Suitability of the manta tow technique for estimating relative and absolute abundances of crown-of-thorns starfish (Acanthaster planci L. ) and corals

Abstract

The manta tow technique has been used mainly for broad-scale surveys of large coral reef systems. The present study comprised two separate investigations of the technique. One involved investigating its repeatability by comparing the performance of three observers towed together along a reef. The other compared the counts of crown-of-thorns starfish from manta tow and SCUBA swim surveys undertaken within 30 large areas on eight reefs. Overall, the technique was found to be highly repeatable, particularly for crown-of-thorns starfish and live coral cover. It was less repeatable for assessing the cover of dead coral, although this was still within acceptable limits. A strong statistical relationship (r²_adj = 0.91) was found between the starfish counts obtained from manta tows and those obtained from SCUBA swim surveys. Consequently, the former can be calibrated to predict the latter. The crown-of-thorns starfish is known to be cryptic and to have an aggregated distribution. However, the present study showed that the manta tow technique is a reasonably accurate and cost-effective option for determining relative changes in the distribution and abundance of noncryptic starfish where the unit of interest is a whole reef (or a large part thereof). As a comparison, approximately 14 manta tows (each lasting 2 min) will give the same level of accuracy as 10 SCUBA swim surveys of the same fixed area. The sensitivity of the manta tow technique in detecting relative changes in abundance will depend on the number of tows that are conducted. As a guide, however, a typical manta tow survey (involving approximately 50-60 tows) is capable of detecting a 20% change in the abundance of an out-breaking population of crown-of-thorns starfish. Given these findings, the manta tow technique also may be useful for assessing relative changes in the distribution and abundance of other organisms.