Body length is absolute - defined as the distance from the tip of the snout to the outermost rays of the caudal fin. It is used to indicate the size limits of the fish.

The length of the whole fish (zoological or absolute) is measured from the top of the snout to the vertical end of the longest blade of the caudal fin when the fish is horizontal.

To measure fish, various instruments and devices are used: measuring tapes, measuring boards, calipers, rulers, centimeter tape measures. On ships, especially during the rocking, to measure fish use a measuring trough. It is made of slats with small openings between them, through which water and slime flow down. Measurements of small fish, as well as more accurate measurements of head length and body height are made with a caliper. When measuring, the fish is placed on the measuring board with its right side, head to the end edge.