Abstract: The kinematics of steady forward swimming of Gymnarchus niloticus are described. The geometric features of the body and locomotive characteristic and parameters of the flexible dorsal fin are discussed. A simplified kinematic model on locomotion of the undulatory long dorsal fin propulsion is based on the the observation and experimental data. The hydromechanical performances of the undulatory long dorsal fin propeller of Gymnarchus niloticus are estimated applying the large-amplitude elongated-body theory. The hydromechanical efficiency of the undulatory long dorsal fin system ranged from 82.284% to 87.455% over a speed rang of 0.728~0.985 lengthes·s–1. It is suggested that the undulatory long dorsal fin propulsion is an adaptation to swimming with high hydrome-chanical efficiency.

Key words: Dorsal fin, Hydromechanical efficiency, Kinematic model, Undulation