Controlling crown of thorns

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Controlling crown of thorns

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.

Success of Controls

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 program needs to be of a realistic scale - about 2 to 4 hectares is the maximum reef area that can be protected using the available manual control techniques (the size of the area that could be protected will depend on the resources available);

  • Adequate funding and resources must be allocated for the task;

  • The program needs to be initiated as soon as possible after an outbreak has been detected;

  • Areas should be surveyed before controls are started to gauge the extent of the problem and the likelihood of other starfish moving into cleared areas (GBRMPA can provide advice on survey techniques and we may also be able to organise for surveys in your area);

  • Surveys should be conducted regularly to monitor the success of the program;

  • Cleared areas must be visited regularly (initially every 4-5 days) to 'mop up' starfish that were missed and immigrants to the area; and

  • There must be a commitment to continue the program until starfish densities are reduced to below the area's capacity to cope with the numbers present

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

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). –details in appendix 1

Cutting up

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.

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.

Coral reefs likely to get cot infestations when coral reefs are stressed

What sorts of things stress reefs? People blast reefs with dynamite, and the crown of thorns, and even their swimming larvae, are attracted to the metabolytes reseased by damaged corals. People break coral when walking on reefs, to get shells for tourists, to get coral rocks for building material.  Reefs are overfished. At least two species of large fish eat the crown of thorns. They are gone from most reefs near people. People poison the reefs with various chemicals to kill or stun fish. Agricultural chemicals applied to island gardens or used to control mosquitos wind up in the sea, and in the corals. This weakens the corals. When the crown of thorns eats the coral they accumulate these chemicals in their own tissues (it does not seem to harm them, the larvae actually survive better with DDT in the water). The predators of the crown of thorns, including triton shells and a beautiful little shrimp, may die or encounter breeding problems from the concentrated poisons in the starfish's flesh. Increased concentrations of carbon dioxide in the sea water reduces calcium deposition, slowing coral growth.  

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