The Swimbladder and the Vertical Movements of Teleostean Fishes

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Abstract
The vertical movements of a teleostean fish may be restricted by the presence of the swimbladder, which will increase or decrease in volume when the fish moves up or down in the water. It is shown that the restriction that the swimbladder imposes to vertical movements involving a reduction in pressure will depend on physical factors such as (1) The resistance that the bladder and body wall offer to the expansion of the bladder gas. (2) The percentage volume of the swimbladder and the density change of the fish when it is subjected to a reduction in pressure. (3) The pressure reduction that leads to the rupture of the bladder wall. A distinction is made between rapid and slow movements. In the former the compensatory ability of the fish must be considered and in the latter the speed with which the fish can accommodate itself to pressure changes. An equation is derived from which the minimum speed at which a physoclist can migrate from deep to shallow water can be calculated. To solve the equation two factors must be determined experimentally. Various experiments are described which were made on the perch, Perca fluviatilis, the wrasse, Crenilabrus melops, the rockling, Onos mustela and the dragonet, Callionymus lyra. The results showed that there was a relation between the relative size of the swimbladder and the change in the density of a fish when it is subjected to a reduction in pressure; that the bladder and body wall of the perch offer little resistance to the expansion of the bladder gas; and that the danger of the bladder wall rupturing might restrict the extent of rapid movements made by the perch. Experiments on the restriction that the swimbladder imposes to the rapid and slow vertical movements of the perch will be described in a following paper.

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