1. Perch can only swim freely above their plane of equilibrium within a narrow zone whose height is equivalent to a 16% (one-sixth) reduction in the total pressure to which they are adapted. Excursions from this zone would not involve more than a 33% reduction of the total pressure.

2. Calculations were made of the minimum time in which perch could make slow vertical movements involving reductions of 10-90% in the total pressure to which they were originally adapted, and it was concluded that perch would not make diurnal migrations in which the reduction was more than 40% of the total pressure.

3. It is unlikely that teleosts with closed swimbladders would be able to make diurnal migrations as extensive and rapid as that shown by the deep scattering layer unless the swimbladder was reduced in size, the bladder wall thickened to resist expansion of the gas, or the rate of gas resorption 20-30 times quicker than in the perch.

This content is only available via PDF.