Physical Properties and Hydrostatic Function of the Swimbladder of Herring (Clupea harengus L.)

Abstract

Living herring at the depth of adjustment had a mean sinking factor of 1003, density of 1.026 g/ml, relative sensitivity of 0.8 and percentage swimbladder volume of 4.2. Neutral buoyancy was attained at a mean pressure reduction of 5.5% from the adjusted pressure. Swimbladder gas was under an average excess pressure of 1 cm Hg. Gas was released through the posterior swimbladder duct during pressure reduction in 105 out of 109 herring observed. Gas release occurred at a mean pressure decrease of 6% in rapidly swimming herring, at 32% in moderately swimming fish and brought the herring to within 19% of perfect adjustment to a new reduced pressure within half an hour. Herring could compensate for their increased buoyancy during pressure decrease until this was reduced by gas release. Decompression at rates up to 123 cm Hg/sec was not fatal after 16 hours at the greater pressure. No recovery of buoyancy after gas loss occurred in herring held 24 hours in running sea water even if fine air bubbles were present. Recovery occurred if these fish had access to the surface. Gas production by bacterial activity as a means of restoring buoyancy was not established. Herring responded to rapid pressure increases by swimming upwards. They could compensate for their increased density following pressure increase of 300% and survive increases of 430%. Herring from 10 to 25 feet depth at sea were positively buoyant at surface pressure when anaesthetized. Thus in nature herring are adjusted to pressures greater than surface pressure. It is suggested that they take in air when feeding at the surface at night and slowly pass this to the swimbladder on returning to greater depths by day.