Tech Talk 22
To successfully transport fish requires an understanding of the physiological aspects of what happens to the fish as well as their temporary environment–the hauling box water. If the fish die shortly after delivery, it's usually the hatchery that takes much of the blame but, all too often, it is the hauler's fault.
Typically, at least 24 hours before the fish are to be moved, they are taken off feed. This reduces fouling of the hauling tank water. The fish are then counted by weighing (a pretty accurate method) and placed into the appropriate chamber of the hauling truck. It is here that attention to detail can make the difference between a good delivery and a debacle.
Overcrowding can describe symptoms that occur from having too much of something (ammonia or CO2) or not having enough of something (oxygen). For average travel times, the general rule of thumb is no greater than 2 pounds of fish per gallon of water. Some fish, like muskie or walleye, do better at only one pound of fish per gallon. These densities typically require the use of pure oxygen systems over the agitator or compressor/diffuser aerators. However, short trips with cool water (55°F or below) will allow higher carrying densities with just an aerator. We’ve heard of fish haulers carrying up to 5 pounds of catfish per gallon of water on short trips to the processors! Remember that these fish are not intended to live very long.
Cooler water temperatures play an important part in keeping fish in their best condition. As cold-blooded animals, their metabolism slows as the temperature drops. This means oxygen intake decreases, along with ammonia and carbon dioxide. Of course, just the opposite happens as the temperature increases.
To counter the stress associated with fish transport, many haulers rely upon salt to boost the fish's electrolytes and enhance mucous production, which will ward off bacterial infections and parasites. A rough rule of thumb for dosage rates is three pounds of non-iodized salt per 100 gallons of water. For non-food fish, haulers often use anesthetics, ammonia removers and defoamers.
There are three main types of oxygen replenishment techniques:
Agitators/aspirators, compressors with air diffusers and pure oxygen. Agitators/aspirators rely upon a mechanical disturbance to splash water or create bubbles to drive oxygen into the water. They are not very efficient, take up space in the tank and may have mechanical problems because of exposure to the elements. Compressors with air diffusers are the most popular method of aerating low stocking densities or for short haul trips. Easily sized for the situation, diffuser systems rely upon bubble size and bubble contact time to transfer oxygen. If more oxygen is needed, more diffusers are added. You can never "over aerate" by putting too much air into the water. The maximum stocking density for this type of aeration is one pound of fish per gallon of water.
Pure oxygen systems are intended for high stocking densities, long trips and sensitive fish. To give you an idea of how well pure oxygen works, remember that bags of tropical fish are shipped around the world for periods of time up to 30 hours in a plastic bag containing 1/3 water and 2/3 oxygen. The key to pure oxygen efficiency in the hauling tank is in the diffuser. The finer the bubble, the better the diffusion into the water. The best micropore diffusers now on the market are less than 50 percent efficient in their ability to transfer oxygen into the water at these shallow depths. Simply put, for every dollar's worth of oxygen bought, you get fifty cents worth of it in the water. But it does allow very high carrying capacities, where the cost is far outweighed by the return in healthy fish delivered. Larger trucks and/or longer runs can benefit from the compact nature of liquid oxygen.
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