HATCHKR^ REQUIREMENTS 9 



Theoretically, gas bubble disease can be caused by any supersaturated 

 gas, but in practice the problem is almost always due to excess nitrogen. 

 When water is supersaturated with gas, fish blood tends to become so as 

 well. Because oxygen is used for respiration, and carbon dioxide enters 

 into the physiology of blood and cells, excess amounts of these gases in the 

 water are taken out of solution in the fish body. However, nitrogen, being 

 inert, stays supersaturated in the blood. Any reduction in pressure on the 

 gas, or localized increase in body temperature, can bring such nitrogen out 

 of solution to form bubbles; the process is analogous to "bends" in human 

 divers. Such bubbles (emboli) can lodge in blood vessels and restrict 

 respiratory circulation, leading to death by asphyxiation. In some cases, 

 fish may develop obvious bubbles in the gills, between fin rays, or under 

 the skin, and the pressure of nitrogen bubbles may cause eyes to bulge 

 from their sockets. 



Gas supersaturation can occur when air is introduced into water under 

 high pressure which is subsequently lowered, or when water is heated. Wa- 

 ter that has plunged over waterfalls or dams, water drawn from deep wells, 

 or water heated from snow melt is potentially supersaturated. Air sucked in 

 by a water pump can supersaturate a water system. 



All fish — coldwater or warmwater, freshwater or marine species — are sus- 

 ceptible to gas bubble disease. Threshold tolerances to nitrogen supersat- 

 uration vary among species, but any saturation over 100"" poses a threat to 

 fish, and any levels over 110"n call for remedial action in a hatchery. Nitro- 

 gen gas concentrations in excess of 105% cannot be tolerated by trout 

 fingerlings for more than 5 days, whereas goldfish are unaffected by con- 

 centrations of nitrogen as high as 120% for as long as 48 hours and 105% 

 for 5 days. Whenever possible, chronically supersaturated water should be 

 avoided as a hatchery source. 



CARBON DIOXIDE 



All waters contain some dissolved carbon dioxide. Generally, waters sup- 

 porting good fish populations have less than 5.0 parts per million carbon 

 dioxide. Spring and well water, which frequently are deficient in oxygen, 

 often have a high carbon dioxide content. Both conditions easily can be 

 corrected with efficient aerating devices. 



Carbon dioxide in excess of 20 parts per million may be harmful to fish. 

 If the dissolved oxygen content drops to 3-5 parts per million, lower car- 

 bon dioxide concentrations may be detrimental. It is doubtful that freshwa- 

 ter fishes can live throughout the year in an average carbon dioxide con- 

 tent as high as 12 parts per million. 



A wide tolerance range of carbon dioxide has been reported for various 

 species and developmental stages of fish. Chum salmon eggs are relatively 



