Gas bubble disease:

Tiny bubbles can mean big troubles

Did you know that gas bubble disease, induced by the rapidly changing barometric pressure over the path of hurricane Andrew in 1992, is considered a major culprit in the death of almost 10 million marine fishes valued at $7.8 million?

Did you know that oxygen supersaturation can be caused in ponds in the late afternoon due to phytoplankton blooms?

Did you know that Dr. Howard Dryden in Scotland claims that excess concentration of nitrogen in fish culture water may be directly or indirectly related to 20% of all fish mortalities in aquaculture?

Do you know why your carbonated drink fizzes over when you add salted peanuts to it? (I just figured this one out during the course of writing this article. More on that later.)

G. R. Bouck defined gas bubble disease (GBD) as a non-infectious, induced process caused by high total dissolved gas pressure, which produces primary lesions in the blood (emboli) and tissues (emphysema), and subsequent physiological dysfunction.

This is a pretty straight forward definition for a phenomenon that plagues aquacultural producers, both expert and novice alike. Recognizing and understanding the fundamentals of GBD is a good first step in minimizing its risk.

Saturation

Water of different temperature, salinity, and pressure holds differing amounts of oxygen, nitrogen, and other gases called air.

When the concentration of a gas, such as nitrogen, is the exact amount the water can hold for its pressure, temperature, and salinity, the water is considered "saturated." When there is more gas than the water can hold, it is "supersaturated." If there is less gas, it is "undersaturated."

Pressure can increase the amount of gas per unit volume that water contains, so, the deeper the water, the more gas the water can hold. Water that is pumped through pressurized systems will hold more gas than unpressurized systems or unpressurized regions of a system, such as culture tanks.

Water at higher temperature holds less gas per unit volume because it is less dense. The higher the salinity, the less gas water can hold. This is because the dissolved salts take up room that the gas could be using.

Dissolved gases are typically measured in milligrams per liter. However, the level of saturation is typically presented as a percent of saturation. This is the percentage of the total gas concentration relative to "saturated" conditions. (See the equation, Fig. 1, below.)

So, if we say that the percent of saturation is 115%, this means that the actual dissolved gas concentration is 15% greater than the saturated dissolved gas concentration at the given temperature, pressure, and salinity.

Gas supersaturation is described by total dissolved gas pressure (TGP). (See the equation, Fig. 2, right.)

The actual -P experienced by the fish is the difference between TGP and local pressure, where local pressure is the barometric pressure plus the hydrostatic or submergence pressure around and inside the fish.

Given time to dissolve into solution, air (gas) is oblivious to the air-water interface that exists over fish culture tanks and the liquid-tissue interface that exists in fish gills and bloodstream.

Thus, the "gas" pressure in the tank will equalize and become the same pressure as the air over it. And, subsequently, the gas pressure in a fish's tissue and bloodstream will become the same as in the water. Actually, the air pressure is the sum of the partial pressures of the individual gases (primarily nitrogen, 78%, and oxygen, 21%) that make up air.

Of course, this is a highly simplified view of fish physiology and aquaculture water chemistry, but it allows us to review some important principles with a minimal and manageable number of process variables. Intensive culture conditions actually bring in oxygen and carbon dioxide consumption and generation issues related to fish respiration, nitrification, and alkalinity and pH control.

Oxygen moderately above saturation in water is not typically a problem because fish use oxygen to breathe. However, since nitrogen is the most common of the inert gases in fresh and salt water systems and is not metabolized by fish, it is the gas most commonly associated with gas bubble disease. Nonetheless, oxygen supersaturation due to phytoplankton blooms, coupled with late afternoon pond warming, is quite common.

Induced process

According to Henry's Law, when the pressure of a gas over liquid is decreased, the amount of gas dissolved in that liquid will also decrease. This can happen slowly with very little physical effect or it can happen rapidly with sometimes disastrous physical effects.

One of the best practical demonstrations of this law is offered by opening a bottle of carbonated beverage. When the cap is removed from the bottle, gas is heard escaping, and bubbles can be seen forming in the drink. This is carbon dioxide gas coming out of solution as a result of sudden exposure to lower barometric pressure.

Similarly, nitrogen is an inert gas normally stored throughout an animal's body (tissues and fluids) in physical solution. When a fish is exposed to decreased hydrostatic and/or barometric pressures, the nitrogen dissolved in the tissues and fluids becomes supersaturated and comes out of solution. If the nitrogen is forced to leave the solution too rapidly, bubbles form in different parts of the fish, causing a variety of signs and symptoms.

Supersaturated conditions can also result if a gas such as nitrogen is forced into solution, creating a partial gas pressure or tension in excess of the normal or surrounding partial pressure. In air, the pressure of a specific gas is called partial pressure. In water, the pressure of a specific dissolve gas is called tension.

A typical problem that can lead to excessive tension or supersaturation is the presence of air leaks on the suction side of a pump. This air is forced into solution by the action and pressure of the pump. Fish exposed to this supersaturated water are susceptible to GBD emboli and emphysema, which were mentioned previously.

There are more insidious ways for excessive tension to occur. Water temperature has a direct effect on the degree of saturation. Warm water will hold less of a dissolved gas in solution than will cold water. Thus, if cold water with a normal dissolved gas content is heated, supersaturation conditions can be created, increasing the risk of GBD.

