Buoyancy Response to Changes in Water Velocity and Its Function in Creek Chub (Semotilus atromaculatus)

Abstract

Creek chub (Semotilus atromaculatus) reduce buoyancy by decreasing swim-bladder volume when exposed to water current. Fish were near neutral buoyancy (0.956–0.997 mL∙g−1) in still water but in current small fish (< 73 mm) were more buoyant (0.668 mL∙g−1) than larger fish (0.540 mL∙g−1). Extent of buoyancy alteration was influenced by acclimation temperature, water velocity, and an interaction of both these factors. Reduction of buoyancy on exposure to water current required 3–6 h and the rate of decrease (0.035–0.068 mL∙g−1∙h−1) was independent of acclimation temperature and water velocity. The hypothesis that creek chub reduce buoyancy when exposed to water current to compensate for the lift created due to deflection of water over the dorsal surface of the body was supported by the following findings. When initially exposed to current, fish lowered the head assuming an angle of attack of 9–12° from horizontal. A horizontal (< 1°) angle of attack was gradually assumed during buoyancy adjustment. Fish acclimated to moderate current for 12 or 24 h tired less readily in strong current (80% took 43 and 44 min, respectively) than those not acclimated or acclimated for 6 h (80% took 33 and 25 min, respectively). Fish induced to swim in still water reduced buoyancy to a similar extent as those in current. Key words: swim-bladder volume change, temperature, hydrodynamic lift, swimming angle of attack, time to fatigue