Taxonomy of Finfish

2.3.4 Definitions of Linear Measurements

Overall length measurements are made between perpendiculars along the median longitudinal axis from the snout (U, the position of the maxillary symphysis). Measurements from L are taken with the mouth closed. The other measurements to be taken are:

1. Standard Length: Taken from U to the tip of the hypural bone (urostyle). This varies from species to species.
2. Fork Length: Measured from U or L to the cartilaginous tip of shortest or median caudal ray.
3. Total Length: Measured from U or L to the longest caudal fin ray, upper or lower, or an average of both of them.

Longitudinal measurements other than overall length are also made between perpendiculars using measuring board with, for example, a sliding cursor. When these are made radially from point U, calipers are recommended. Point to point measurements are sometimes made on big fishes such as tunas by tape. These would be indicated by the word ‘surface’ as these are not generally recommended. All measurements from LX to LM and also their ‘upper’ equivalents are grouped under the general name ‘total length’ LT. LM has been called ‘bilobular length’ and ‘total auxiliary length’. The word ‘Extreme’ is used in LX. 

Fig No: Morphology and measurments of teleost