused or exacerbated by human activity, controls should be limited to small-scale tactical measures in areas important to tourism or science. A permit is needed to conduct crown-of-thorns starfish controls on the Great Barrier Reef.
Despite numerous attempts throughout the Pacific Region, no large-scale control programs (involving more than 20,000 A. planci) have successfully protected corals or eradicated the starfish. The main reasons why large scale control programs failed included the high cost; long delays in getting the programs underway; inefficiency of currently available control methods which involve treatment of individual starfish; movement of starfish into cleared areas; and early termination of programs.
To give control programs the best chance of success the following points need to be considered:
The Cost of Controls
Costs of control programs vary enormously, depending on the situation (area to be protected; distance from provincial centres; land- or shore-based; vessel availability and requirements; number of divers involved; labour costs; depth, number and dispersion of the starfish etc.).
On the Great Barrier Reef costs of controls ranged from about A$0.5 to A$40 per starfish. A survey of the area to estimate the number of starfish and to assess their extent of aggregation and location will help to establish the likely cost of the program.
Control programs are rarely once-off operations. Because starfish will move into cleared areas and new individuals will settle on the reef over time the effort must be sustained, in some cases for several years. Preservation of a limited area (2-4 hectares) of reef on the Great Barrier Reef might cost between A$20,000 to A$200,000 over 3 years.
Labour and Organisation
Three categories of personnel have been used to conduct control programs: bounty collectors (where people are paid an amount for each starfish collected), volunteers and paid staff. Generally the bounty system has been the most efficient method for large outbreaks but it has not worked well for smaller outbreaks. It has been most successful when the area is accessible by road (or transportation is provided) and the starfish are close to shore in shallow water where they can be collected by wading or snorkelling. Bounty collectors are usually not thorough and they concentrate on large aggregations which provide the quickest return. When starfish numbers are lower or the starfish are dispersed, collection becomes an uneconomical proposition for the collectors even though there may be enough starfish to continue to damage the reef.
Volunteers have been successfully used in a number of situations but they tend not to be particularly efficient or thorough. Inexperienced divers typically don't have the necessary discipline and long-term commitment and their 'kill rates' are usually much lower than professional divers. Trained Armed Services personnel have been involved in control programs on the Great Barrier Reef to overcome these problems. Although volunteers may be less costly than paid staff in the short term, they still require substantial support and, because they are less efficient, there is a greater risk of the program failing.
For local controls, paid divers are likely to be the most efficient and effective option. The biggest problem is recruiting a sufficiently large team quickly to eliminate the starfish before they cause significant damage. Someone must take responsibility for organising and supervising controls. This may be the operator, a dive master, boat skipper or a contracted diver. In all cases it is extremely important to maintain continuity of the supervision (and keep accurate records) to avoid duplication of effort or missing large numbers of starfish.
A variety of techniques have been used to kill crown-of-thorns starfish. All involve treatment of individual starfish. Injection with poison is the most cost-effective.
Injection of Poison
This is the most efficient technique, taking just a few seconds to inject each starfish. Rates of treatment of up to 120 starfish per diver hour have been recorded during trial control operations.
Until recently, the recommended poison was copper sulphate (CuSO4), also called 'blue stone'. It was recommended because it was effective, inexpensive, widely available and safe when handled correctly.
Other poisons that have been used include formalin; concentrated aqua ammonia solution and hydrochloric acid. These aren't as effective as copper sulphate, they tend to damage the injection guns and they are usually more hazardous to use (particularly in a rocking boat). Getting the dose right was critical - using less or more than the recommended doses resulted in fewer starfish dying from the treatment.
The main concern over using copper sulphate was the potential for heavy metal pollution in areas where control operations were conducted. Even relatively small doses of heavy metals can affect the health of reef animals and plants and with sufficient exposure result in death.
