Types of culture systems
The successful tank culture of any fish specie is highly dependent upon maintaining good water quality. This is accomplished by aeration, oxygenation, and frequent or continuous water exchange to renew dissolved oxygen (DO) content and remove waste products. Culture systems that discard water after use are called flow-through systems, while those that filter and recycle water are called recirculating aquaculture systems (RAS), recycling or water re-use systems. Each type has advantages and disadvantages.
Flow-through tank systems
Flow-through tank systems depend on constant or periodic water exchange to flush out fish waste products. Exchange rates are determined by the available water quality and quantity, the fish biomass, and feeding rates. As a rule, the volume of water needed for a facility is the amount required to replace 100 percent of the tank water every 90 to 120 minutes.
Flow-through systems often are not suitable for commercial tilapia tank culture. Tilapia are warmwater fish that grow best when the water temperature is in the low to mid-80 °F range (approximately 27 to 29 °C). Unless incoming water is from a geothermal source or is warmed, it will be too cool for optimum growth. Warming large volumes of incoming water is generally not economically feasible. Operations with a constant source of heated water, such as a geothermal or low-cost waste heat source, might be economically viable.
Using surface waters for tank culture is not advisable, although there may be exceptions. The quantity of surface water available may vary during a drought, and its quality can vary because of rainfall runoff, agricultural activity or other development activity in the watershed area. Groundwater is a better source, but it is advisable to gather as much history as possible on the water quality of a site before developing the culture operation. Water from shallow wells may contain organic matter and unacceptable levels of ammonia or hydrogen sulfide gas. Geothermal water sources may have levels of dissolved minerals that affect fish health. It might be possible to treat groundwater before using it, though the operator would need to determine whether treatment is economically feasible.
Water discharged from flow-through tank systems may pollute receiving waters with nutrients and organic matter. Under the regulatory structure of the U.S. National Pollutant Discharge Elimination System(NPDES), the discharge of effluent water may require a permit, with required periodic testing and oversight, based on the following two criteria: 1) annual production must total at least 100,000 pounds (45,400 kg) of product, and 2) discharge of effluent water must occur for more than 30 days annually. These are federal rules, but states are allowed to determine whether one or both conditions must be met before requiring a permit.
Large facilities that exceed the production level of the first criterion should try to use the effluent for crop irrigation or treat it on site and not discharge it off site for more than 30 days of the year. A permit may also be required, at the discretion of the state regulatory agency involved, if any complaint is received. It is prudent, therefore, for the prospective aquaculturist to know the rules of the controlling regulatory agency during the early stages of facility planning and design.
Recirculating aquaculture systems (RAS)
Recirculating aquaculture systems, despite earlier failures, have become more common and more economically viable because of advancements that accomplish the required unit processes: solids removal, biological nitrification, oxygenation, and dissolved gas management. Recirculating systems must be designed to accomplish these processes with less new water input in an economically sustainable manner. Sand filters have largely been replaced by better types of solids removal devices such as bead filters and screen filters. Granular plastic media (bead) filters have undergone extensive development and improvement and are readily available from commercial suppliers. Screen filters that use rotating drums or discs are also widely used. These filters remove solids well, lose very little energy as water passes through them, and discharge very little water during the cleaning process.
Recirculating systems for tilapia culture have a number of advantages. They can be located in areas that do not have sufficient water resources for pond aquaculture. They can be located closer to markets and infrastructure, such as highway connections and utilities. Indoor operations protect the fish stock from seasonal variations in temperature, allowing year-round production that satisfies constant market demand. Depending upon the manager’s adherence to good operating procedures and the quality of incoming stock, isolating recirculating tanks from each other promotes biosecurity by controlling the introduction and spread of diseases or parasitic organisms. In some states it is easier to obtain tilapia culture permits for indoor, recirculating systems because there is less chance that these non-native fish will escape.
There are also disadvantages to recirculating aquaculture systems. Probably the greatest problem is the large capital investment required for building and starting up facilities. The need for specialized equipment and thermally efficient buildings significantly raises the cost of getting into the business. Economic analyses for the southern region have shown that the minimum production required for proper cash flow is about 200,000 to 250,000 pounds of live weight tilapia annually. The cost of constructing and starting up a facility with this capacity can exceed $750,000. If funds must be borrowed, a large debt to equity ratio may not be economically viable.
More detailed information about recommendations for the design and operation of recirculating aquaculture systems can be found in SRAC publications 450-459, available at: http://srac.tamu.edu/.
Culture tank design
Tanks for culturing tilapia can be of different sizes and shapes as long as they allow for the effective removal of waste solids. Tilapias produce a solid waste that is well-suited for removal from culture tanks. When fed commercial fish feeds, they produce fecal strings held together in a mucous membrane that maintains the feces in a relatively large, filterable size. The longer these solids remain in the tank, the smaller they become (as they disintegrate) and the more waste ammonia they generate, so tanks should be designed for the rapid and efficient removal of waste solids. All manner of tank shapes have been used for culturing tilapia, including square, rectangular, round, oval, octagonal and “D-ended” or “racetrack” configurations. The most desirable tanks are those that effectively remove solids at an affordable cost while using valuable floor space efficiently.
Round or octagonal tanks have a circular flow pattern that moves settleable solids toward a central drain, which is usually screened to prevent fish escape. In tanks with an internal standpipe, the center drain can be fitted with a larger outer pipe (sleeve) with notches at the bottom to remove water from the bottom of the tank where solids are concentrated. External standpipes are easily adjusted to accommodate increased water flows during the culture cycle, while maintaining the same water level in the tank. Tank drains must have screens, slots or some type of open area to prevent clogging that might cause the tank to overflow. Double drains, which create an effluent flow stream with greater solids concentration, perform well and are widely recommended.
If tilapia fecal material is not quickly removed from the tank, the mucous membrane will trap gases generated by bacterial decay and cause the fecal string to float. For this reason, some tanks have an additional surface drain at the center (standpipe) or on the tank sidewall to remove surface water and floating solids.
Research has shown that round or octagonal tanks with flat bottoms have better solids removal than those with sloped or cone-shaped bottoms. The slope may prevent solids from moving to the center drain. Tanks with flat bottoms also are simpler and less costly to build.
Continue to Tank Culture of Tilapia (part 3)