Although cetaceans can sometimes be easily mistaken for large fish, such as sharks, they are actually quite different. Whales, dolphins and porpoises are mammals. This means that they are warm-blooded, have lungs, not gills, and must surface to breathe air (the ‘spouting’ or blows associated with whales is warm air being exhaled). In addition cetaceans usually give birth to one calf every few years and nurse and care for the calf until it is able to survive on its own, unlike fish which can have thousands of offspring every year and give little or no parental care.
Click above to see the differences between cetaceans and large fish at www.arkive.orgFish and cetaceans also look different (see the differences yourself in the "Visual Differences" links above). An easy way to tell the difference between a cetacean and a fish is by observing the orientation of their tails. A fish’s tail is vertical and moves side to side while a cetacean’s tail is horizontal and moves up and down. When you observe closely you will notice that cetaceans have one or no dorsal fins and two pectoral fins in addition to their caudal fin (tail), whereas fish tend to have several additional fins. Also, you can observe gills at the side of a fish's head but not on a cetacean, as they breathe through one or two blowholes (their nostrils leading to lungs) at the top of the head.
Less obvious is the difference in their skins: fish have scales or dermal denticles, but cetaceans have soft skin instead. If you are ever able to feel the skin of a cetacean and scaley fish or shark you will notice at once that its smooth texture is different from the scales or rough, sand-papery skin of the fish.
Apart from appearances, fish, whales and dolphins also make different sounds. Some fish grunt or click while others seem silent. Cetaceans make many different noises which are generally divided into echolocation clicks (only toothed whales are known to make these sounds) and social noises such as whistles and squeaks. Some whales, such as the humpback whales sing long, complex mating songs.
The male humpback whale sings to attract a mate.