Ponds

Pond size, shape, and construction technique varies widely (7,8). Ponds tend to be shallow, less than 1.5 m deep, and vary in area between less than 100 m2 to more than 10 ha (Fig. 2). The pond bottom is sometimes covered with concrete or plastic liners to prevent water losses to infiltration. The ponds may be built by excavation, by construction of dikes, or more commonly by a combination of the two techniques. In all cases, pond design should incorporate struc-

Figure 2. Pond being used for the culture of shrimp with supplemental mechanical aeration.

tures for water inflow for filling and maintenance flows, and for effluents including flows during pond drainage and emptying.

Raceways

Raceways are relatively narrow, long channels with water constantly entering through one end of the channel and leaving through the opposite end (Fig. 3). Raceways are often built of concrete, and the normal dimensions are in a ratio of approximately 30:3:1 (length:width:depth). Raceways are the most common holding system for the production of trout and are usually approximately 30 m long. Flow in a raceway approaches plug flow characteristics, creating a gradient of water quality along the raceway; water quality is best near the head of the raceway where the water enters and degrades toward the drain side of the raceway. Raceways are often constructed in series of up to six raceways with reaeration between raceways to replenish some of the oxygen used by the fish.

Tanks

Aquaculture tanks are normally designed to optimize the movement of water around the center of the tank. As such, common shapes are round, square with rounded corners, hexagonal, and octagonal (5,9,10) (see Fig. 4). The choice is often made on the basis of space availability and construction materials, as there aren't large differences in the hydraulic properties of the various shapes. Tank size ranges from an equivalent diameter of less than 1 m to more than 10 m. Water depth is normally maintained between 0.5 m for the smaller tanks and 1 to 2 m for the larger tanks. The two most common materials used for tanks are fiberglass and concrete.

The size and shape of tanks used in aquaculture vary greatly, but the common characteristic is that flow approaches the ideal continuously stirred tank reactor (CSTR) model in which water quality within the tank is approximately uniform. To achieve this uniformity, water is introduced along the periphery of the tank through one or more inlets that normally inject the water tangentially, and the water effluent is collected from the center of the tank (10,11). Contrary to raceways, it is possible to achieve a certain degree of independence between water flow rate and water velocity in a tank by manipulating the inlet and outlet characteristics and location. An important feature is that by adjusting the circulation pattern and water velocity to maintain particulate matter in suspension, tanks may be made to be "self-cleaning" (10,11).

A recent development in tank culture is the introduction of "double drains" in which two water streams are created in the drain, one constituting a large percentage of the flow containing a very small fraction of the solids released to the tank water (fish feces and uneaten feed), and the second stream conveying no more than 5 to 10% of the total flow from the tank, but containing up to 90 or 95% of the tank solids (10,11). These new double drains offer opportunities for improved water utilization efficiency and water treatment systems, with the high flow-low solids stream being relatively easy to treat for reuse, and the low flow-high solids stream being suitable for treatment prior to disposal. Flow rates in tanks are normally selected to result in residence times ranging from 10 to 15 min to several hours, and the value used depends on the level of water treatment (normally aeration) taking place within the tank and on the fish biomass being held.

Figure 3. Farm that uses raceways for the culture of trout. The raceways are laid out in series and in parallel to optimize land and water use.

Figure 4. Round concrete tanks used for the culture of striped bass. Water in the tanks is reoxygenated using pure oxygen injection systems and aerated for carbon dioxide removal.

Figure 3. Farm that uses raceways for the culture of trout. The raceways are laid out in series and in parallel to optimize land and water use.

Figure 4. Round concrete tanks used for the culture of striped bass. Water in the tanks is reoxygenated using pure oxygen injection systems and aerated for carbon dioxide removal.

Cages

Although cages have been used in aquaculture for many years, they have received a great deal of attention recently, especially with the development of the salmon industry in Norway and other countries (Fig. 5). Conventional cage technology is based on a rigid frame that is either floating or fixed, and to which a cage made of some type of netting material is attached. Traditionally, cages had been rectangular and relatively small, with total volumes of less than 10 m3. The recent growth of the industry, the availability of new materials and technologies, and the presence of competing users for coastal resources have resulted in the development of large cage systems suitable for open sea deployment (6,12). Some of these cages may be more than 50 m in diameter, 30 m deep, with production volumes of more than 50,000 m3, and are designed to hold up to 1,000,000 kg offish (6,12). Engineering contributions into the design, construction, and anchoring of cages becomes very specialized as the cage size increases and as cages are placed in more exposed locations. Cages are subjected to static and dynamic loads. Static loads are relatively easy to quantify and are caused by the weight of the fish and the structure itself, but transient loads caused by wind, waves, and currents may be orders of magnitude greater than static loads and are very difficult to determine accurately (6). Frequent cage maintenance is required to prevent fouling of the net from restricting the flow of water. Exchange of water through the net must be sufficient to maintain adequate water quality for the fish.

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