HYDROSTATIC STABILITY OF FISH WITH SWIM BLADDERS - NOT ALL FISH ARE UNSTABLE

The center of mass and the center of buoyancy were found to occur at t he same relative longitudinal location along the body of the elongate- bodied European eel (Anguilla anguilla), the fusiform yellow perch (Pe rca flavescens), and two gibbose species, rock bass (Ambloplites rupes tris) and bluegill (Lepomis macrochirus). The locations of these cente rs 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 ma ss was dorsal to the center of buoyancy in yellow perch. The observati ons on bluegill show that neutral hydrostatic rolling equilibrium is p ossible. Our conclusions on similarity or separation between the locat ions of the centers of mass and buoyancy derive from tests of signific ance and differ from conclusions reported elsewhere on the basis of me an Values only. In general, fish with swim bladders can be at or close to neutral hydrostatic equilibrium but any disturbance will be destab ilizing. This probably explains continuous fin-beating of fish in appa rently still water.