Fish inhabit most water bodies, from high mountain streams to deep oceans. In terms of the number of described species, fish exhibit the greatest species diversity among vertebrate groups.
With a mountainous landscape, Hong Kong has hundreds of fast-flowing hill streams as well as slow-flowing estuaries, providing a wide range of freshwater habitats for diverse freshwater fish. Hong Kong also has a rich diversity of marine fish despite its small marine area. Notably, the area of Hong Kong's territorial waters (1,650 km2) is only 0.05% of the South China Sea (~3.4 million km2), yet it has around 30% of the marine fish species recorded in the South China Sea.
Freshwater Fish
There are 196 freshwater fish species in Hong Kong which are mainly from the families named Cyprinidae, Gobiidae, Cobitidae and Cichlidae. Freshwater fish are classified into 4 groups: primary freshwater fish, brackish species, marine vagrants and diadromous species. Primary freshwater fish are strictly confined to freshwater environments during their entire life cycle. 71 species belonging to 19 families found in Hong Kong are primary freshwater fish, representing 36% of the total species recorded. They belong to the families of Cyprinidae, Balitoridae, Poeciliidae and Channidae. Common examples are Predaceous Chub (Parazacco spilurus), Broken-band Hillstream Loach (Liniparhomaloptera disparis), Mosquito Fish (Gambusia affinis) and Blotched Snakehead (Channa maculata). Brackish species usually occur in estuaries. About one quarter of freshwater fish recorded locally are brackish species, of which Gobiidae and Eleotridae are the dominant groups. Examples include Dusky Frillgoby (Bathygobius fuscus) and Sharphead Sleeper (Eleotris oxycephala). Marine vagrants spend most of their lives in coastal or oceanic waters. A total of 60 marine vagrants have been recorded in Hong Kong, comprising 31% of our freshwater fish species. Mangrove Snapper (Lutjanus argentimaculatus), Glassy Perchlet (Ambassis gymnocephalus), Jarbua Terapon (Terapon jarbua) and White-spotted Rabbit Fish (Siganus canaliculatus) are common examples. Diadromous fish shift between marine and freshwater environments during different stages of their lives. This group comprises 9% of our freshwater fish species. Local amphidromous species include Stiphodon atropurpureus and Stiphodon multisquamus. Japanese Eel (Anguilla japonica) and Giant Mottled Eel (Anguilla marmorata) are catadromous examples.
Most of the freshwater fish in Hong Kong are distributed in upland streams, lowland streams, estuaries and reservoirs. Upland streams are formed by numerous tributaries that typically have riffles, cascades and deep pools. Species such as Broken-band Hillstream Loach (Liniparhomaloptera disparis), Sucker-belly Loach (Pseudogastromyzon myersi), Striped Loach (Schistura fasciolata) and Rhinogobius duospilus are inhabitants of these environments. Lowland streams are less shaded by riparian vegetation with turbid water. Species commonly encountered are Predaceous Chub (Parazacco spilurus), Chinese Barb (Barbodes semifasciolatus), Barcheek Goby (Rhinogobius similis), Whitespotted Walking Catfish (Clarias fuscus) and Blotched Snakehead (Channa maculata), whereas rare species such as Chinese Rasbora (Rasbora steineri) and Metzia lineata can also be found. Estuaries with finely-grained bottom substrates provide unique habitats for brackish species and marine vagrants, such as gobies, gudgeons and snappers, amongst which include uncommon species such as Giant Mottled Eel (Anguilla marmorata) and Largesnout Goby (Awaous melanocephalus). Reservoirs in Hong Kong provide different types of aquatic habitat, ranging from shallow and bushy marginal zones, to deep and open water for various freshwater fish, including rare ones like Rose Bitterling (Rhodeus ocellatus) and Metzia formosae.
Marine Fish
Throughout the 175 years of Hong Kong ichthyological history, more than 1,000 marine fish species of some 135 families and 27 orders have been recorded, and the list has continued to grow over the past decades. Marine fish in Hong Kong waters are from two major taxonomic groups, the cartilaginous fish (Chondrichthyes) and the bony fish (Osteichthyes). The 10 most abundant fish families in Hong Kong are Gobiidae, Labridae, Serranidae, Carangidae, Lutjanidae, Scorpaenidae, Pomacentridae, Apogonidae, Sciaenidae and Chaetodontidae, in descending order.