Trials of alternative poisons by GBRMPA staff resulted in the identification of a commonly available swimming pool chemical 'Dry Acid' (sodium bisulphate) as the poison of choice for killing crown-of-thorns starfish. It is widely available, inexpensive (about $4 per kilogram), considered to be safe to handle (when manufacturer's instructions are followed) and, most importantly, it eventually breaks down in seawater to completely benign components (it won't harm anything).
Cutting starfish into a number of pieces was one of the first methods tried, however its use was stopped because of concerns that the pieces would regenerate creating an even bigger problem. Some starfish (like the blue starfish Linckia) do regenerate entire bodies from substantial pieces but the survival of A. planci after having been cut into separate quarters in the field appears to be very low. Tests of the effectiveness of this method were conducted by GBRMPA staff. When starfish were cut into quarters or their central disk was removed, most of the pieces were still alive after 2 weeks. We don't know if these remains would have regenerated but it seems unlikely.
The biggest problem with this method is that the starfish have to be extracted from the coral to be treated. This will usually damage the corals as the starfish can be entwined securely around branching forms. The risk of a diver being spiked is quite high (see section on First Aid). Because poisons are not involved the method has the advantage of being environmentally 'friendly' (as long as coral is not damaged in extracting the starfish) and special equipment is not needed.
Trials conducted by GBRMPA showed that this method is 10 times slower than injecting poison and the diver doing the cutting up was spiked 3 times during the treatment of 20 starfish.
Removal and Burial Ashore
Strong sharpened sticks, barbeque tongs or a hooked steel rod are best for pulling starfish out from under corals. The collected starfish can then be taken to a strategically placed floating or sunken bin for transfer to a small boat. Long-handled barbeque tongs are best for carrying starfish (above- and under-water). Because of the multiple handling of each starfish removed, manual removal is highly inefficient and there is a high risk of serious spiking of the divers and people involved in the transfers in and out of the boat.
The need to continually remove starfish that move into cleared areas of reef adds significantly to the cost of control programs. Research conducted for GBRMPA found that the most effective barrier to crown-of-thorns starfish was a fence made of rigid steel wire mesh (12.5 mm square), 1 metre in height and having a horizontal 60 cm overhang at the top. Materials for this fencing cost about A$10 per metre.
Fences have not been tested on a large scale but they could provide an answer to the problem of adult starfish moving into cleared areas. However, there are a number of problems with the fencing approach:
It is important that the area in which there is a problem, and the surrounding reef, are surveyed regularly during an outbreak to determine the scale of the problem and the success of control action. Manta towing is the quickest way of seeing a large section of reef. A snorkel diver holds onto the grips of a manta board which is towed 5-10 m behind an outboard powered dinghy (rubber dinghies are best) at a speed of about 1-2 knots. Because of rapid ascents when manta towing, never use SCUBA. The diver can steer and dive by turning the manta board to carefully scan the bottom to note the number of crown-of-thorns starfish and the amount of white recently dead coral. The dinghy operator should stop after 2 minutes of towing (a distance of about 150-200 m) and record the path on an aerial photograph of the reef (laminated in plastic to make it waterproof). The diver should record (or tell the dinghy operator who then records) the number of starfish seen, and the amount of dead coral. Because of the speed of the diver's movement and the tendency of starfish to hide, probably less than 5% of the adult starfish in an area will be seen using this technique. Juvenile starfish (smaller than 15 cm diameter) are very rarely seen.
The manta tow technique works reasonably well for continuous areas of reef but not for patch reefs or bommie fields in back reef areas or where the water visibility is less than 10 m. Where the reef is very patchy, short (20 minute) spot SCUBA dives are best. The number of starfish and an estimate of the amount of recently dead coral should be recorded during or immediately following the dive.
A combination of manta towing and spot dives will probably give the best description of the situation. Manta towing can be used to locate the aggregations of starfish and spot dives can be used to get a good estimate of the number of starfish at those sites.
