The Buoyancy of the Sun-fish

Abstract

ABOUT the end of August numbers of sun-fish (Orthagoriscus mola) make their appearance off the north coast of Ireland floating passively on the surface of the sea. The positive buoyancy, while not so great as to prevent the fish from submerging when attacked, constitutes a constant upward drag which brings it back to the surface as soon as it stops swimming. So far as we know, the bodies of all other fish have a specific gravity greater than that of sea-water, and the swim-bladder, where it exists, contains the necessary amount of gas to compensate for this and bring the body of the fish to neutral buoyancy.