Mechanisms of buoyancy adjustment and effects of water velocity and temperature on ability to maintain buoyancy in fathead minnows, Pimephales promelas, rafinesque

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1. 1. Fathead minnows ( Pimephales promelas) increase buoyancy when water velocity declines by simultaneously gulping air at the surface and passing it into the swimbladder and by secreting O 2 and CO 2. When water velocity increases resorption of gases (initially O 2 and CO2) is used to reduce buoyancy. 2. 2. The negative buoyancy reached in flowing water (10–30 cm/sec) can be maintained for at least 42 days in temperatures from 10 to 30°C. 3. 3. In flowing water buoyancy remained unchanged when water temperature was increased to near lethal levels but when water velocity was increased and temperature held constant, buoyancy declined linearly. When water velocity and temperature were increased concurrently their effects were antagonistic and buoyancy remained unchanged.