exemplified
ROBERT C. PLUMB
Gas-Bubble Disease of Fish1
show gas bubbles under the skin, in the roof of the mouth, and in fin membranes, and one can observe hemorrhaging inside the eye. The treatment for gas-bubble disease in humans is recom~ression: the fish's resnonse to the discomfort nf eas bubble disease is sounding to greater depths and hi& pressures. Sometimes natural conditions, more frequently man's impact on the environment, cause the supersaturation with nitrogen. Freshly melted snow, having been in intimate contact with air, contains about 18.9 ml of Nz/l of HzO, but warm i t to 5°C and the equilibrium solubility is reduced to about 16.7 ml/l. If it is not re-equilibrated with air the resultant 113% saturation with nitrogen is enough to cause gas huhhle disease. Similarly river water may be saturated with nitrogen. However, pass i t through a heat exchanger of a factory or power plant and discharge it into the river without re-equilibration and living conditionsfor fish in the thermal plume will he intolerable. Water passing over a fall and plunging into a deep pool will entrain air which will dissolve a t the high pressures deep in the pool. The water will he supersaturated as it returns to the surface and will remain so until re-equilihrated with the air-e.g., by tumbling down over rocks. Similarly build a dam with a spillway plunging water into a deep pool; when there is rapid runoff with a large overflow the fish downstream will be exposed to supersaturated nitrogen. Is your town water unsatisfactory for rearing pet fish? If you have deep wells the water is often supersaturated hecause the subterranean rivers and lakes may be under pressure. Or if your town pumps water under pressure into storage tanks it may be supersaturated by excess gas entering the intake of the water pump. The solution is sim.ple-bring the water to equilihrium with the atmosphere hy boiling and re-aeration, by standing exposed to the atmosphere for a day or so or by mechanically mixing the water with air at room temperature for enough time to reach equilihrium.
INustrating principles of gas solubility and nucleation Information prouided by Wesley J. Ebel, National Marine Fisheries Service. Seattle, Washington A bright-eyed child brings some goldfish and a fishbowl home-conscientiously feeds and changes the water-the fish sicken and die. A river is dammed, heavy runoff of water occurs, and the fish population downstream-e.g., salmon or trout-suffer high percentages of kills. If you own a goldfish bowl or home aquarium, or are a fisherman or conservationist then keeping fish alive and healthy is of concern to you. You will see in this exemplum how a simple chemical process is a common cause of fish kills; knowing the chemistry leads directly to solutions to the problem. Gas-bubble disease in humans is known as decompression sickness; i t may develop when humans surface from deep-water diving and from underground compressed-air work or when, e.g., an aviator enters subatmospheric pressures. The physico-chemical principles responsible for decompression sickness in humans are simple. According to Henry's law when a liquid is in contact with a gas the equilihrium soluhility of the gas in the liquid is directly proportional to the pressure of the gas. Thus in the process of decompression the hody fluids become supersaturated. If the gas is not removed bubbles form in the hlood stream and other parts of the hody. Oxygen bubbles are not usually a problem because oxygen is consumed in metabolism. A physiologically inert gas such as nitrogen however can only he removed through the respiratory system. If decompression is too rapid huhhles of gas will form in the hody. Fish are not normally subjected to rapid changes in pressure. How can a fish he afflicted with gas-bubble disease? Fish gas huhhle disease occurs when the water in which they live is supersaturated with nitrogen. The fish breathes by taking water in through its mouth and forcing it out through its gills. Oxygen and any other dissolved gases are extracted by a fine network of hlood vessels in the gills and carried to other parts of the hody. When the water is supersaturated with nitrogen the hody becomes supersaturated with nitrogen and bubbles may form a t nucleating sites. Commonly, fish killed by huhhle disease
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'The following publications provide technical details on this problem: Rucker, R. E., "Gas-Bubble Disease of Salmonoids," Bureau of Sport Fisheries and Wildlife Technical Paper 58, Feb. 1972. Ebel, W. J., Fishery Bulletin, 68, 1 (1969). Ebel, W. J., Dawley, E. M., Monk, B. K., Fishery Bulletin, 69, 833 (1970). Beiningen, K. T., and Ebel, W. J., Trans.Am. Fish. Soe., 99, 664 (1970). Ebel, W. J., NOAA Technical Report NMFS SSRF-646, US. Government Printing Office, Washington,D.C., 1971.
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Volume 50, Number 8, August 1973
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559
