Coral colony abundances and sizes as indicators of reef health in subtropical Hong Kong waters

Previous studies of the coral communities in Hong Kong focused on the characterisation of benthic cover, with little attention given to colony size structure. We conducted transect surveys at 52 sites across a gradient of water quality, identified 6774 coral colonies to 23 genera and measured the size of each colony. The southern waters harboured less diverse (1-9 genera) and fewer corals (2-28 colonies) than the eastern waters (3-16 genera and 17-427 colonies) per site. The density of juveniles (< 5 cm diameter) was low (0.06-4.23 colonies 100 m^-2), indicating recruitment limitation and/or post-recruitment mortality. Corals in the estuarine waters were typically more right-skewed in size structure compared to those in the oceanic waters, indicating recruitment failure and/or high juvenile mortality. Even in the oceanic waters, the size structure of several genera differed, indicating coral sizes can reflect the impact of not only water quality but also environmental disturbances such as bleaching and overgrazing by sea urchins.