Morphometric features

Morphometric refers to measurable characters or length-based
measures of specific body parts of fish, such as total length, head
length, eye diameter, or ratio between such measurements. These
characters are usually measured in the millimeter scale. Although
the morphometric measures can be compared to each other
directly, their proportions give more accurate information.
The body of fish can be divided into three parts: head, trunk
and tail. There is no neck in fish as in the land animals. The gill
opening is the boundary between the head and the trunk (Figure
4.17 & 4.18). Similarly, the vent is the dividing line between the
trunk and the tail. The relative proportion of these parts varies
greatly in different fishes. Some morphometric measures used in
fish identification and taxonomic studies are described below:
i. Total length: The maximum distance from the tip of the snout
to the tip of the caudal fin. This includes long pointy beaks, as
in Xenentodon, and long trailing tail filaments, as in Bagarius.
ii. Standard length: The distance from the tip of the snout to the
base of the caudal fin, not including the tail.
iii. Body depth or height: The distance from the highest part of
the dorsal surface to the ventral surface in a straight, vertical
line.
iv. Head length: Head length is measured from the tip of the snout
or upper jaw to the most posterior (preferably bony) portion of
the operculum.
v. Head depth: Sometimes called height of head at occiput, this
measurement goes from the top of the head (occiput) to the
bottom of the head or breast.
vi. Snout length: It is taken from the most anterior point on the
snout or upper lip to the front margin of the orbit.
vii. Eye diameter: The distance between the margin to margin of
the bony orbit.
viii. Pre-orbital: Widely used on cichlid studies, this is a bone at
the front of the head.
ix. Post-orbital: The region behind the eye to the rear of the
Introductory Ichthyologyoperculum.
x. Pre-dorsal length: A straight measurement from the tip of the
snout to the first ray of the dorsal fin.
xi. Post-dorsal length: A straight line measurement from the base
of the posterior of the dorsal fin to base of the caudal fin or end
of vertebral column.
xii. Caudal peduncle depth: It is measured along the vertical line
at its narrowest part between posterior base of anal fin to the
base of the caudal fin.
xiii. Caudal peduncle length: It is measured from posterior base
of anal fin to the base of the caudal fin.