Pressure distribution on the body surface of swimming fish.

Although there are a number of papers on how fish swim (Gray, 1957; Aleev, 1969; Lighthill, 1969), we have been unable to find any measurements of the pressure on the body surface measured while a fish is swimming. A preliminary calculation using the Pitot equation indicated that the pressure against the anterior surface of the fish, expressed in centimetres of water, would be approximately equal to the square of the speed of the fish in miles per hour. A fish one foot long, swimming at six miles per hour, would have a pressure of about 36 cm H2O against the front of his head. Such pressures, if actually present, might be large enough to affect the body functions, particularly circulatory, and ultimately influence the body structure of aquatic animals. They are of the same order of magnitude as the hydrostatic pressures due to gravity acting on land animals of equal body size, and as the blood pressure in fish (Satchell, 1971). In addition, the determination of the pressure distribution on the body surface of swimming fish may be of help in explaining the mechanism of fish locomotion. Compared to pressure distribution on the body surface of dead fish, or casts, the difference may be responsible for the otherwise unexplained ability of some fish to sustain locomotion with relatively little energy expenditure (Gray, 1957; Schmidt-Nielsen, 1972). In this study we measured the pressure on the body surface of swimming bluefish, and we speculate as to the physiological implications of the pressures which were found.