WEBB, P.W.: Mechanisms for the control of posture and swimming trajectories of fishes

Fish with a high density ratio can achieve stability on the substratum. Morphological design criteria stabilizing station holding and maximizing current speeds displacing fish are well known, but optimal designs for all criteria are not possible. In the water column, fish are inherently unstable. Posture of fish in the water column is not stable because the metacentric height is typically negative and the centers of mass are separated longitudinally. When gas inclusions are used to regulate density, altitude is not stable. Neutrally buoyant fish are unstable in coasting. Biological materials are deformable, and some control surfaces are forward of the center of mass both of which tend to amplify disturbances. When swimming, stability depends on dynamic controls, generated by the same range of body and fin appendages that introduce instabilities for maneuvering. Dynamic forces may be passive, resulting from flow over the control surface due to the motion of the body, or self-powered. Powered controls are required to stabilize altitude and swimming trajectories at low speeds, but these are energetically expensive. Passive controls are effective and more economical at higher speeds and can be self-correcting. Predictable disturbances, such as recoil, may be damped using passive systems, but powered control is required to improve the smoothness of swimming trajectories. Unpredictable disturbances are a greater challenge, especially when self-correction is not possible or inadequate, and when response delays occur. Some preliminary observations suggest reliance on powered control affects the distribution of various fish morphs in hydrodynamically variable habitats.