Swim bladder gas composition and control of buoyancy by fathead minnows (Pimephales promelas) during exposure to hypoxia

Abstract

In normoxic water, swim bladder gases of the fathead minnow consisted of 1.5% CO2, 10.2% O2, and 89.2% N2, and remained unchanged over 24 h. Fathead minnows were exposed to hypoxic water with and without access to the surface and changes in swim bladder lift and gas composition were recorded. After 12 h of hypoxia with surface access, mean swim bladder lift (buoyancy) increased from slightly negative to strongly positive with extreme variation between individuals. Lift declined slightly between 36 and 60 h in hypoxia. O2 and CO2 proportions were maintained at levels similar to those found in normoxic waters for 12–36 h but then these declined such that N2 was 96% of the swim bladder volume after 60 h of hypoxia. Without surface access, swim bladder lift and gas composition remained unchanged. Changes in swim bladder lift were due to addition and (or) losses of gases and were not due to changes in internal pressure of swim bladder gas. Mechanisms of controlling swim bladder volume (lift) in hypoxic water and the significance of changes in swim bladder gas composition are discussed.