Re ECF Project: 2008-25

Project Title: Impact of Sea Urchin Bioerosion on Coral Communities in Hong Kong

Applicant: Hong Kong Baptist University (Dr. Jian-wen QIU)

Total Approved Grant: $249,940

Duration: 1/8/2009 to 30/9/2010

Project Status/Remarks: Completed

Scope:

  • To determine the abundance and size-structure of Diadema steosum and coral-eating gastropods, as well as percent coverage of corals
  • To determine the size-dependent Diadema steosum rate
  • Quantify the contribution of various biotic and abiotic factors to sea urchin bioerosion; and
  • To assess whether management intervention is required to control sea urchin bioerosion in the study sites

Summary of the Findings/Outcomes:
Bioerosion by the sea urchin Diadema setosum had been reported to contribute to the collapse of massive coral heads in Hoi Ha Wan Marine Park in recent years. However, bioerosion rate in this marine park and elsewhere in Hong Kong remained unknown. The aim of this study was to determine the spatial pattern of bioerosion by D. setosum in Hong Kong.

SCUBA diving surveys at eight sites with different levels of coral coverage in the north-eastern, eastern, eastern coastal and southern waters have been conducted to determine benthic substrate and hydrological characteristics, density of the coral eating gastropod Drupella rugosa, and population structure of D. setosum. D. setosum was collected; the gut content was measured, and the gut turnover rate was determined in the laboratory in order to calculate the individual bioerosion rate. D. setosum density and individual bioerosion rate were then used to calculate the gross bioerosion rate for each site.

The study revealed differences in the density of D. rugosa among the eight sites, ranging from 0.1 individuals m-2 at Sharp Island to 8.2 individuals m-2 at Sharp Island. The densities were low compared to those reported in feeding clusters formed during Drupella population outbreaks. Low densities of D. rugosa indicated that this species did not contribute significantly to coral damage during our survey period. There was also a clear site difference in mean D. setosum density, which ranged from 0.5 individuals m-2 at Sharp Island to 6.3 individuals m-2 at Moon Island. Annual bioerosion rate varied by 10 times among sites and was highest at Moon Island (1.2 kg CaCO3 m-2 yr-1). Except for Moon Island, annual bioerosion rate was below 0.5 kg CaCO3 m-2 yr-1 at all sites. Annual bioerosion rate was positively correlated with D. setosum density, but not with any other biotic and abiotic factors measured (coral coverage, D. rugosa density, sediment deposit, water clarity, slope angle, wave exposure).

These results supported our field observation of the relative serious coral damage at Moon Island, and indicated bioerosion by D. setosum could be an important contributing factor to this damage. At other seven sites surveyed, however, our field observation showed that coral communities were healthy, and our data showed that urchin bioerosion minor. However, factors responsible to the site differences in D. setosum density and bioerosion remain unknown. Similar urchin bioerosion studies at other coral communities, as well as studies on other aspects of the coral biology, especially coral growth, and bioerosion caused by other groups of coral-associated animals are required before we can fully evaluate the health status of coral communities in local waters.