A dorsal fin is a fin located on the backs of some fish, whales, dolphins, and porpoises. Depending on the species, an animal can have up to three of them. Wildlife biologists often use the the distinctive nicks and wear patterns which develop on the dorsal fins of large animals, especially whales, to identify individuals in the field. The main purpose of the dorsal fin is to stabilize the animal against rolling and assist in sudden turns. Some species have furthermore adapted their dorsal fins to other uses. In anglerfish, the anterior of the dorsal fin is modified into a biological equivalent to a fishing pole and a lure known as illicium or esca. Many catfish can lock the leading ray of the dorsal fin in an extended position to discourage predation or to wedge themselves into a crevice. Some animals have developed dorsal fins with protective functions, such as spines or venom. For example, both the spiny dogfish and the Port Jackson shark have spines in their dorsal fins which are capable of secreting poison.
Tench Dorsal Fin - WWC Archives