The Hydrodynamic Drag of Swimming Bluefish (Pomatomus Saltatrix) in Different Intensities of Turbulence: Variation With Changes of Buoyancy

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Abstract
Thrust and drag were calculated for ten bluefish using Newton's law, F = ma, the measured acceleration of the fish, and one essential assumption, without which only the sum of thrust and drag could be determined. Any time the thrust is zero, the drag equals the mass of the fish times its negative acceleration. For reasons given in the full text, the thrust is believed to drop to zero twice per swimming cycle, and never become negative. Zero thrust thus occurs at the negative acceleration peaks. For acceleration waveforms that are symmetric, zero acceleration lies at a level half way between the levels of the positive and negative peaks, so the peak negative acceleration is half the peak-to-peak height of the waveform. It is further assumed that the instantaneous drag thus measured at the negative acceleration peaks equals the mean drag averaged over the whole swimming cycle, which in turn equals the mean thrust. Turbulence, induced in the water tunnel by two flat metal plates placed in the stream in front of the fish, or by rotation of a mixing rod, did not change the drag, or the fish's ability to produce thrust, as indicated by the maximal swimming speed. The attachment of two pressure transducer enclosures to the tail did not alter the maximal swimming speed. Changes of buoyancy were produced by inflation or deflation of balloons sewn into the swim bladders of three fish. The thrust calculated from fluctuations in the forward acceleration of the fish measured at zero speed during neutral buoyancy was 0.0 to 0.14 N. If the non-zero values are genuine, they must be balanced by rearward forces from the pectoral fins. The thrust increased linearly with speed at the rates of 0.40, 0.64, and 0.71 N per m.s−1, respectively, at speeds between 0 and 0.8 m.s−1. The mean tail thrust of a negatively buoyant fish at zero water speed equalled or exceeded the body's weight in water. Its forward component must be balanced by a rearward component of the lift force from the flapping pectoral fins. Upward components of the two forces support the weight of the fish. With 45 ml air removed from the swim bladder of a bluefish, the tail thrust diminished from 0.64 N at zero water speed to a value of 0.46 N as the water speed was increased to between 0.2 and 0.4 m.s−1, and then increased to 0.71 N at a water speed of 1 m.s−1.

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