The main difference between a dolphin and a porpoise is size; porpoises are generally smaller, reaching a length of up to 2.5 metres. The smallest dolphin, the Maui Dolphin, reaches just 1.2 metres. The largest member of the dolphin family is the Killer Whale or Orca, that can be up to 9.5 metres long. The Orca, although called a whale is actually a member of the order Cetacea, which includes dolphins and porpoises.
Porpoises have a triangular dorsal fin, whereas dolphins’ dorsal fins are more hooked shaped. Porpoises also have flattened, spade shaped teeth, whereas dolphins’ teeth are conical in shape. Porpoises’ heads are more blunt and less rounded than dolphins’ heads. Porpoises also have a stouter, more compact body shape.
Porpoises are less adaptable to captivity than their dolphin or orca cousin; so you won’t find many in public aquariums. There are however, more endangered or threatened species of dolphins than there are of porpoises. The Vaquita porpoise, native to the Northern California coast is endangered; with as few as 100 left in the world. Both the Ganges and the Yangtze River Dolphins are Critically Endangered or Functionally Extinct – no individual Yangtze River Dolphins have been documented. If an animal if Functionally Extinct, there are either: no individuals found, or the ones in existence are too advanced in age to be viable reproductive specimens.
The Ganges River Dolphin is actually one of two subspecies of the South Asian River Dolphin. The other is the Indus River Dolphin. These two dolphins have not interbred in many hundreds, and possibly thousands of years. The Indus River dolphin may have as few as 1000 individuals. The Ganges River Dolphin may have a couple thousand individuals; but there habitat is extremely threatened with dam building projects and pollution.