Physostome

Abstract: The swimbladder of fishes is readily compressed by hydrostatic pressure with depth, causing changes in buoyancy. While modern fishes can regulate buoyancy by secreting gases from the blood into the swimbladder, primitive fishes, such as sturgeons, lack this secretion mechanism and rely entirely on air gulped at the surface to inflate the swimbladder. Therefore, sturgeons may experience changes in buoyancy that will affect their behavior at different depths. To test this prediction, we attached data loggers to seven free-ranging Chinese sturgeons Acipenser sinensis in the Yangtze River, China, to monitor their depth utilization, tail-beating activity, swim speed and body inclination. Two distinct, individual-specific, behavioral patterns were observed. Four fish swam at shallow depths (7–31 m), at speeds of 0.5–0.6 m s−1, with ascending and descending movements of 1.0–2.4 m in amplitude. They beat their tails continuously, indicating that their buoyancy was close to neutral with their inflated swimbladders. In addition, their occasional visits to the surface suggest that they gulped air to inflate their swimbladders. The other three fish spent most of their time (88–94%) on the river bottom at a depth of 106–122 m with minimum activity. They occasionally swam upwards at speeds of 0.6–0.8 m s−1 with intense tailbeats before gliding back passively to the bottom, in a manner similar to fishes that lack a swimbladder. Their bladders were probably collapsed by hydrostatic pressure, resulting in negative buoyancy. We conclude that Chinese sturgeons behave according to their buoyancy, which varies with depth due to hydrostatic compression of the swimbladder.

Abstract: Techniques were developed to determine which life stages of fish are vulnerable to barotrauma from expansion of internal gases during decompression. Eggs, larvae, and juvenile hatchery-reared white sturgeon (Acipenser transmontanus; up to 91 days post hatch; d.p.h.) were decompressed to assess vulnerability to barotrauma and identify initial swim bladder infla tion. Barotrauma-related injury and mortality were first observed 9 d.p.h., on the same day as initial exogenous feeding. However, barotrauma-related injury did not occur again until swim bladder inflation 75 d.p.h. (visible at necropsy and on radiographs). Swim bladder inflation was not consistent among individuals, with only 44% being inflated 91 d.p.h. Additionally, swim bladder inflation did not appear to be size dependent among fish ranging in total length from 61 to 153 mm at 91 d.p.h. The use of a combination of decompression tests and radiography was validated as a method to determine initial swim bladder inflation and vulnerability to barotrauma. Extending these techniques to other species and lifehistory stages would help to determine the susceptibility of fish to hydro turbine passage and aid in fish conservation.

Abstract: Rearing experiments were made to determine the mechanism of diel swim bladder inflation and deflation in the red sea bream Pagrus major larvae. Swim bladder volume index (V/L3•10-6) was found to increase remarkably with growth during the larval stage, and remain nearly constant in the juvenile stage.The volume index was higher at night than in the day time under natural light conditions. However, under constant 24-h illumination such differences could not be observed. By changing the light conditions from dazk to light, the volume index decreased to half within 2h. Furthermore, with the reverse changing from light to dark, it increased to maximum level about lh after the onset of dark. Sealing the water surface of the tank with a layer of liquid paraffin, did not interfere with swim bladder inflation under dark condition and larvae clearly displayed the diel rhythm of night-inflation and day-deflation under natural light conditions.These results suggest that increase in the swim bladder volume in the physoclist species of red sea bream, is not caused by swallowing air at the water surface such as takes place in the physostome species of anchovy, but by the internal gaseous exchange mechanism of the swim bladder according to environmental light conditions.