Gills
Located on either side of a fish’s head, gills remove oxygen from the water and diffuse
carbon dioxide from the body. The gills are covered by a flexible bony plate called the
operculum. Some fish have spines located on the operculum as a defense mechanism to protect
them from predators.
Lateral Line
Running down the length of a fish’s body is the lateral line. It is made up of a series of
microscopic holes located just under the scales of the fish. One of the fish's primary sense
organs, the lateral line can sense low vibrations in the water, and is capable of determining the
direction of their source.
Nares
All fish possess a sense of smell. Paired holes called nares, are used for detecting odors in the
water, and are located on a fish’s snout. Some fish, such as some shark varieties, catfish, and
eels, have a very sharp sense of smell.
Scales and Slime
Most fish have scales covering the length of their body. Scales protect fish from injury, much
like skin on the human body. On top of these scales is a mucous covering known as the slime
layer. Slime protects fish from bacteria and parasites in the water. Anglers should be careful not
to remove the slime layer when handling a fish.