Marine fish are closely linked to the habitats they inhabit and play important roles in maintaining a balance in the marine ecosystem. Through their interaction with other organisms and the environment, the species community and habitats are shaped. Rich marine fish diversity also forms the basis of fisheries resources. More than 150 species found in Hong Kong are commercially valuable and thus provide support to the food supply and the livelihood of people in the fisheries industry.
· Sharks and Rays
The cartilaginous fish, which are relatively easier to find in Hong Kong are sharks and rays. They are one of the oldest taxa of vertebrates on the Earth, having existed for more than 400 million years. They both belong to the subclass Elasmobranchii. 'Elasmo' means plate and 'Branchii' means gills, and hence these species are recognised by the multiple paired gill openings on the sides of their head. This group differs from more commonly observed bony fish, in body structures and modes of life. For example, the skeletons of all cartilaginous fish are made of cartilages instead of bones; they are covered by placoid scales (i.e. tooth-like scales), which give a rough feeling to their bodies. Depending on the species, they breathe through five to seven pairs of gills. Generally speaking, pelagic species must continuously swim to move oxygenated water through their gills, whilst demersal species can actively pump water in through their spiracles and out through their gills. All species practice internal fertilization but ways vary depending on the species. The young of most species are usually born alive (viviparous) but some reproduce through shelled eggs (oviparous). Interestingly, a very few of them procreate through a rare form of reproduction, by producing young in shelled eggs that hatch inside females' bodies (ovoviviparous). Many species have electroreceptors that help detect electric fields in water for finding prey, navigating and sensing temperature.
The exact number of shark and ray species that have been observed in Hong Kong waters is not known because of limited records and research. Some of the previous records were based on specimens stranded along our coastline or found in the fish markets, yet the native habitats of the individuals could be miles away from Hong Kong. In addition, the relatively large habitat area and migratory habits of many species also makes it difficult to distinguish between residents and regular visitors. Historical accounts of the fishing industries of Hong Kong mentioned that sharks were plentiful and were found everywhere in Hong Kong, including waters off Tai O, Cheung Chau, Lamma Island, Junk Island and Tunglung Island the whole year round in the 1940s. However for a number of decades, a large number of species have been rarely observed, possibly due to overfishing or reduction of suitable habitats. Today, there are only two small species commonly seen in the territory, namely the White-spotted Bamboo Shark (Chiloscyllium plagiosum) and the Pacific Spadenose Shark (Scoliodon macrorhynchos). Both species prey on small fish and crustaceans and generally pose no harm to humans. In contrast, rays are still relatively common in local waters, examples include Blotched Fantail Ray (Taeniura meyeni) and Pale-edged Stingray (Dasyatis zugei).
· Reef Fish
Reef fish is one of the well-known groups of bony fish, which usually refers to those species that spend part or entire lives on or close to areas of hard substrata, including rocky reef and coral communities. However they may also spend their time in surrounding habitats in different life stages, thus making a universal definition of this group particularly difficult. For example, Juvenile Mangrove Snapper (Lutjanus argentimaculatus) spends their time in soft-substrata near mangrove and estuary, while the adults are usually found in reefs. In contrast, some species are heavily dependent on the continued survival of corals or rocky reefs throughout their entire life stage, such as parrotfish (Scarinae spp.) which prey on hard corals. Compared to fish in other habitats, reef fish are usually characterised by their vivid appearance. This likely helps them to blend in with the colourful and complex surroundings, confusing both the predator and prey.
Reef fish usually inhabit the eastern and southern waters in Hong Kong with higher salinity and coral coverage. The species richness of reef fish in Hong Kong is relatively high, being greater than that of Hawaii, for instance, but similar to other locations in the South China Sea such as Hainan Island. To date, more than 340 reef fish species have been recorded in Hong Kong. Some well-known representatives from this group include Clark's Anemonefish (Amphiprion clarkii), butterflyfish (Chaetodon spp.) and groupers (Epinephelus spp.).
