Taxonomy of Finfish

2.3.2 Methods of Measuring

Measurements are made with special measuring boards. Length measurements are usually made with the fish lying on its right side snout to the left, on a measuring board consisting essentially of a wooden or metal base carrying a centre scale and having a headpiece (nose block) against which the snout is to be pressed (Holden and Rait, 1974). The mouth of the fish should be closed, the fish body and tail are straightened along the mid-line and the readings are to be recorded from the scale. The measurements should be recorded to the nearest 0.5 mm with a fine draftsman dividers using a fresh fish in a near to relaxed condition as far as possible. Rays and other dorso-ventrally flattened fishes may be measured by lying straight on their ventral surface. Disc width rather than overall length is sometimes used as linear dimension of rays. Large fishes could be measured with calipers or from point to point along the body surface with a tape.

If a fish is to be measured in centimeter units, a board with 1 m long is sufficient. For a larger specimen, an extension piece of 30 cm long can be clipped or hinged to the board. For fish measured in half-centimeter units, a board of 50 cm long is usually sufficient. The scale must correspond to the measurements being recorded. It is also not possible to measure fish to the nearest centimeter below on a board ‘marked 2 cm intervals’. Too many divisions in a scale will, either lead to mistake or waste time in recording characters to the nearest division.