The major advantages of pond fish culture are the low capital cost of earthen ponds and the reliability of pond fish production. Disadvantages are the need to continuously manage pond oxygen concentration and other fluctuating water quality variables, prevent off-flavor, control predators and disease, and provide labor for harvesting. These management difficulties, combined with land, water and environmental constraints, have driven the search for technological improvements in pond aquaculture. One solution has been to shift to more energy-intensive systems, either by increasing aeration (1 to 20 hp per acre) in low-cost ponds or by abandoning ponds altogether in favor of recirculating tank or raceway processes.

Partitioned Aquaculture Systems
The Partitioned Aquaculture System (PAS) developed at Clemson University combines the process control advantage of recirculating aquaculture with the lower cost of pond aquaculture. Many of the management problems of conventional pond culture are linked to the daily diurnal cycles of oxygen production and the uncontrolled algal photosynthesis in ponds. 

The PAS superimposes a water velocity field upon the pond, making it possible to reconfigure the pond into separate, controllable compartments for the processes of fish culture, gas exchange, algal growth and waste treatment. A high rate of photosynthesis makes possible a solar powered biological waste treatment capacity that is sustainable (unlike fossil fuel systems). The PAS adapts algal production to produce a sustainable, low impact, high-yield, and more controllable fish production process.

Beginning in 1989, scientists at Clemson University attempted to develop a system that offered the advantages of high-density raceway culture while coupling waste treatment to the kind of high-rate algal systems previously developed for treating domestic wastewater.

Central to the economic success of the PAS is an energy efficient means of moving large volumes of water at low velocities uniformly throughout the pond.

Uniform water velocities can best be achieved with the use of low rpm (1- to 3-rpm) paddlewheels. This technique of coupling algal growth basins (or high-rate algal ponds) and high-density fish raceways with a paddlewheel-driven water velocity field was termed the Partitioned Aquaculture System (PAS). The PAS partitions pond fish culture into a series of physical, chemical and biological processes linked by computer-controlled, uniform water velocity.

The low-speed paddlewheel moves water through the algal basin and fish raceways so the operator can control oxygen and carbon dioxide concentrations in the water. The uniform water velocity field: 1) mixes and disperses nutrients into the entire water column, 2) rapidly turns over algal biomass into the light zone to ensure maximum algae production, and 3) controls water quality across the raceway. A solid settling basin at the discharge end of the fish raceway is where solid waste and/or flocculated algal solids are removed. A relatively shallow (1.5-foot) algal culture basin (waste treatment site) enhances the rate of photosynthesis, thus increasing fish production.

The PAS technique (at the highest yields) depends on the co-culture of filter-feeding fish, shellfish or detritivoirs. As these species continuously harvest algae they control the age of algae, the total standing crop of algae, the rate of algal respiration, and the rate of oxygen production. Co-producing filter feeders makes nutrient removal environmentally friendly and sustainable, while yielding a secondary fish crop.

PAS Design and Operation
The PAS design has evolved over 15 years. During the early years, research focused on optimizing algal culture basin depth, mixing speed, gas exchange and high-density (cage) culture of catfish using four 1/36-acre PAS units. In 1995, six 1/3-acre PAS units were installed. From 1995 to 2001, the limit of the fish carrying capacity was examined and the use of tilapia co culture explored. Finally, in 1999, a single 2-acre commercial scale prototype was installed.

System Paddlewheels
The PAS units use low-speed (1- to 3-rpm) paddlewheels driven by oil hydraulics to move large volumes of water at relatively low head (0.5 to 2.0 inches ∆H).  In the final design, the power requirement of the motor should be based on the “worst case” expected power (highest velocity) and increased to adjust for mechanical losses. Because of the desire for a wide range of water velocities and the use of a single power unit to power two sets of paddles, the 2-acre PAS prototype was fitted with a 3-hp motor, well in excess of actual required power. One of the important modifications of the 2-acre PAS prototype was the use of a second paddlewheel system to independently control raceway water quality (i.e., independent from the algal reactor). This arrangement allows for more precise computer control of dissolved oxygen, ammonia and carbon dioxide concentrations in the fish raceway. Cost was reduced by eliminating the multiple channels in the algal culture basin originally used in the 1/3-acre and 1/36-acre PAS units. The 2-acre system uses a gang of paddlewheels with a single 70-footwide by 600-foot-long algal growth channel.

