introduction to fisheries science

Flow-through system

A flow-through system involves the movement of water through a series of raceways or tanks. The waste is flushed out of the system into a receiving body of water. In many cases, the raceways or tanks may simply be downstream of a diverted river or stream. This can be a simple system if there is a sufficient amount of clean water entering the system.

The raceways or tanks give better control of the feeding and allow for better handing of the fish. It also is easier to grade or sort the fish by size. As with cage system, the raceways make it possible to harvest and market fish over a longer period of time, which improved the farmers marketing position. Some farms have pumped well water instead of using streams or springs. The advantage of pumping from wells is that water purity can be controlled more easily and there is less chance for disease and parasites contaminating the fish.

The major advantage of the flow-though system is the cost of pumping the large amount of water that is required. The optimum fish density for this system is limited by the flow rate of clean water and the amount of waste the receiving water can absorb while maintaining acceptable water quality.

Recirculating system

Two problems common to the pond, cage, and flow-through system are that they all require a large amount of clean water, and the environment must absorb a large amount of waste. Since the 1960s, much research and experimentation has been done on reciruculating system.

A recirculating system means that the aquatic organisms are grown in raceways or tanks and the waste is the removed from the water and the water re-used. Oxygen is added to the water at a constant rate by using compressed air or liquid oxygen, or via cascade flow. The recycling system uses natural processes to remove the waste, but confines and intensifies the processes in order to get more waste removed in less time and using less space. Mechanical filters such as settling tanks, pad filters, or rotating screens called drum filter remove the solid waste. Another type of filter is the biological filter. This filter removes ammonia and nitrite from the water, which come from the fish body waste. These two chemicals are particularly toxic to fish. The filter uses bacteria to digest the ammonia and nitrite, with the bacteria growing on surface inside the filter. The surface area is critical to the efficiency of the filter. The more surface area, the more bacteria and the more bacteria, the more waste that can be removed. The surface area is usually shredded plastic, plastic rings, or plastic beads. Plastic is usually used because it is durable and can be cleaned and re-used.

The major advantage in this system is that large numbers of aquatic organisms can be raised in very little space and using a small amount of water. Another advantage is that the farmer can manage the organisms, the water quality, the water temperature, the feed ration, and the grading much more closely, especially if the tanks are in a building. This allows the farmer to plan for year round production, which is a strong  marketing advantage. It also solve the predation problem and weather related problem. The problem of waste removal is easier to manage with a close system. The waste can be totally removed from the system and spread on land as fertilizer, rather than dumped into a receiving body of water.

A major disadvantage to recirculation system is the capital cost construct the system. A system that can produce 100,000 pounds of fish a year can cost up to US$500,000, not including the land building. Another major problem is the energy cost to run the system, as well as the higher cost of fish food, which must contain a much higher level of protein (up to 60 percent) than, for example, cattle food, and a balance amino acid composition as well.

A third area of concern is disease and parasites. When fish densities are high, the risk of infections by parasites like fish lice; fungi (saprolegnia ssp;) internal worms, such as nematodes or trematodes; bacteria (e.g, Yersinia ssp, pseudomonas ssp.) and protozoa (such as DinoFlagellata) is much higher than in animal husbandry because of the ease in which pathogens can invade the fish body (e.g. by the gills). Once system is infected, it is very difficult to purge the system. Most chemicals that will kill bacteria, virus, and parasite will also kil fish, or will contaminate them and cause a problem when they are consumed. Salt can be effective in treating parasite in fresh water fish and there are a few other chemicals that are approved for use in treating fish disease. The best way is to prevent an infection by keeping the tanks and equipment clean and by being careful about introducing new organisms from other farm into the system. Other means to treat the water being tried, including ultraviolet light and ozone. These treatments can be effective, but they are very expensive. This  type of aquaculture requires light monitoring and a high level of expertise.