Hong Kong provides a marginal marine environment for coral growth due to its high latitude in addition to massive freshwater run-off from the Pearl River Delta. Previous studies have reported that Hong Kong waters nurture 84 species of scleractinian corals in 28 families distributed in various locations, especially the protected bays in the eastern waters. However, very little is known about the benthic composition and health of coral communities. This study aimed to 1) determine the benthic composition of local coral communities and understand the environmental determinants of coral coverage and coral community composition; 2) record coral colony size frequency distribution across these 33 sites to understand the patterns of coral recruitment in recent years; 3) quantify coral bioerosion and corallivory by the long-spined sea urchin and explore the feasibility of remediating the coral damage by a coral-associated portunid crab. Surveys were conducted at 33 sites in Hong Kong, which cover sites with the highest coral coverages that are mainly located in the north-eastern, east and south-eastern waters. A belt-transect photo quadrant method was applied. 22 hard coral genera were identified, among which the genera Porites, Platygyra and Pavona were found to be the most abundant. Most of the study sites were dominated by few genera of massive corals which led to a low diversity. Coral coverage was negatively associated with nutrient levels including nitrogen, phosphorus and organic matter deposition rates based on sediment trap data. Apart from sedimentary parameters, coral coverage was also found to be strongly negatively correlated with the density of the long-spined sea urchin Diadema setosum. Study sites were categorized into four different conservation classes with sites of higher diversity assigned a higher conservation value. These data could serve as a baseline for measuring changes in benthic composition in the future, and as a reference for management planning such as designating new marine parks. Determining the size structure can help predict how a population may change in the future and whether conservation efforts are effective in promoting the increase in numbers of individuals. To determine coral size structure in local waters, a video transect method was adopted to capture videos on the benthic substrates of the 33 study sites. In the laboratory, the video clips were analyzed to extract information on the size and growth form of all coral colonies along the transects. Size-frequency distribution plots generally showed a highly positive skewness, which indicated a dominance of small-sized (i.e. 10 - 30 cm) colonies, yet low in recruitment-sized (i.e. 5 cm) colonies. An examination of the size distribution of the most common genera showed that the distribution patterns were more genus-dependent rather than site-dependent. Also, massive corals were the most dominant growth form, while branching corals were the least common which was different from healthy tropical reefs. Apart from establishing a baseline of coral communities, coral bioerosion was further studied. Previous studies found that coral coverage and urchin density were negatively correlated in local waters. Further, severe coral bioerosion had been reported to cause community-level coral damage in several locations. Therefore, impact coral bioerosion by the sea urchin Diadema setosum and whether such impact could be remediated were further investigated in a series of controlled experiments in the field. Although sea urchins were reported to prevent shifting from coral-dominant to algae-dominate phase elsewhere, they were found to cause severe tissue loss and bioerosion at high densities in my study. Thalamita prymna, a common portunid crab in local coral communities, was found to effectively reduce coral damages including bioerosion and surface mortality. Crab predation, an overlooked relationship in coral reefs, can thus be exploited to control urchin corallivory and bioerosion. Prohibiting fish trapping in reef areas could reduce the by-catch of these crabs and protect reefs against urchin attack.

Thesis (M. Sc. (Zoology)) -- University of the North, 1981 / Refer to the document

