Fish measurement

• Standard length (SL) is the length of a fish measured from the tip of the snout to the posterior end of the last vertebra or to the posterior end of the midlateral portion of the hypural plate. Simply put, this measurement excludes the length of the caudal (tail) fin.[1]
• Total length (TL) is the length of a fish measured from the tip of the snout to the tip of the longer lobe of the caudal fin, usually measured with the lobes compressed along the midline. It is a straight-line measure, not measured over the curve of the body.[2]

Standard length measurements are used with Teleostei (most bony fish), while total length measurements are used with Myxini (hagfish), Petromyzontiformes (lampreys), and (usually) Elasmobranchii (sharks and rays), as well as some other fishes.[2]

Total length measurements are used in slot limit and minimum landing size regulations.

In addition, fishery biologists often use a third measure in fishes with forked tails:

• Fork length (FL) is the length of a fish measured from the tip of the snout to the end of the middle caudal fin rays and is used in fishes in which it is difficult to tell where the vertebral column ends.[3]

Other measurements that may be taken include the lengths of various fins, the lengths of fin bases, the length from the snout to various points on the body, and the diameter of the eye.[4]

Abyssal grenadier

• 
  Black drum
• 
  Red snapper

• Ichthyology terms
• Standard weight in fish

• Diagram of fish measurements - Florida Museum of Natural History Ichthyology Department