The dorsal fin is a stabilizer fin
The dorsal fin is located on the top of a fish (as well as some marine mammals). Predominately, the fin is used to stabilize fish in the water and help direct the fish through turns and stops. Some fish, like Striped Bass (Morone saxatilis), have two dorsal fins and others have very highly derived dorsal fins, such as the Ocean Sunfish (Mola mola) which uses its dorsal fin as a primary mode of locomotion.
The Ocean Sunfish has a highly modified dorsal fin
The dorsal fin of sharks is one of the fins targeted in shark finning operations and destined for shark fin soup and traditional Chinese medicines. Shark fin soup is a delicacy in the Chinese culture, served at special occasions, including weddings. As China has become more affluent, the demand for luxury items, such as shark fin soup, has dramatically risen.
Dorsal fins of sharks are targeted for shark fin soup