If the control program is to be successful, nearly all of the crown-of-thorns starfish have to be removed from affected areas. Divers searching for starfish must swim very close to the bottom, paying special attention to crevices, cracks and overhangs where the starfish may be hiding. While doing this it is very easy to get disoriented and lose track of where you've been unless the area is quite small. Systematic searches will add considerably to the effectiveness of the program. Searches should be repeated to minimise the number of starfish missed.
Having a large-scale map or aerial photograph will help in recording the areas checked for starfish. These can be obtained from the Australian Surveying and Land Information Group (AUSLIG) (Freecall telephone number 008 800 173) or from GBRMPA. Where the affected area is mainly patch reefs several divers should work around the sides of the patches at different depths. One diver could search say 0-3 m, another diver 3-6 m and so on to cover the full height of the reef. Obvious white feeding scars indicate that a starfish is close by and can be used to locate hidden starfish.
Where the reef patches are large or where the reef is continuous it is best to lay down weighted ropes so that defined areas between the lines can be searched systematically. The ropes should be about 2.5 m apart and marked with a buoy for easy location. Although this preparation involves extra time and effort it will help to ensure that few starfish are missed and the program is successful.
A permit from GBRMPA and the Queensland Department of Environment and Heritage is needed to conduct crown-of-thorns starfish control programs on the Great Barrier Reef. Permit application forms can be obtained from either of these organisations. Applications should state that the activity is not for commercial purposes (or else it will attract a processing fee) and it should be lodged several weeks prior to when the activity will be undertaken.
Permit holders are required to keep a log of the numbers and sizes of starfish killed or collected. The following additional information may help in developing more effective and efficient local control techniques:
The spines of the crown-of-thorns starfish are very sharp, about 5 cm in length and prone to breaking off in wounds. The covering of the spines produces a venom which is deposited in the tissues on penetration. Skin penetration by the spines is painful and can lead to secondary infection and continued swelling. The pain is usually immediate, very severe and it may persist for a few hours. Vomiting may commence about one hour after the injury and recur every few hours for the next few days. Localised allergic reactions may also occur in susceptible individuals. Repeated spikings (even if months or years apart) may result in progressively more severe reactions.
Birkeland, C. and Lucas, J.S. (1990) Acanthaster planci: major management problem of coral reefs. CRC Press, Boca Raton, USA. 257 pages.
Engelhardt, U. and Lassig, B. (1992) Crown-of-Thorns Starfish Research Update 1991/92. Great Barrier Reef Marine Park Authority, Townsville, Australia. 19 pages.
English, S., Wilkinson, C. and Baker, V. (1994) Survey manual for tropical marine resources. ASEAN - Australian Marine Science Project: Living Coastal Resources. Australian Institute of Marine Science, Townsville, Australia. 368 pages.
Moran, P.J. (1986) The Acanthaster phenomenon. Oceanogr. Mar. Biol. Ann. Rev. 24: 379-480.
Zann, L.P. and Eager, E. (1987) The crown-of-thorns starfish. Aust. Sci. Mag 3: 14-55.
First aid treatment is the same as that given for all venomous fish stings:
Note: Do not apply an arterial tourniquet, nor a pressure or immobilisation bandage.
Although there is no physiological explanation, a widespread traditional remedy for spinings is to hold the starfish's lower surface (with the suction feet) against the wound. This allegedly relieves pain and disability.
Sodium bisulphate, or 'dry acid', is the best choice. Add about 1/3 cup (140 grams) of the chemical to each litre of seawater. One litre of the solution is enough to kill about 40 adults starfish. Because the solution is colourless and difficult to see underwater, add a food colouring as well.
The mixture can be injected using a standard agricultural injection gun. In Australia, the DuPont Veldspar Spot Gun fitted with a longer 50 cm needle and 5-litre plastic bladder is recommended. To inject the starfish, set the dose meter on the gun to 2 ml. Push the needle under the skin of the central disk of the starfish and pull the trigger. Inject the starfish three times this way.