Hong Kong's Subtropical marine environment supports species of both tropical and temperate climates. Local waters contain a wide diversity of fishes, crustaceans, molluscs and other marine life, of which at least 150 species are of significance.

Set on the eastern bank of the Pearl River's estuary, Hong Kong receives freshwater discharged from the river, especially in its western waters. The eastern waters, on the other hand, are little influenced by the Pearl River outflow and have a predominantly oceanic characteristic. This special hydrographic condition helps to contribute to the diversity of marine life found in Hong Kong.

Despite being close to the northern limit for hard corals, Hong Kong supports some 80 stony coral species. This diversity of corals is considered quite rich by international standards. A variety of marine fishes also breed in Hong Kong waters. Typical of eastern waters is the red sea bream, one of the several sea bream varieties whose fry are abundant along the shore of Mirs Bay in early spring.

Despite the small extent of Hong Kong waters, two marine mammal species can be found locally throughout the year. The better-known one is the Indo-Pacific humpback dolphin, also known as the Chinese white dolphin. The other is the finless porpoise. The humpback dolphin prefers the estuarine environment and inhabits the western waters of Hong Kong while the finless porpoise lives in the eastern and southern areas, which are predominantly oceanic waters.

There has been concern over the degradation of the marine environment by pollution, coastal development works, over-fishing and destructive fishing practices. To counteract the disturbance to inshore marine resources, the AFCD has installed artificial reefs to enhance fisheries resources and the biodiversity of the marine environment. The Marine Parks programme continues to serve as an important scheme in protecting and conserving sites of special ecological and conservation value.