By

Dr.Howard T Dryden

Much is said of the requirement for adequate dissolved oxygen in aquaculture, its importance is implicit to the successful culture of fish and aquatic animals of all species. However oxygen represents only 21% of the gas constituents of air, the principal remaining gas being nitrogen, the importance of which is often overlooked. This is a surprising statement. Equally surprising is that nitrogen, or rather the excess concentration of nitrogen in fish culture water, may be directly or indirectly related to some 20% of fish mortalities in aquaculture.

The above is a rather bold claim, however at Dryden Aqua we have been measuring nitrogen gas concentrations on fish farms for nearly 20 years and frequently observe sub-lethal conditions. Because of the nature of the symptoms such as recurring myxobacterial gill infections in mild cases, the underling cause of the problem is oft en missed. Nitrogen supersaturation can occur as a result of both man-made and natural events. For example air and water mixed together at pressures greater than atmospheric, e.g. in pumps drawing air in from the suction side, venturi injection systems, spillways, monks or pressurized oxygenation systems using oxygen generators, will cause varying levels of gas saturation. Supersaturation will also occur when water is heated in a boiler, or even by the sun when a length of black pipe is used above ground. Ground water and borehole water is nearly always supersaturated. Even when an atmospheric depression passes over a fish farm, the drop in air pressure of 1 - 2% will result in an equal increase in the total gas pressure in the water, which will diminish only slowly as equilibrium is re-established.

Fish sense high gas pressures, and like a diver, will go deeper in the tank to compress the gases and thereby prevent nitrogen bubble formation in their blood. When water is at 102% gas saturation, for example, the compensation depth, or depth at which bubbles will not form in the blood of the fish, is 200mm (8 inches). As a guide, for every 1% increase in gas pressure, the fish have to swim 100mm (4 inches) deeper in the water to equilibrate. If the total gas pressure increases to 110% and the depth of the tank is only 500mm (20 inches), the fish can not escape and the consequences will be 100% mortality in about 30 minutes. If the gas pressure increases to only 103% the fish will survive but will be subjected to sublethal stress.

At low sublethal concentrations, whenever the fish swim up into the danger zone, there will be some bubble formation in the surface capillary vessels of the fish. The organs most at risk are the gills where any damage will result in both bacterial and fungal infections that keep recurring. The distal ends of the fins, the eyes, swim bladder and intestine can also be affected. For example, fish can become very exited by sudden changes in light intensity, they may swim with the head down and may die after feeding because of their inability to digest food.

A more insidious problem can occur where fish are exposed to elevated gas pressures while in shallow hatchery tanks. Exposure of young fish, even for a short period could potentially damage the developing capillary vessels and nervous system. The effects may go unnoticed for many weeks or months since an animal damaged at an early stage in development may not exhibit symptoms until later. A classic example of this is exposure of salmon fry in a hatchery, which results in mortalities during smoltification or on transfer to sea cages. Exposure to low level gas supersaturation can also predispose the fish, or act as a precursor to other infections such as anemia and PKD. The importance of nitrogen gas supersaturation should not be ignored. One fish farm we worked on described it as being like exposure to radiation; once the fish have been exposed they will never be the same.

One simple way to check for supersaturation is to fill a clean glass bottle with the fish culture water, but leave the bottle in the water with the top off, and with the neck sticking out. After 30 minutes gently lift the bottle out of the water and inspect for gas bubbles adhering to the inside. If bubbles are present you definitely have gas supersaturation, if the oxygen levels are at 100% or less then the gas will be nitrogen and the fish will be suffering from gas bubble trauma. Even if no bubbles are observed does not mean that the water is safe since low saturation levels will not be detected by this technique. To be certain, an accurate total gas meter should be used to check the water.

The function of a degasser is to restore the equilibrium partial pressures of dissolved gases in the water to be the same or lower than the partial pressure of the same gases in the atmosphere. In order to achieve this, the water is trickled down though a media-packed column. The water flowrate for a 1 metre diameter vacuum degasser column is 100 m³/hr which translates to a 10% water hold up within the packed bed. For example if the bed volume is 2000 litres, no more than 200 litres of water will be falling through the column at any one time.

Water vapour exerts a gas pressure on the water equivalent to approximately 2% of the total gas pressure. Passive open degassing columns will not compensate for water vapour pressure, so their best performance will be 102% gas saturated water. Passive degassing columns are never 100% effective so there will always be a slightly elevated residual nitrogen partial pressure in the water. In active degassing columns in which air is blown up through the column, the act of blowing the air through the bed means that there will be a slightly elevated positive pressure within the column. Such an arrangement will cause gas supersaturation that could lead to chronic sub-lethal problems among the fish, and should be avoided.

In the Dryden Aqua columns air is pulled (rather than pumped) through the column creating a slight negative internal pressure, which compensates for the water vapour pressure. A large volume of air is drawn counter-current against the flow of water. If water flowrate is 100 m³/hr, then 100 m³/hr of air will be pulled through the degasser. This volume of air is not required to effect degassing of the water. However water which has a high nitrogen pressure (e.g. ground water) will normally have high nitrogen content, high carbon dioxide and volatile organics such as hydrocarbons, and low levels of oxygen. By pulling a large volume of air against the flow of water, the water is scrubbed of all volatiles, increasing the dissolved oxygen level and providing for a very stable partial vacuum in the column. Typically the total gas pressure on the discharge from the column can be held at 97 ± 0.5%.