The sailfish is widely regarded as the fastest swimmer in the ocean.
Sailfish have been credited with speeds around 110 kilometres per hour, but precise speed claims are difficult to measure in the open sea. Scientists therefore often describe them more cautiously as among the fastest fish rather than treating every historic figure as exact.
Their distinctive dorsal fin, called a sail, can be raised during displays and feeding. Sailfish may herd schooling fish with their bills and fins, then strike individual prey. Their streamlined bodies and powerful tails help them accelerate rapidly.
The black marlin and some sharks are also frequently named in speed comparisons. Such rankings depend on whether researchers measure a brief burst, sustained swimming, or an estimate from body mechanics, which explains why sources sometimes differ.