- Standard length (SL) refers to 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 fin.
- Total length (TL) refers to the length 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.
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.
In addition, fishery biologists often use a third measure in fishes with forked tails:
- Fork length (FL) refers to the length 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.
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.
- "Term: standard length". FishBase.org. 2004-11-18. Retrieved 2006-11-02.
- "Term: total length". FishBase.org. 2004-11-18. Retrieved 2006-11-02.
- "Term: fork length". FishBase.org. 2004-11-18. Retrieved 2006-11-02.
- "Standard Measurements of Bony Fish diagram". Florida Museum of Natural History Ichthyology Department. Retrieved 2007-06-06.
- Diagram of fish measurements - Florida Museum of Natural History Ichthyology Department