Gunnar Sundnes

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 mecha-nism 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