Estimates of pressure differences across the head of a swimming clupeid fish

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Abstract
This paper is concerned to estimate, for a regularly swimming clupeid fish, the effective pressure difference that drives those motions in the subcerebral canal which can stimulate the lateral-line neuromasts (see the preceding paper by Denton & Gray 1993). Hydrodynamic analysis indicates that pure sideslip of the head (at observed sideslip velocities) would generate a pressure difference so great that the neuromasts would be saturated; however, simultaneous yawing can enormously reduce the effective pressure difference. For this purpose the angle of yaw would need to be kept in phase with sideslip velocity, with a magnitude only a little less than the ratio of sideslip velocity to swimming speed, making the `crossflow' of water across the yawed head small. These moreover are conditions which tend to avoid any serious distortions of the boundary layer on the fish's surface by `crossflow', such as are known from other evidence to increase significantly the resistance to the fish's motion. It is noted that the lateral-line sensors would provide an appropriate feedback signal into a possible system for controlling yaw by oscillatory neck deflections so as to minimise the effective pressure difference and any associated crossflow effects. It is suggested that swimming clupeid fishes may use such an `active' mechanism for reduction of hydrodynamic resistance. The same ratio (around 0.87) of yaw angle times swimming speed to sideslip velocity is estimated: (i) to annual the signal sensed by lateral-line neuromasts; and (ii) to remove crossflow in the boundary layer over the head. The succeeding paper (Rowe et al. 1993) gives evidence, both that yaw is kept in phase with sideslip velocity, and that the above ratio (see their figure 4) remains close to 0.87, in a swimming herring.

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