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| SOURCE OF HIGH GAS PRESSURES | |
Low and high levels of nitrogen gas pressures will cause fish, molluscs and crustaceans sublethal stress and mass mortalties. Elevated gas
pressures occur in water of the following types;
The following paragraphs are concerned primarily with gas supersaturation of water with nitrogen
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| TOXIC LEVELS | |
| Eggs can tolerate a relatively high total gas pressure (110% to 115%), however as the eggs hatch they become very sensitive to elevated gas pressure. Fish moralities can be expected at gas (nitrogen) pressures above 105%. However at lower levels ranging from 102% to 105% moralities might also be expected. The low gas pressures may not cause moralities directly, however they will stress the fish and predispose them to infection and possibly long terms problems. | |
| GAS PRESSURE AND DISEASE | |
| Salmonids and most aquatic animals will tolerate up to 102% gas supersaturation with nitrogen. However long term damage may be inflicted on the organisms. For example in the case of juvenile salmon, exposure to low level gas pressure may damage the gill structure, bone formation, blood capillaries and the nervous system. These conditions will render the fish susceptible to a high rate of infection in the hatchery. |
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| Symptoms | |
| Recent evidence has shown that levels up to 102% can cause an increased incidence of myxobacteria gill infections, the fish also become
sensitive to light and mass moralities may be experienced shortly after feeding the fish. Mass moralities may also be experienced during periods of low
atmospheric air pressure or if water level in the tanks are reduced.
There is also some evidence which suggests that exposure of salmon to elevated gas pressures for a short period of time after egg hatch may reduce the ability of the smolt to osmoregulate when transferred to the sea. It may therefore be the case that exposure to elevated gas pressures at an early stage in the Atlantic Salmon life cycle could be a possible reason for 'Failed Smolt Syndrome, or it could predispose the fish to increased incidence of disease or other as yet un-recognized problems. |
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| Carbon Dioxide | |
| Gas supersaturation can also occur with carbon dioxide in certain water types, such as spring or bore hole water in which the geology of
the surrounding area is predominantly of carbonates.
High levels of carbon dioxide can also occur in recirculation systems due to the respiration of the fish and bacterial filters. The haemoglobin in the blood of fish has a high affinity for carbon dioxide, and at levels of 25mg/l free carbon dioxide the ability of the fish blood to absorb oxygen is reduced by 50%. The carbon dioxide will also tend to acidify the water, thus pH buffering is essential in recirculation systems. Maintaining stable pH or slightly alkaline conditions will greatly assist in reducing the concentration of carbonic acid. However, assisted degassing by ventilated bacterial nitrification or vacuum degassing may also be required in systems with greater than 95% water reuse |
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