Production Methods in Aquaculture
Aquaculture production methods vary greatly, but regardless of method, a good understanding of water quality and chemistry, species requirements, and systems design and operation will facilitate good management. Production can be divided into two major categories: land-based and open-water systems. Land-based facilities use tanks and ponds to house and raise fish. Tank systems can be flow-through, in which continuous replenishment of “new” water comes from a well, reservoir, or other central water body; recirculating, with reuse of system water after filtration to remove nitrogenous wastes and dissolved and suspended particulates; or a combination of the two. Indoor, land-based tank systems afford much greater control over water quality, predators, and pests than outdoor facilities, but often house fish at much higher densities.
Design and management of recirculating aquaculture systems (RASs) requires greater technical knowledge than for flow-through systems (see Southern Regional Aquaculture Center [SRAC] Fact Sheet 4708 [https://srac.tamu.edu/index.cfm/getFactSheet/whichfactsheet/238/]). Most RASs typically incorporate biologic filtration and mechanical filtration components, although some also include some type of chemical filtration and in-line sterilization. Biologic filters rely on the establishment of two sets of nitrifying bacteria on biofiltration surfaces to transform ammonia, a by-product of protein and nucleic acid metabolism by fish from food, first into nitrite (a toxic intermediate), and then into nitrate, considered a much less toxic compound (although levels of nitrate may be problematic for invertebrates and, at higher levels, fish). More advanced systems include a denitrification component that transforms nitrate into nitrogen gas that is released into the atmosphere. Mechanical filters remove particulates to reduce their negative effects on water quality and fish health, and to improve water clarity, which facilitates observations. Chemical filtration is used to change the concentration of specific ions or compounds in the water (eg, ion-exchange resins such as those used in softeners to remove calcium, or zeolite to bind ammonia). Ultraviolet (UV) sterilization units and ozone are common choices for aquaculturists who wish to include in-line disinfection of system water. UV sterilizers act optimally at a wavelength of 254 nm and disrupt pathogen DNA. Systems with these components typically include a system bypass loop to feed a portion of total system water into these disinfection units. UV sterilizing units must be designed and rated to deliver a specific “zap dose” to target a specific pathogen or pathogen group. Component maintenance, water clarity, wattage, and water flow through the UV sterilizer all contribute to the efficiency of the unit. Although there are exceptions, in general the larger and more complex a pathogen is, the greater the zap dose required to kill it. Ozone units use highly reactive O3 to break apart microorganisms. However, because ozone can be very harmful to fish, it must be removed from the water before it is returned to the system. Ozone is also a human health hazard, so protocols to safeguard human health should be implemented. See also Aquatic Systems.
Pond systems include earthen and lined ponds typically filled from wells, aquifers, or surface water bodies. Ponds have minimal water exchange and rely on good management of their more “natural” systems to maintain acceptable water quality and control aquatic weeds, algae, and other pond life. Nitrifying bacteria are found on surfaces in the pond. Important key water quality parameters—oxygen, carbon dioxide, and pH—can vary widely throughout the day in a pond because of photosynthesis by phytoplankton (algae) and other plant life. During the day, photosynthesis increases oxygen levels. However, during the evening, when photosynthesis has ceased, oxygen is consumed by all organisms in the pond, including algae and higher plants, resulting in lower levels. At the same time, increased production of carbon dioxide (without consumption through photosynthesis) results in lowering of pH. During the day, these trends are reversed. Any ammonia levels in a pond will have increased toxicity as pH rises throughout the day. Other challenges in pond production include pests and predators, such as snails, otters, birds, amphibians, and reptiles, as well as nontarget fish, all of which can also act as reservoirs or vectors for pathogens or intermediate hosts. See also Aquatic Systems.
Channel catfish, tilapia, hybrid striped bass, shrimp, crawfish, baitfish, and ornamental fish are raised in ponds, tanks, or a combination of the two. Some species, including rainbow trout, require higher oxygen concentrations and are raised in land-based raceways.
Open-water aquaculture includes production in net pens or sea cages placed in oceans, bays, lakes, or reservoirs. Open-water systems rely on good flushing, dilution, and natural processes occurring within the given water body to maintain acceptable culture parameters. In many cases, land-based systems are used for reproduction and early life-stage growout for “open water aquaculture” species. Open-water systems do have a number of challenges. Water quality in the main body of water cannot be controlled. Access (by boat in many cases) and logistics are more complex because of location within a larger body of water. Biofouling (growth of algae, bacteria, other invertebrates) on nets can reduce water exchange and degrade water quality within the net or cage. Pests and predators can be more difficult to control, as can human theft or vandalism. Disease management is also more complex because of increased environmental restrictions in natural water bodies and treatment logistics (eg, modifications such as “tarping” (placing a large polyethylene liner over or under a net to hold water for temporary bath/immersion treatments).
Open-water systems are used for final grow-out stages of clams, oysters, mussels, Atlantic salmon, flounder, pompano, cobia, and other marine species. Production of some freshwater species (eg, tilapia) may also use net pens set in lakes, ponds, or reservoirs for growout.