Demersal fish are dependent on the sea bottom for food and shelter and it is upon this group of fishes that the commercial fisheries are based, although interest is also shown in the exploitation of some pelagic species.
Species living on the continental shelf (depths to 600 ft or rather more) are extremely varied in shape and colour, but typically they are less streamlined and slower swimmers than the pelagic group and are usually red, brown, or grey – colours which act as good camouflage in their various environments. They do not migrate to any great extent. Their food consists of such items as shellfish, crabs, worms, and seaweed.
Sea-bottom topography also has an influence on the general distribution of fish. Rocky sea bottom attracts the greatest variety of species, but some are common over sandy or muddy bottom.
Most species living in rocky areas are weak swimmers, some being capable of only short, darting movements; many are solitary, especially the larger members of a species, while some swim in small schools just above the sea floor. Some are brilliantly coloured (e.g., the wrasses or false parrot fishes); some are eel-like (hagfish or blind eel, conger eel, moray eel); some have large heads and jaws (groper, John Dory); several species are spiny (gurnard, scorpionfish); while some do not resemble fish at all (seahorse, pipefish). The small fish of intertidal pools are adapted to surviving under continuous wave action, either by clinging to rocks by means of a ventral sucker (clingfish) or by sheltering under rocks or stones (blennies).
Fish characteristic of a sandy or muddy bottom tend to be flattened from above and are typified by flounders, rays, and stargazers. Their colours are usually grey or brown. Flounders are further protected from enemies by being capable of colour changes to match the sea floor. They also partially bury themselves in sand, a habit that is carried further by stargazers, sand eels, and lancelets.
Some coastal species, although classed as demersal, spend considerable periods in the middle depths. They resemble pelagic fish in their swimming ability and in their general colouring and shape, but they are mainly bottom feeders and they form small schools. A number of our commercial species are included in this class, such as snapper, tarakihi, mullet, moki, and trumpeter.