The purpose of this study is to investigate the factors that are responsible for the distribution of coral communities in the lagoon of the Dongsha Atoll. Previous surveys proposed that there was significant difference of coral cover between the east and west sides of the lagoon. Two hypotheses, seawater temperature and water depth, were proposed to explain the spatial variation of coral distribution. In addition, the growth rate of poritid corals, coral recruits, sexual reproduction, and the occurrence of coral predators and diseases were also studied. Ten patch reefs 5 on the west and 5 on the east sides in the lagoon were selected and the corals on reef tops (1-3 m) and deep reef bases (10-15 m) were investigated. The growth rates of poritid corals on reef tops were higher on reef bases than those on reef tops, and higher in eastern lagoon than those in western lagoon. On average, the growth rate is 0.9 cm/year. A total of 10 families of small corals were recorded in which Faviidae (61.6%), Fungiidae (16.6%) and Poritidae (9.7%) constituted the majority. Diversity index comparisons indicated that reef bases have higher diversities than reef tops. The densities of small corals, ranging between 0.1-3.0 ind./m2, are higher on reef bases than on reef tops. In fungiids, individuals in the eastern lagoon were larger than those in the western lagoon. Tissues of corals were sampled in June 2009 for examination of gonads. However, no reproductive tissues were found after decalcification and histology. The morphological classes, live-coral coverage, dead-coral coverage and dead-coral ratio among the comparisons of reef bases vs reef tops and western vs eastern lagoons, only dead-coral coverage was found to be higher in eastern than in western lagoon. The live-coral coverage was 0.3-46%, dead-coral coverage was 8%-76% and dead-coral ratio was 10%-100%. Among the 10 patch reefs, patch reefs 6, 7, 9 and 10 are represented by mostly K type competitors and belongs to Conservation Class 2, patch reefs 1, 2 and 3 are presented by mostly S type stress-tolerated corals and belongs to Conservation Class 1. The remaining 3 patch reefs 4, 5 and 8 belong to the highest Conservation Class 4 and are represented by diverse types of corals. The water temperature exceeded 30¢XC in 36% of the time at reef tops, and 13% of the time at reef bases, during the summer period, i.e., between June and September, 2009. And the water temperatures were found in 95% of the time to be ~ 1¢XC higher at reef tops than at reef bases. The water temperatures were also higher in the west than in the east of the lagoon that 29% of the time exceeded 30¢XC in the west and 9% in the east only. No coral diseases or pests were found that may pose a large-scale threat in the near future. The variations of coral fauna found among habitats in the lagoon are consistent with the temperature patterns, i.e., the higher the temperature the poorer the coral condition. To the two hypothesis, in the eastern or western lagoon and the different depth are the factors of coral distribution in the lagoon.

Marine protected areas (MPAs) have been established along the East African coast to protect coral communities from human and natural disturbance. Their success is dependent on the degree to which resource populations are self-seeding or otherwise connected. Estimates of contemporary gene flow on or between south-east African reefs are thus required to reveal the interdependence of the South African coral communities and those to the north. Accordingly, the ecologically relevant (1 or 2 generations) connectivity of two broadcast-spawning corals, Acropora austera and Platygyra daedalea, was assessed on reefs in the region, from the Chagos Archipelago to Bazaruto Island in Mozambique and Sodwana Bay in South Africa, using hyper-variable genetic markers. Analysis of genetic diversity and differentiation provided evidence for the existence of four discrete genetic populations of A. austera and five of P. daedalea in the sampled area. Higher genetic diversity was found on northern South African reefs (Nine-mile Reef and Rabbit Rock) and migration patterns inferred from assignment tests suggested that, at ecological time scales, South African reefs are disconnected from those in Mozambique and might originate from a source of gene flow that was not sampled. The analysis of fine-scale genetic connectivity conducted on Two-mile Reef (TMR) demonstrated the existence of significant spatial genetic structure at the reefal scale that might be related to the non-random dispersal of coral larvae, putatively explaining the genetic discontinuity observed in the region. Altogether, the results are consistent with the isolation observed in other studies using less variable markers, and support the hypothesis that there is demographic discontinuity between the coral populations along the south-east African coast. More importantly, Acropora austera and P. daedalea represent different life strategies in the South African reef communities yet manifested similar genetic patterns, suggesting that these corals are responding similarly to forces that are driving genetic connectivity in the region. For management purposes, the genetically distinct populations identified at each of the spatial scales analysed in this study may correspond to management units, or evolutionarily significant units. Furthermore, since some reefs appear to act as “landing-sites” for migrants (Nine-mile Reef) and there is evidence of significant within-reef genetic structure (TMR), an adaptive management framework would be the best option for the MPA in the region. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2014.