Studies of a physostome swimbladder by resonance frequency analyses

The resonance frequency of the swimbladder was measured for intact living charr at different depths. The acoustical behaviour of the charr swimbladder was similar to that of a free bubble of gas varying with depth in a similar manner. However, due to shape and tissue effects, the resonance frequency of the charr swimbladder was about 40% higher than for a comparable free bubble. The swimbladder volume was calculated from the resonance frequency of the bladder, and the volume changes which occurred with time were recorded for fish kept at a certain constant depth without access to the surface. Four different patterns of volume/time relationships were observed; a) the calculated swimbladder volume could show a sudden decrease, probably due to gas release through the pneumatic duct; b) a gradual decrease of swimbladder volume occurred with time, probably because of loss by diffusion; c) an apparent constant swimbladder volume occurred, indicating that the gas lost by diffusion might be fully compensated; d) in one case the apparent volume of the swimbladder increased 44% in 20 h. Charr therefore possesses a gas filling mechanism in addition to the swallowing of gas at the surface. Since charr have no gas gland, this gas filling mechanism may be different from that described for physoclists.