What is the function of a dorsal fin on a perch?

The dorsal fin keeps the perch's body from rolling from side to side and assists the fish in making sudden turns and stops. It plays no role in propulsion.

Except for the tail fin, perch fins have no connection to the skeleton and are held in place by muscles. The tail fin helps propel the fish and provides up to 40 percent of the fish's forward thrust. The remaining thrust is generated by the muscular body as it flexes from side to side. Perch angle the paired fins on their bodies to swim upward or downward and also use these fins to slow down and stop.