Managing Coral Predators
Coral predators are a natural part of a healthy coral reef ecosystem. However, excessive densities of some corallivores, such as crown-of-thorns starfish (Acanthaster planci) and coral-eating snails (mainly Drupella spp. and Coralliophila abbreviata) can result in dramatic and widespread declines in coral cover. Loss of corals can have serious implications for reef industries — especially tourism — and coral reef managers may wish to consider options for controlling coral predators.
A range of techniques is available for removing or preventing the spread of corallivores, but these techniques are generally only feasible for local scale controls. For this reason, control of coral predators is normally only attempted at smaller scales (a few hectares or less), such as around high-value tourism sites. Some control programs have focused on sites thought to be key source reefs for outbreaks of coral predators, with the aim of reducing impacts across the wider system. This page introduces techniques and issues relevant to control of crown-of-thorns starfish and Drupella snails. While sea urchins can sometimes damage reefs, they are also often important for controlling algae and maintaining a healthy coral reef ecosystem; therefore sea urchin management is covered in a separate page.
Crown-of-Thorns Starfish (COTS)
Populations of the crown-of-thorns starfish (COTS) periodically boom, resulting in severe outbreaks (or ‘plagues’) capable of killing coral over large areas. While COTS outbreaks are likely to have occurred historically on some reefs, there is strong evidence that they are occurring more frequently and causing more severe damage in recent decades. Overfishing of COTS predators (including the triton snail, titan triggerfish) is likely to be a contributing factor, but recent analyses ref have provided compelling evidence that excess nutrients, leading to changes in plankton productivity, are the major driver of increasing outbreak frequencies. This has important implications for coral reef managers, suggesting that the most important long-term strategy for reducing the risk of COTS outbreaks is likely to be reducing land-based sources of nutrients through improved watershed management.
While watershed management might be the most important long-term strategy, the ecological and economic impacts of major COTS outbreaks have motivated coral reef managers and the reef tourism sector to develop and test methods for control of COTS during outbreaks. The following methods are effective at killing, removing, or preventing spread of COTS, although they have generally proven worthwhile only for protecting small areas (less than 4 hectares) of reef:
- Injection — Injecting bile salts or sodium bisulfate kills COTS within a few days and is non-toxic to other marine life. The single-shot method using bile salts is the most efficient technique, taking just a few seconds to inject each starfish. Rates of treatment of 5-6 starfish per minute using single injections of bile salts have been recorded compared to just one starfish per minute with sodium bisulfate. ref
- Manual Removal — Strong sharpened sticks, barbecue tongs, or a hooked steel rod are best for pulling starfish out from under corals. Collected starfish can then be taken to a strategically placed floating or sunken bin for transfer to a small boat. Because this process requires starfish to be handled multiple times, manual removal is highly inefficient and there is a high risk of spiking (i.e., getting pierced by the poisonous spines of the starfish) to divers and people involved in the transfers in and out of the boat.
- Underwater Fences — The need to continually remove starfish that move into cleared areas of reef adds significantly to the cost of control programs. Tests of underwater fences in the Great Barrier Reef indicated that certain designs can be effective, but they have not been widely adopted due to logistics and efficiency considerations. Fences may be useful in very small, very high-value locations, but are likely to need regular surveillance and monitoring.
- Emerging Techniques — A range of other techniques is being explored in the search for more efficient, broad-scale control techniques. None are currently developed enough to be field tested, but scientists are examining the feasibility of genetic, biochemical, and microbiological approaches to COTS control.
Despite their small size, corallivorous snails can cause serious damage to coral reefs when they reach large densities. Control of outbreaks of snails, like Drupella, has proven challenging, even over small scales, because of their life history, behavior and ecological interactions with corals.
Drupella tend to prefer branching corals with complex three-dimensional structures, where they often cluster around branch bases. Hiding deep within colonies makes them difficult to access. Some tourism operators on the Great Barrier Reef have had success using long tweezers and flexible claw pickup tools to remove snails one by one. This can be very time-consuming, and it is difficult to be certain that all animals are removed from any one coral colony. Experiences to date from Australia and Florida suggest that snail removal can be effective at reducing tissue loss or mortality of target coral colonies, but unlikely to be effective as a method for controlling predator populations.
Detecting and Responding to Coral Predator Outbreaks
Coral reef managers concerned about outbreaks of coral predators could consider having a system for early detection of outbreaks, and for assessing the abundance and distribution of corallivores to guide control programs. Managers might also consider including COTS and Drupella and Coralliophila searches ref in their routine monitoring programs, and developing an incident response plan for coral predators.
Urchin outbreaks are best managed by addressing the underlying causes, such as overfishing of predators or herbivores, or nutrient pollution. In some instances, however, rapid reductions in urchin density may be desirable to facilitate recovery of coral populations as part of a restoration strategy. Management trials, such as those in the Seychelles, have indicated that coral recruitment can increase up to two-fold at sites where urchins were removed. ref In Kenya, experiments also indicated that urchin removal can benefit corals, but that this can be preceded by an initial increase in seaweed abundance and must also be accompanied by protection of fishes that prey on urchins. ref