Quantifying Processes

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As was mentioned in the definition of aquacultural engineering, many engineering disciplines can be applied to the design of aquaculture systems. In this section, some of the critical processes that are unique in an aquaculture system and that define design parameters will be described. The key processes in the design of aquatic systems have to do with the primary water quality factors that affect the cultured organism and with how the organism affects water quality.

Figure 5. Cages used for the culture of Atlantic salmon in Norway.

As a starting point in the description of important processes in an aquaculture operation, one can consider how a fish relates to its environment. Fish consume oxygen from the water and release water products from their metabolism back into the water. The main waste products of concern are ammonia, carbon dioxide, and particulate and dissolved organics. Depletion of dissolved oxygen below a certain level, which is species and size dependent, results in increased stress to the fish and may ultimately result in suffocation. Similarly, the accumulation of waste products may reach toxic levels, as in the case of ammonia and carbon dioxide; it may alter the pH of the water, again as a result of carbon dioxide accumulation; it may affect growth and reproductive behavior as a result of the accumulation of dissolved organics such as hormones; or it may affect the overall oxygen balance of the system by the oxygen demand created by the decomposition of particulate and dissolved organics.

A major difference between aquaculture and land-based food production systems is that in an aquaculture system the animal or plant takes its nutrients and oxygen and releases its waste products into the same environment: the water. Maintaining water quality in an aquaculture system is based on providing sufficient oxygen to satisfy the needs of the fish, and in removing metabolites to prevent their buildup to toxic levels. Design of systems to achieve those goals requires the quantification of processes that take place within the system, especially those related directly to the target species. The relative importance of the various consumption and production processes depends on the type of aquatic production system being considered. In general, the shorter the residence time of the water in the production system, the more important are the fish-related processes with respect to other biological processes taking place. If, on the other hand, the residence time is long (more than one day and as long as one year), as is the case in most ponds, biological and chemical processes that are not directly associated with the target product's metabolism become very important in determining water quality. As an example, dissolved oxygen in a tank with a short residence time will be determined by the concentration in the influent water, the fish biomass being held in the tank, the amount of oxygen being added to the system through aeration mechanisms, and the oxygen consumption by the fish. In a pond, on the other hand, many processes can have significant effects on dissolved oxygen concentration. These processes include photosynthetic oxygen production, respiration by all the organisms in the ponds (phytoplank-ton, zooplankton, fish, bacteria, and sediment decomposition processes), and gas exchange between the water and the atmosphere.

Quantification, design, and management of aquaculture production systems are most predictable in those systems with short residence times, where the majority of water quality changes are associated with fish activity. Pond systems, often with long residence times, are difficult to quantify and their management remains very much an art rather than a science.

Design of systems is ultimately based on mass balances on the critical water quality factors already mentioned. The general form of the mass balance equation for a par

ticular substance (eg, dissolved oxygen or ammonia) in a control volume can be written as

ACCUMULATION = INFLOW + PRODUCTION

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