Hydrostatic stability of fish with swim bladders: not all fish are unstable

Abstract

The center of mass and the center of buoyancy were found to occur at the same relative longitudinal location along the body of the elongate-bodied European eel (Anguilla anguilla), the fusiform yellow perch (Perca flavescens), and two gibbose species, rock bass (Ambloplites rupestris) and bluegill (Lepomis macrochirus). The locations of these centers were not affected by fish size. Therefore the fish were neutral in hydrostatic pitching equilibrium. The vertical location of the centers of mass and buoyancy were the same for bluegill, but the center of mass was dorsal to the center of buoyancy in yellow perch. The observations on bluegill show that neutral hydrostatic rolling equilibrium is possible. Our conclusions on similarity or separation between the locations of the centers of mass and buoyancy derive from tests of significance and differ from conclusions reported elsewhere on the basis of mean values only. In general, fish with swim bladders can be at or close to neutral hydrostatic equilibrium but any disturbance will be destabilizing. This probably explains continuous fin-beating of fish in apparently still water.