Reading a fish's features
Check five things in order: body shape, mouth, fins, tail and markings. Body shape and the fins usually narrow a fish to its family; the mouth and the markings usually settle the species.
The five checks
| Check | What to look at | Example |
|---|---|---|
| Body shape | Long and round, tall and flat-sided, or flattened top to bottom | Pike are long and round; sunfish and crappie tall and flat-sided; flounder lie flat |
| Mouth | Size, how far the jaw reaches against the eye, teeth, barbels | A largemouth's jaw reaches past the eye; catfish and black drum have barbels |
| Fins | One dorsal fin or two, spines against soft rays, an adipose fin | Walleye and perch have two separate dorsal fins; trout have an adipose fin |
| Tail | Forked, square, rounded, or crescent-shaped; spots on the lobes | Tuna have a stiff crescent tail; a bowfin's is rounded; a red drum has a spot at its base |
| Markings | Bars, stripes, spots, and whether marks are light on dark or dark on light | Pike have light spots on a dark body; musky dark marks on a light body |
Reading the fins
The fin on the back is the dorsal fin. In many families its front part has stiff, sharp spines and its back part soft, branched rays. Counting the spines separates some close species: seven or eight in a black crappie's dorsal, five or six in a white crappie's. In bass the two parts are joined; in walleye and perch they are separate fins.
The fin under the body behind the vent is the anal fin. Its shape and ray count separate channel catfish (rounded, 24 to 29 rays) from blue catfish (straight-edged, 30 to 36). Count from a photo with the fin spread.
Quick family signs
- Catfish: barbels around the mouth, no scales, an adipose fin.
- Trout and salmon: an adipose fin, small scales, a single soft dorsal.
- Pike and musky: a long body, a duck-bill snout with teeth, one dorsal set far back near the tail.
- Gar: a long, narrow snout full of teeth and hard, diamond-shaped scales.
- Bowfin: one long dorsal fin running most of the back and a rounded tail.
- Sunfish: a tall, flat-sided body, the spiny and soft dorsal joined, and an ear flap at the back of the gill cover.
- Black bass: a longer body and a big mouth, the spiny and soft dorsal joined behind a notch.
- Drum: an arched back and a lateral line that runs onto the tail.
- Tuna and mackerel: a torpedo body, a crescent tail and small finlets between the last fins and the tail.
Questions and answers
What is the small fin near a trout's tail?
The adipose fin: a small fleshy fin with no rays, on the back between the dorsal fin and the tail. Trout, salmon and catfish have one; bass, sunfish and perch do not. On some waters a clipped adipose fin marks a hatchery fish.
What is the lateral line?
A row of sensory pores along each side of the body that senses movement in the water. Its shape helps with some species: on a king mackerel it drops sharply below the second dorsal fin, on a Spanish mackerel it slopes gently.
Sources for this entry
- FishBase Body shape and fin descriptions per species.
Keep reading
Cite this entry
Global Fishing League. “Reading a fish's features.” Best Fish Identification, updated 9 November 2026.https://bestfishidentification.com/fish-features/