The functions of fins

The fins on flashy 1950s cars were, practically speaking, useless appendages, all style and no substance. The fins on fish, though, are a different matter.

Fins may add to fishes’ beauty – take for example the “sail” on a marlin – but they are also highly functional, as essential as arms and legs are to us humans.

So, what exactly do these fins do? Let’s start with the tail fin, or what the scientists call the caudal fin. It’s mainly for propulsion. If the fish’s muscles are the engine, then the tail fin is the propeller. A few whips of the tail and the fish can be off in a flash, chasing prey or fleeing a predator. The tail fin also contributes to steering.

The dorsal fin, the one that runs along the top of the spine, adds stability during travel, a little bit like the centerboard on a sailboat or the fletching on an arrow. In many fish the dorsal fin also contains spines. It can lie flat or be unfurled, spines vertical.

The expanded dorsal fin can help protect a fish by making it appear, to a predator, larger than it actually is. The spines themselves can also deter attacks. If you’ve ever been “speared” while unhooking a perch, imagine how that would feel to the inside of a larger fish’s mouth. The anal fin, on the fish’s underside forward of the tail, also lends stability.

Deep-bodied fish, like bluegills or crappies, generally have larger dorsal and anal fins because they need more support to hold themselves upright.

Pelvic fins, on the underside coming off the belly, also help the fish stay level, providing stability against rolling from side to side. The pectoral fins, generally on the side of the fish just behind the gills, add stability and help the fish maneuver and control depth. Pelvic and pectoral fins also act as brakes – when flared out they help the fish slow down and stop.

Fin sizes, shapes and configurations vary with fish species, where they live, and how they feed. If you’d like to know more, there's a great online presentation from the the Minnesota Department of Natural Resources.