tive blowers because there is limited contact with the air outside the tank. Lower water temperatures help maintain higher levels of dissolved oxygen, slow fish metabolism and will lower the percentage of toxic unionized ammonia present. Liquid oxygen cannot be stored indefinitely inasmuch as approximately two percent of liquid oxygen is vented and released from the dewar daily. (Below) Loading food size channel catfish into an aluminum tank Carbon dioxide in the hauling water should be monitored when using liquid oxygen and, if possible, kept at concentrations less than 20 mg/L. Fish appear to be able to tolerate gradual increases Diffuser hose grid used with liquid oxygen in carbon dioxide concentrations (Piper et al. 1989). Although carbon dioxide acidifies and reduces the percentage of unionized ammonia in the transport water, it may interfere with the ability of fish blood to carry oxygen. The release of fine oxygen bubbles from the diffusers does not agitate the water sufficiently to drive carbon dioxide gas out of solution. Tank vent funnels and electric (12v) agitators mounted through ports located on the top of transport tanks have been used to help drive off carbon dioxide (Jensen 1990a). Transport Tanks Fish hauling trucks may range from a pickup truck weighing 2,722 kg that can transport 379 L of water, weighing approximately 378 kg, to a tractor trailer that can haul 18,927 L weighing 20,412 kg. Table 3 provides truck sizes and recommended hauling tank capacities (Jensen 1990a). Fish transport tank capacities may range from 283 to 9,464 L. Most tanks Fish loading net attached to a boom truck hold between 379 and 1,136 L. Transport tanks are rectangular and are equipped with insulated top doors for loading. Most tanks are constructed of either aluminum or plywood covered with fiberglass. Aluminum is more durable than fiberglass and aluminum tanks may better withstand the wear and tear of frequent loading and off-loading. However, fiberglass tanks tend to remain cooler during hot weather and may be more appropriate for transporting fish over long distances. Inside the outer shell of the tank, two inches of polyurethane or other insulation material encases the inner compartment liner. The use of insulated tanks is particularly crucial for transporting fish over long distances in hot weather. Non-insulated tanks made of steel, wood or fiTable 3. Truck size and recommended hauling tank capacities (Jensen 1990a). Truck size (t) Tank Capacity (L) Tractor -trailer 1 8,927 2.27 4,542 1 .36 2,271 - 3,028 0.91 1 ,514 0.68 1 , 136 0.45 379 WORLD AQUACULTURE 43
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