Published literature is loaded with references to GBD problems that are commonly encountered when fish are cultured in the heated effluents of power plants or where heat is recovered from steam generating facilities.

Shellfish culturists should be concerned if their hatchery source water is a shallow bay that can quickly change temperature. Since the water is typically pumped from the bottom (higher saturation pressure due to depth), it is supersaturated before it heats up and becomes even more supersaturated.

High total gas pressure

Nitrogen enters a fish through its gills, just like oxygen. It is then carried to the tissue by the blood. Once distributed, nitrogen remains in the tissue while oxygen is consumed. If the nitrogen enters the animal's tissue in a supersaturated concentration and accumulates, it will come out of solution into the fish in the form of bubbles. Divers call this decompression sickness or the bends. In fish, it is called gas bubble disease.

As Dr. Dryden points out, fish sense high gas pressures. Like a diver, they will go deep in the tank to compress the gasses and thereby prevent nitrogen bubble formation in their blood.

At a pressure of 102% gas saturation, the compensation depth, or depth at which bubbles will not form in the blood of the fish, is 8". Thus for every 1% increase in gas tension, the fish must "dive" 4" to compensate for the elevated pressure.

If the total gas pressure is 110%, it's kind of hard to dive 40" in a 20" deep tank. Obviously, such dimensions are on the scale of fry and fingerling production systems.

Review of aquaculture literature shows that uncompensated supersaturation as low as 101% or 102% can be lethal to early life-stage fish. The literature also shows that adult fish are more tolerant of excessive total gas pressure conditions, on the order of 115%-120%. But then their tanks are typically deeper.

Picture this: heated ground water pumped into a flow through system, a shallow tank, no degassing, poor oxygenation, floating feed. Cruel. Get out your checkbook.

Tolerance levels

James Avault has suggested that a supersaturation of 110% may be detrimental to sac-fry and supersaturation of 115% may be detrimental to adult fish.

Bisker and Castagna have shown that hard clams (Mercenaria mercenaria) are not susceptible to GBD up to approximately 109% saturation. At a 115% saturation level, growth is stopped. Other clam species have died at 8%-14% supersaturation. Lacking information on smaller seed, it is thus recommended that hatcheryand nursery systems keep gas saturation levels below 105%.

Howard Dryden states that fish eggs can tolerate a relatively high total gas pressure of 110%-115%. After the eggs hatch, however, they are very sensitive to supersaturation damage with mortalities expected at gas (nitrogen) pressures ranging from 102% to 105%.

Dryden further states that at low sublethal concentrations, whenever fish swim up close to the surface, there will be some bubble formation in their surface capillary vessels. The organ most at risk of damage is the gills. Any damage to the gills will result in both bacterial and fungal infections that will reoccur until the cause is eliminated.

Huguenin and Colt state that lethal dissolved gas concentrations can occur at diffuser submergences as small as 3' for sensitive larval fish. Risk of GBD is more serious in marine larval systems due to increased sensitivity of larvae and the required high concentration of oxygen. While criterion will vary depending on species, life-stage and specific gas, a maximum criterion of 2.6% supersaturation can be used for many organisms. They further state that water heated more than 10°F should be degassed prior to use.

Huguenin and Colt have also suggested maximum submergence depths for air stones and diffusers to avoid gas supersaturation problems. Course and medium bubble diffuser submergence depths in excess of 6' could cause excessive gas saturation problems. Fine bubble diffuser submergence in excess of 3' could cause problems.

Prevention, mitigation

Here are a few practical tips for preventing and dealing with GBD problems.

• Aeration One of the simplest ways to prevent GBD is to bounce, splash, spray, and/or cascade your culture water through air before it reaches your fish. This chaotic mixing and aerating process will promote the release of supersaturated gases.

• TGP monitor When in doubt, total gas pressure should be monitored using a TGP meter or tension-o-meter, which measures the -P by the membrane diffusion method. And

• Degassing Column A degassing column is a more efficient way to "bounce, splash, spray, and/or cascade" water through an air or oxygen enriched atmosphere. A typical configuration includes a column packed with media such as bio-barrels, a water distribution plate or spray nozzle, and, often times, an exhaust fan to draw air in a countercurrent direction through the packed media. Sometimes, oxygen is injected in the bottom of a packed column and vented at the top along with the stripped gases. This is especially effective for removing nitrogen and also provides beneficial oxygenation of the culture water.

Peanut-induced fizz

I'm probably showing my age when I tell you this, but as a young person growing up in South Carolina, my friends and I used to add salted peanuts to our Pepsi. This would cause an immediate and messy release of foam from our drinks. Some fun! I now know that it is the dissolving salt that triggers the instant, localized super-supersaturation that causes the carbonation to immediately and violently come out of solution.

And now you know the rest of the story.

Paul Hundley is president and principal engineer for Applied Aquatics Inc. in Charleston, SC. He has more than 25 years experience in the design, construction, and operation of commercial, industrial, and institutional facilities and has been working in aquaculture for over 10 years. Hundley can be reached via: phone (843)971-9639; fax (843)971-9641; or e-mail <[email protected]>. Copies of this and other Systems Engineering articles are available at <www.AppliedAquatics.com>.

Note: Some of the information contained in this article has been generated by independent sources. Such information is accurate to the best of the author's knowledge, but the author disclaims any liability for the accuracy or completeness of such information or the reliance by any party on such information. Reader comments and corrections are welcomed.

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