Adipose fin
A small fleshy fin without rays on the back near the tail, found in salmon, trout, catfish and characins.
The adipose fin is a small, soft flap on the back between the dorsal fin and the tail. It has no bony rays. It is found in salmon and trout, many catfishes, tetras and some other groups, and is missing in most others. Its job was long thought trivial, but studies on trout suggest it carries nerves that may sense water flow. Hatcheries in North America clip the adipose fin of farmed salmon before release, so that anglers and scientists can tell hatchery fish from wild fish.