Standard length: not quite so standard

Abstract

A variety of morphological, behavioral, physiological, and biochemical characteristics are used to identify and classify fishes. In practice though, it is more common to use morphometric measurements (i.e., body length, body depth, head length, eye diameter, jaw length) and meristics (i.e., fin ray, scale, teeth, gill raker, and lateral line pore counts). These morphometric measurements are usually presented as a proportion of either standard or total length. Unfortunately, in early works, it was often vague as to how lengths were taken. In addition, a protocol of how to measure standard length has never been standardized; consequently, several different definitions and interpretations exist. If a standardized protocol was implemented this could make comparisons of new measurements with older ones more difficult if the specimens are damaged, in a fragile state of preservation, or no longer exist. Standard length as a measurement has been in use for over a century, only the name is modern. This study reviews the chronological development of the concept of standard length and provides suggestions as to how ichthyologists should proceed into the next century.