Role of water current and related variables in determining buoyancy in the sticklebacks Culaea inconstans and Pungitius pungitius

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The sticklebacks Culaea inconstans and Pungitius pungitius alter buoyancy (lift from the swim bladder) in the field primarily in response to changes in water velocity. Potential adjustment is extensive and is only realized when fish are exposed to current for several days. Buoyancy measured on fish from the field showed an inverse relationship with water velocity and temperature. Buoyancy was also highest in fish living among vegetation. However, vegetation was restricted to still water or areas of very low velocity. In the laboratory, water temperature affected buoyancy of C. inconstans in current only; there, at 6 °C, buoyancy was greater than at higher temperatures. In the field an unknown factor(s), dependent on temperature, affects buoyancy. An inverse relationship was found between buoyancy and water velocity in the laboratory, and adjustment of buoyancy was slow. The time required for buoyancy to decrease from maximum to minimum levels was 4 and 7 d for P. pungitius and C. inconstans, respectively. In the field, minimum buoyancy levels were not reached because fish were exposed to weak current velocities for short intervals. The ability to reduce swim bladder lift (volume) in strong velocities and replace it with hydrodynamic lift and vice versa is seen as an adaptation to environments where water velocity varies in time and space permitting fishes to use the most effective source of lift.