From shimmer to structure: The anatomy of scales in Idaho's fish

One of the most fascinating features of many fish is their scales. These tiny, often overlooked structures play a big role in a fish’s survival, protection and movement through water. It's important to know there are three different types of scales that are present in Idaho’s fishes. They can have no scales on their body like Pacific lamprey, or they can have one of the following three types, being cycloid, ctenoid, or ganoid.

 Cycloid (si-kloid) (Figure 1) scales are found on salmon and trout species. These scales are very circular and are the thinnest among all the scales.

The ctenoid (te-noid) scale (Figure 2) which is found on fishes like perch, bass, and walleye. This scale can have a comb-like structure that forms on the top (most commonly on walleye scales) and is larger and thicker compared to cycloid scales.

Ganoid scales are the least common type present in Idaho Fish, and can be found on ancient species, such as sturgeons. Sturgeon possess a modified ganoid scale called a scute (figure 3). Scutes are armor like, denser and bonier than scales found in other Idaho fish. These scales are discovered on more ancient fishes and have an extremely dense layer of bone that makes it seem a lot more like human bone compared to the other scale types. 

Fish scales are not just what you see from the outside, a larger portion of the scale is embedded into the fish and then hidden under the exposed section of other scales. Like the tip of an iceberg, only the smaller portion of the scale is visible on the exterior of the fish. Most of the scale is beneath the surface, overlapped by other scales.

The hidden section of a fish’s scale reveals a pattern of incremental ridges that look like rings following around the scale. These rings can provide growth and age information, similar to how you would age a tree. If you want to learn more about how to age fish scales and why we age fish in Idaho, take a look at this blog: Read between the scales: An introduction to aging fish and why it's important | Idaho Fish and Game.

Not all fish scales are the same, but all are unique and beautiful adaptations that serve important functions for Idaho fishes. These functions include a regenerating armor that protects fish from the environment and predators. Scales also make fish more efficient swimmers and can serve as a reservoir of important minerals when food is less available.