The species and family of fish in trade practice are established by further external signs:

1) body shape (spindle-shaped, flat, compressed from the sides, arrow-shaped, serpentine);

2) type of scales;

3) disposition of the skin (scaly, naked);

4) shape, number and arrangement of fins;

5) presence (or absence) of adipose fin, adipose century;

6) shape and size of snout, location of mouth cavity;

7) number of antennae near the mouth;

8) the disposition of the lateral line, the number of scales along it, their shape and size.

When determining the species, first of all examine the mouth of the fish to see if it has jaws or if it has a suction cup shape, which is characteristic only for the lamprey family.

If the fish are jawed, the skin is examined. Fish with five rows of bony scutes belong to the sturgeon family. When determining the species of sturgeons, similar signs are compared: body shape, size and shape of the snout in individual species, location of the mouth and antennae, number of antennae and longitudinal rows of bone plates on the body, shape of the caudal fin, and features of its scales.

The species of sturgeon are identified by specific marks, such as:

– in beluga and calluna – the comparative height of the first dorsal scute and the shape of the antennae;

– in starred sturgeon – shape and length of the snout, shape and features of the antennae, number of dorsal, abdominal and lateral scutes, presence of starred plates on the sides of the body;

– in Siberian and Russian sturgeon – length and shape of snout, location and shape of antennae, number of lateral scutes, presence of stellate plates between dorsal and lateral scutes;

– Baltic (Atlantic) sturgeon – surface pattern of lateral scutes, presence and location of small rhomboidal plates between the rows of dorsal and lateral scutes;

– in sterlet – snout shape, number of lateral scutes, shape of antennae, presence of pectinate small grains on the body.

If the fish are scaly, attention is paid to the shape of the body when determining the family. For example, the serpentine body is found only in eels. For scaly fish with a non-snake-like body, only flatfish have eyes that are asymmetrical. When determining the species of flatfish, attention is paid to the body structure and shape, the location of the eyes, the presence of spines on some flatfish body surface, the colouring features of the body sides. Species of flatfish are determined by specific characters, which are: flounder – body colour, peculiarities of colouring the sides of the body, yellow-belly flounder – colour of the front side of the body, the common halibut – the location of the lateral line and dorsal fin, its length, the presence of a spine in front of the anal fin; in the black halibut – the colour of both body sides, the location of the upper eye and lateral line. The other scaly, non-snake-like fish with a symmetrical eye arrangement are distinguished by the presence of an eye on both sides of the body and the position of the upper eye and lateral line.

are distinguished by the presence or absence of a fat fin on the backbone near the tail. This fin is only present in fish of the salmon, smelt and grayling families. When determining the species of salmonid fish, attention is paid to the shape and size of the scales, the shape of the dorsal fin and the number of rays in the dorsal fin, and the size of the eyes. The species of Far Eastern salmon (chum, pink salmon, Chinook salmon, coho salmon, coho salmon and others) is determined by the position of the maxillary bone, shape of spots on the body surface (except for chum), scale size, number of transverse rows of scales in the lateral line and branched rays in the anal fin. The species of whitefish is judged by scale size and number of transverse rows of scales in the lateral line, size and position of the mouth.
The family of scaly fish without fat fins is defined by further signs: many codfish have three dorsal fins and mackerel fish have more than 3.

The cod family is distinguished by the arrangement of the pelvic fins, the shape of the caudal fin, the number of dorsal fins and their features, the number of anal fins, the shape of the scales. The marks specific to individual cod species are established: in cod – number of dorsal and anal fins, size of jaws, location and number of antennae, features of lateral line, body colour of different species, difference in shape of head, body and caudal fin in different species; in haddock – colour of lateral line, characteristic colour of both sides of body below lateral line on first dorsal fins; in saithe – features of location and shape of lower jaw, features of lateral line, size and shape of antenna; in hakes, merlus – two dorsal fins, no antenna.

The family of scaly fish with two dorsal fins is determined by the presence of the lateral line, which the mullet does not have.

Belonging to the family of scaly fish with two dorsal fins and lateral line is determined by the disposition of the first dorsal fin from the head: it is soft and flexible – in laboules and gobies, and strong – in perch (gill covers without spines) and scorpina (with spines on the gill lids).

In characterizing the perch family, attention is drawn to the scales, position of the mouth, shape of the gill lids, caudal fin, and the presence of characteristic rays in the anal fin. In determining the species of perch consider the distinguishing marks: in common perch – height of the first and second dorsal fins, characteristic colouration of the first dorsal fin, lateral sides of the body, size of the upper and lower jaws; in Balkhash perch – size of the dorsal fins and jaws, colouration of the body and first dorsal fin; in pikeperch – location, shape and size of dorsal fins, forehead width in relation to eye diameter, lower jaw features, body colouring, specificity of head surface, cheeks; in burbot – difference from pikeperch in head shape.

When characterizing the scorpine family, establish the coloration of the body, eye size, head shape, peculiarities of the dorsal and anal fins and gill covers.

If a fish has a single dorsal fin, then in determining the family first of all pay attention to the presence or absence of scales. The catfish has smooth, scaleless skin.

Belonging to a particular family of scaly fish with one dorsal fin is established by the presence of the lateral line: fish without the lateral line belong to the herring. When identifying fish of the herring family, attention is paid to the shape of the abdomen, the number of dorsal fins, and their structure.

If a fish with one dorsal fin and a lateral line has a mouth with teeth, then it belongs to the pike family, while a fish without teeth belongs to the carp family. Determine types of pike fish by body shape and length, size of head and snout shape, size and location of teeth, dorsal fin, shape of the caudal fin, the features of the head cheek surface. Species of carp are determined by the nature of scales, location of the lateral line, noting: in carp, carp – scale size, location of the mouth, presence and number of antennae, shape and features of the dorsal fin, the number of rays in the dorsal and anal fins; in bream – body shape and anal fin, mouth location, number of scales on the lateral line and branched rays in the anal fin.