Managing crown-of-thorns starfish outbreaks

Crown-of-thorns starfish are a natural part of the Great Barrier Reef ecosystem. Outbreaks of the starfish may also be a natural part of the reef ecosystem. So, even if it were possible to eliminate crown-of-thorns starfish, would it be appropriate?

To develop the best control measures, it is important to understand the distribution of crown-of-thorns starfish. Therefore, the Great Barrier Reef Marine Park Authority (GBRMPA) has supported the development of several survey techniques to monitor starfish distribution and numbers.

There are few options to manage outbreaks of crown-of-thorns starfish. Several techniques have been developed to control starfish numbers. However, they are labour-intensive and expensive, and are only practical in small areas, for example, areas that are visited frequently by tourists who expect a high cover of hard coral.

Some human influences, such as overfishing and poor water quality, have been suggested as playing a role in crown-of-thorns outbreaks. Regardless of whether such factors cause outbreaks, they are already being addressed by management responses because they are important issues in the Great Barrier Reef.

Survey methods for crown-of-thorns starfish

The distribution of crown-of-thorns starfish on the Great Barrier Reef (GBR) has been monitored regularly using two different and complementary methods: broad-scale reef surveys by the Long-Term Monitoring Program supported by the Australian Institute of Marine Science (AIMS) and CRC Reef; and the fine-scale reef surveys carried out by Reefwatch Australia, funded by CRC Reef.

Long-term Monitoring Program

Manta tow Photo: GBRMPA

The Long-Term Monitoring Program is designed to monitor broad-scale changes in several parameters that are indicators for reef health across the Great Barrier Reef. The program is undertaken by scientists from the Australian Institute of Marine Science (AIMS) who have been conducting surveys since 1985.

Each year, the team uses manta tows to survey the perimeters of about 100 reefs. During a manta tow, a diver using a ‘manta board’ is towed behind a small boat around each reef. The boat stops every two minutes so the diver can record evidence of crown-of-thorns starfish (sightings or feeding scars) and estimates of reef-wide coral cover. During the Long-Term Monitoring Program, sites on 48 of these reefs are surveyed more intensively for coral cover and resident fish populations.

Divers using manta tows can see a proportion of adult crown-of-thorns starfish that are bigger than about 15 cm in diameter. The numbers of starfish seen during two-minute tows are used to estimate the status of the crown-of-thorns starfish populations. An ‘incipient’ outbreak is the density of starfish at which coral damage is likely: 0.22 starfish per two-minute tow. During an ‘active’ outbreak, densities reach >1.0 starfish per two-minute tow which would certainly damage reefs. It is interesting to note that some reefs have been unaffected by crown-of-thorns starfish since the beginning of the monitoring program in 1985.

Information about the current status of crown-of-thorns starfish on the Great Barrier Reef from the Long-Term Monitoring Program is available on the AIMS website.

Fine scale surveys

Since 1994, up to 21 reefs between Cooktown and Townsville have been surveyed each year using the fine-scale survey method. As the name suggests, the fine-scale surveys look in detail at small areas of reef and reveal the structure of the crown-of-thorns starfish populations.

At each reef that is surveyed, SCUBA divers search for crown-of-thorns starfish in 40 transects (50m long x 5m wide) at 20 different places on the reef. The divers count all the starfish in each transect, including juveniles (up to 13cm diameter, estimated to be one year old), sub-adults (14-25cm diameter, estimated to be about two years old) and adults (more than 26cm diameter, estimated to be three years old or more). The smaller starfish cannot be seen during manta tow surveys.

Incipient outbreaks are recorded when sub-adult and adult densities together reach more than 30 per hectare (10,000 square metres). Active outbreaks are defined when there are more than 30 adult crown-of-thorns starfish per hectare (10,000 square metres).

Comparing long-term monitoring and fine-scale surveys

An analysis of the difference between manta tows and fine-scale surveys showed that:

• Manta tow surveys underestimate the numbers (densities) of crown-of-thorns starfish compared with fine scale surveys on the same reef.
  
• Both techniques show a similar progression of the outbreak and rate of southerly drift of the outbreak.

Therefore, the fine scale surveys are most useful to calculate densities of crown-of-thorns starfish on a small-scale. Manta tows are better for following broad-scale changes in crown-of-thorns starfish density across the entire Great Barrier Reef.