Report Title

Crown of Thorns Seastars – Threats Posed to Mahé’s coral reefs

Objective

The monitoring and eradication of the crown of thorns seastars around the northwest region of Mahé island, Seychelles

Summary

GVI and SNPA SCUBA divers partner up for crown of thorns (COT) eradication dives, these are arranged based on the analysis of data recorded by GVI staff and volunteers, who document COT sightings and alert SNPA to any explosions in population condensed in a certain area. The eradication/removal of the COTS is an important step to assisting the restoration of the reef.

Report

GVI has been partnering with the SNPA once again, in efforts to assess the magnitude of the threat the coral reefs of Mahe, including Baie Ternay, are under due to predation from crown of thorns sea stars, Acanthaster planci.

This sea star is a corallivore, almost exclusively consuming live sclerectinian corals. An average sized adult (40 cm) can kill up to 478 square cm of live coral per day through its grazing activities (Glynn, 1976; Keesing and Lucas, 1992; Wilson, et al., 2008). Signs of its presence are obvious; the coral skeleton is left behind as the result of starfish feeding and stands out sharply as patches of pure white, which eventually become overgrown with algae (Chesher, 1969. Crown-of-thorns outbreaks thus result in an alteration of the coral community structure (Kayal, et al., 2012).

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During every dive, GVI staff and volunteers note incidental sightings, which can include marine megafauna and many unusual invertebrates, one of which is the crown of thorns. The recordings are presented to Seychelles National Parks Authority (SNPA) and action is taken when noticeable numbers are found in a condensed area.

Over the past few months GVI have been noting the increase in COT numbers when monitoring the reef in and outside of Baie Ternay. Our dives span from our most southerly site, site 23; Therese Island, all the way to Lilot in the very north, site 18. These can be seen below in Figure 1.

April time saw a very successful eradication dive in Baie Ternay Marine Park, where over 170 COTs were removed from the park. The marine park has shown to not only be a very important breeding ground for many marine species found of the coast of Mahe, but also to hold the highest abundance and diversity of both fish and coral species. Therefore the removal of these coral grazing sea stars is of exceptional importance.

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Figure 1- GVI Survey sites

The latest eradication team up was during May, where a few staff members met early morning at Beau Vallon beach. The morning began slowly, with not too many being located, this was due to scanning new areas of reef that had not been surveyed. However after a few hours crawling along the coastline, the team began to find more promising areas, and by the afternoon the team had struck gold. A flurry of 8 divers, the clashing of steel swords and toxic spines across the reef, the feverish swimming and hauling back and forth from the sea floor to the boat and over 16 collective hours underwater, ended in a total of 294 crown of thorns being removed.

Although these seem like large quantities being eradicated, these coral predators reproduce at a prolific rate which requires continually attention from the parks authority if the coral reefs of Mahe are to thrive.

References

Glynn, P. 1976. Some physical and biological determinants of coral community structure in the Easern Pacific. Ecological Monographs, 46 (4): 431-456.

Kayal M., Vercelloni J., Lison de Loma T., Bosserelle P., Chancerelle Y., Geoffroy S., Stievenart C., Michonneau F., Penin L., Planes S., Adjeroud M. Predator crown-of-thorns starfish (Acanthaster planci) outbreak, mass mortality of corals, and cascading effects on reef fish and benthic communities. PLoS One. 2012;7(10):e47363

Keesing, J., J. Lucas. 1992. Field measurement of feeding and movement rates of the crown-of-thorns starfish Acanthaster planci (L.). J. Exp. Mar. Biol. Ecol., 156: 89–104.

Wilson, S., S. Burgess, A. Cheal, M. Emslie, R. Fisher, I. Miller, N. Polunin, H. Sweatman. 2008. Habitat utilization by coral reef fish: Implications for specialists vs. generalists in a changing environment. Journal of Animal Ecology, 77 (2): 220-228.

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