5542 Engineer Dr.
Huntington Beach, Ca 92649
by D. Weaver
When receiving live aquatic animals packed in closed bags at high densities, the receiving water used for acclimation of the incoming stocks should match the temperature, salinity and pH of the shipping water.
Basis for procedure:
The most common method of packing aquatic animals for shipment via air freight is to place the animals in a bag which has about 1/3 water and 2/3 either air or pure oxygen. When air is used, it is possible to create low oxygen conditions and stress the animals as the metabolism of the animals consumes the oxygen. The use of pure oxygen increases the total amount of oxygen available by a factor of 5. But, more importantly, the use of pure oxygen increases the mass transfer capacity from the gas to the liquid by a factor of almost 10. This allows the oxygen in the bag to get into the water fast enough to handle the metabolism of high density aquatic animals.
As part of the animals metabolism, waste products are produced. In particular (assuming the animals are not fed during shipping), ammonia and CO2 are produced. The toxicity of ammonia is primarily from the unionized form of ammonia in the water which intern is a function of the pH. The lower the pH, the less toxic the ammonia. With pH being a log scale, the difference between a pH of 7.5 and 6.5 is almost a factor of 10 in toxicity.
Normal metabolism produces more CO2 that ammonia, we have a situation where the CO2 being produced by the animals will reduce the pH (through the carbonate system) in a closed bag. With most reasonable alkalinities on the shipping water, the pH will decrease fast enough in a shipping bag to prevent any ammonia toxicity from occurring while the animals are in the bag. If animals have been in the shipment bag and the ammonia has built-up in concentration, opening the bag and allowing the CO2 to leave the water can result in ammonia toxicity and animal stress, tissue damage, disease and death. This can all be calculated given the carbonate chemistry K values, the ionization constant for ammonia, alkalinity and other water chemistry variables along with the CO2 and ammonia generation rates for the animals at the temperature in question.
This means that the animals will not die in the bag from ammonia toxicity or lack of oxygen, it is usually free CO2 toxicity, which can be expressed as partial pressure of CO2. This also means that the details of how the fish are unpacked is critical to their health and survival.
Procedures that are used by Scientific Hatcheries:
- Float the bag in the receiving water to equalize temperature. This usually requires about 15 minutes.
- Carefully open bag without letting the gas out of the bag.
- Pull a small sample from the bag and reseal the bag (rubberband).
- Immediately measure the pH of the sample. In a heavy pack, it should be in the 6.7 range.
- Adjust the pH of the receiving water to the pH of the bag. We use HCl, but other acids will work. With a reasonable alkalinity in the receiving water, acid addition will decrease the pH of the receiving wate. The pH will then increase as the CO2 leave the tank as part of the re-equilibration of the carbonate system. The rate of increase is determined by the mass transfer rate of CO2 to the atmosphere (adding aeration increases the rate of increase). It is best not to allow very much of the CO2 escape before adding this water to the bag.
- Adjust the salinity, if necessary. For fresh water animals that are 2,000 ppm or less, this is not necessary .
- Add pH, salinity adjusted water to the bag.
- If the receiving tank is large, and on a large filtration system, just turn the bag loose in the tank.
- If the receiving tank is small, net out the animals and place in receiving tank.
The details can be modified to fit a specific situation as long as the basics of what you are trying to accomplish is maintained. You want to purge the animals in low ammonia water long enough to clear their tissue before the pH is increased to normal. You don't want to subject the animals to rapid changes in temperature, salinity, or pH.
If the pH was just brought down in the receiving tank for the delivery, the pH will naturally increase as the CO2 comes off with aeration. This will give a very slow rise to the pH.
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