New Kinematic Data From High Speed Cine Film Recordings of Swimming Cod (Gadus Morhua)

Abstract

1. We described a method to determine the values of the parameters of the body wave: speed v, wavelength λ b , wave period T b and of the sinusoidal track in space of different points of the body of a swimming fish: speed u, wavelength λ s , wave period T s and amplitude A. 2. These characteristic wave parameters were measured from high speed cine recordings of straight forward swimming behaviour of one 0.42 m Cod. 3. The instantaneous propulsive force of the tail was determined using LIGHTHILL'S (1971) large amplitude elongated body theory. 4. The same theory predicts and data in this paper affirm, that, to increase propulsive force per unit virtual mass, a fish should increase the lateral speed of the tail tip and should keep the angle between tail blade and direction of movement not far from values between -10° and + 10°. 5. Results show that swimming at higher speeds as well as acceleration is always attended with an increase of v as a result of a decrease of the period of the waves of the body. The body wavelength λ b does not change with speed. The wavelength of the tracks in space, λ s , slightly decreases with an increase of speed. 6. Relationships between some of the characteristic features of the cycles of swimming movements are discussed. With the aid of a simple model, limits of swimming abilities of the fish studied could be determined. 7. We came to the conclusion that propeller efficiency during steady swimming at cruising speeds decreases with increasing speed.