Rocky and sandy shores comprise most of the natural shoreline in Hong Kong. Rocky shore is defined as areas of stable (non-mobile) rocks larger than cobbles between the high and low tide marks covering more than 50% of the area. Rocky shores vary in degree of wave exposure they receive: some rocky shores, particularly on the eastern coast of Hong Kong, are exposed to the full strength of oceanic waves, eg the Ninepins, whereas others are protected, eg those within the inlets of Tolo Harbour. The wave energy experienced by these shores, combined with their geology, creates specific habitats. More exposed shores tend to be composed of jagged pinnacles of eroded rock whereas more sheltered areas usually have low-lying, smooth, rock platforms and large boulders.

Species assemblages exhibit characteristic distributional patterns based on the degree of exposure experienced by the shore habitat. Individual rocky shores also display zonation patterns with a progression of different species along the vertical gradient from terrestrial to marine environments. Artificial shoreline may serve a similar habitat function to natural rocky shore if it mimics a natural gradient and provides microhabitats. Boulder seawalls and dolossee blocks are known to provide some habitat (Binnie Consultants Ltd 1996a and 1997a), however, other artificial shorelines (eg vertical seawalls) provide little or no habitat value.

All the survey sites of the outlying islands, including North Ninepin Island, Basalt Island and Bluff Island, were considered of high conservation interest and an ecological value of high* (upgraded from indicative medium ecological value) was assigned. These rocky shores are extensive and support comparatively higher species diversity and abundance than other rocky shores in Hong Kong. Disturbance to these habitats due to human activities was not observed because of their remoteness from urban areas. These rocky coasts are also of geological interest and received SSSI designation in 1979.