Marine fish are fish that live in the salt water of oceans, seas or salt lakes. There are about 11.6 thousand species in total. They approach shores only during the breeding season, avoiding desalinated bays and river mouths (pelamid, shark, tuna, anchovy, mackerel, etc.). Marine fish are subdivided into pelagic fish, which live in the water column, and bottom fish, which live near the bottom (rays, flounder).

Fish that live and breed permanently in seawater with a salinity greater than 12°/oo (average 33-35°/oo), i.e. fish whose osmotic pressure - the amount of dissolved salts in blood plasma and other body fluids - is equal to or lower than the osmotic pressure of the medium. 

Maintenance of osmotic pressure of body fluids of marine fishes is accomplished by active retention of water and removal of excess salts from the body (in bony fishes), or active balance of soluble and insoluble ammonia compounds - uric acid, urea (in cartilaginous fishes), or both mechanisms together (in latimeria). 

Bony fishes have true bone tissue in the skeleton, derived from bone cells, and are subdivided into shovelnose fishes (keysteppers and bivalves) and beamnose fishes (bony ganoids and bony fishes). 

Cartilaginous fishes (marine and freshwater) have no true bone tissue in their skeleton (sharks, rays, chimaeras). Marine fishes include almost all species of sharks and rays (550 species), chimeras (about 30 species), killifish (Latimeria) and more than half (58.2% or 12,000) of bony fish species. 

Marine fishes inhabit all seas and oceans from the polar regions of the Arctic (cod fishes) to the shores of Antarctica (nototepaea, whiting fishes). Marine fish live from epipelagial (depth up to 200 m) to the maximum ultraabyssal depths. So far it is possible to catch only at depths of up to 9000 m, but with underwater apparatus saw fish in the Mariana Trench at a depth of more than 11 thousand meters. 

Marine fish are divided into pelagic - constantly living in the water column, and bottom - more or less associated with the bottom. Now there are still special groups of marine fish - benthic (benthic-pelagic), living in the water column above the bottom, and cryopelagic - permanently living under the lower surface of drifting and pack ice. 

The greatest species diversity of these fishes is found in warm waters of tropical and subtropical areas, especially among coral reefs, but the numbers of each species are not particularly high here. In temperate waters, the number of species is much smaller, with many species reaching huge numbers, such as herring, sea bass, codfish, flounder, etc. Species inhabiting areas of high biological productivity (anchovies, mackrurus, jack mackerel, etc.) reach very high numbers. 

The size of marine fishes varies. Dwarf gobies Pandaki and Mistichthys, 7.5-14 mm long, inhabiting the shallow waters of the Philippine Islands, are the smallest vertebrates. The whale shark, a marine giant, is up to 15-18 m long and weighs up to 20 tons. Length of the giant shark 10-12 m, weight 3-5 tons, both species are plankton-eating fish. 

Among the bony fish the largest sizes reach tunas, swordfish, sailfish (length up to 3 m, mass over 500 kg) and especially marlins, among which the blue marlin has a length of more than 5 m with a mass of about 1 ton. The average size of bony fish from a few tens of centimeters to 1, rarely 2 m. The shape of the body of marine fish the most diverse: from streamlined cigar-shaped in pelagic fish (sharks, tunas, swordfish, mackerel) to flattened from above (rays, sea devils) or from the sides (flounder). Many fish, especially those living among corals and algae, as well as deep-sea fish have a bizarre, not fishy body shape (kuzovki, whistle fish, sea needles, seahorses, etc.). The coloration of pelagic fish is usually darker, often blue, silvery on top, light on the sides and white underneath. Bottom fish often have mottled, brightly colored, especially tropical species. The coloration of bathyal and abyssal marine fishes is usually gray or black-gray. Many of them have luminous organs - photophores. Among marine fish are poisonous: sea dragons, wings, stingrays-tails, prick sharp prickle which has at the base of the poisonous gland, causes severe poisoning in humans, and prick warthog, living in many tropical seas, - even fatal outcome. Dangerous to humans many species of large sharks, especially white sharks up to 11 m long and weighing several tons, tiger shark, hammerhead shark and others. There are known cases of attacks on humans by barracudas. 

Many marine fishes are an important object of fishing: cod, herring, anchovies, tuna, nototenia, flounder, sea bass, and others. Some marine fish species have become the object of mariculture (e.g. flounder, mullet, eels).