A dorsal fin is a fin located on the backs of some fishes, whales, dolphins, and porpoises, as well as the (extinct) ichthyosaurs. Its main purpose is to stabilize the animal against rolling and assist in sudden turns. Some animals have developed dorsal fins with protective functions, such as spines or venom. 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.
Dorsal fins come in a variety of shapes and sizes.
They are infamous for being the sign of an approaching shark.
Whales, dolphins, and porpoises
In relation to the size of the creature, the dorsal fin of the male orca is quite large, as much as 1.8 m high. (The female orca has a shorter dorsal fin that is more curved.) Many (30-100%) captive male killer whales (orcas) experience collapse of the dorsal fin, possibly because lack of exercise through turning leads to diminished muscle tone, other possible factors include change in diet, sex drive and pressure in the pool due to counter-clockwise swimming, this reacts with the sodium nitrate in the whale's pituitary gland and leads to the loss of connective tissue in the dorsal area (Only about 1% of wild orcas have collapsed dorsal fins.) The dorsal fins of most other whales are relatively small. The bowhead whale has no dorsal fin at all, as an evolutionary adaptation to its life spent cruising under icepack. The dorsal fins of whales develop distinctive nicks and wear patterns with time, and this fact is used by wildlife biologists to identify individuals in the field.