The effect of swimbladder deflation on pressure sensitivity in the saithePollachius virens

The saithe,Pollachius virens(L.), makes substantial diel vertical migrations like many other teleost fish. Adulst saithe offshore are known to ascent at dusk from 100 to 10 m (Schmidt, 1955) during which they experience a pressure decrease from 11 to 2 atm. If the swimbladders of these fish were fully adapted to the daytime depth, the gas of the swimbladder (which ‘obeys’ Boyle's Law) would expand 4½ times and rupture the swimbladder wall (Tytler & Blaxter, 1973). Even if not fullly adapted it is likely that some increase in volume would be experienced, giving not only the danger of the swimbladder bursting, but also a serious equilibrium problem as the fish become more positively buyant during the ascent. Jones (1952) for example suggested that the physoclistous perch adapted to a particular pressure could only cope with a 33% decrease in pressure without rising to the surface out of control.