Fish Raceways
The 2-acre system uses a three-channel raceway with the central channel serving to distribute water into the fish raceway. The flow distribution to the raceway is “distributed plug flow,” where individual fish sections are maintained at uniform water quality by introducing side flow velocities in direct proportion to the oxygen demand of each section. Water velocity across the fish raceway is provided by a separate paddlewheel. In this way, the flow and water quality in the fish raceways can be controlled independently of the algal basin. High densities in each fish raceway result in uniformly mixed raceway conditions.

In 1999, stocking density experiments showed that 8 to 10 lbs./ft3 of catfish could be sustained with no adverse effect on growth, suggesting that overall system costs can be further reduced by using a single high-density raceway and fewer paddlewheels. 

Other advantages of high-density raceway culture are that inexpensive netting can be stretched across the top of the raceway sections to keep out predators, and fish confinement allows for more efficient use of chemicals to treat or prevent fish diseases. The walls of the prototype were constructed of concrete-filled cinder block, although formed concrete or wooden raceways have been used with similar success.

Algal Basin Flow Patterns
To reduce construction cost, the algal culture basin channels were reduced from six in the 1/3-acre units to two in the 2-acre units. This reduced the partition/wall requirement from 3,000 feet per acre to 600 feet per acre. The central dividing wall was constructed of 4- foot-wide, 0.032-inch, black polyethylene attached to a conventional 6-inch mesh “hog wire” fence buried 1 foot in the soil and supported by steel posts. This forms a plastic curtain wall 3 feet high. Flow experiments were conducted in 1999 in the 2-acre unit to determine the uniformity of the water velocity field that can be sustained with different combinations of paddles and paddle rpm. The results suggest that sufficient mixing and flow velocity in the algal channel can be maintained with 50 percent of the flow path driven by paddlewheels in the algal channel.

Primary Production in the PAS
The most important difference between conventional pond aquaculture and the PAS is the increased algal growth achieved in the homogenous water velocity field. This 3 to 4-fold increase in photosynthesis is the basis for the increase in fish carrying capacity.

Algal Biosynthesis
The algal biosynthesis that removes carbon dioxide and ammonia from the pond and adds oxygen to it is the key to the continuous waste treatment, and reuse of pond water, in the PAS.

The PAS uses co-culture as opposed to polyculture.  In polyculture, different species of fish are stocked into the pond and allowed to feed at large. In PAS culture, the target species are physically separated from the filter-feeding fish.

Fish Production and Feed Application Rate
Net catfish production in the six 1/3-acre units increased from an average of 3,078 pounds per acre in 1995 to 17,263 pounds per acre in 2000. By 1997, maximum daily feeding rate reached 200 pounds per acre, with an average daily feeding rate for the season of 94 pounds per acre. Maximum fish carrying capacity reached 17,001 pounds per acre in 1999.

Fish Health Management
When fish are confined in raceways, chemical treatment of diseases is easier, less expensive and more effective than when fish are in ponds. Controlling the oxygenation of the water also is more effective and less costly than in ponds. Early in the PAS trials (before tilapia co-culture), it was necessary to treat the whole pond for toxic cyanobacteria. When fish in the fish raceway flow are isolated from the algal culture basin such treatments can be made if needed. Columnaris was the most significant disease problem in the PAS. It was controlled effectively with medicated feed and, because of system design, with potassium permanganate. Periodic Costia and monogenetic trematodes were controlled with 1-hour exposure to 250 ppm formalin in the fish raceways. Infrequent occurrences of brown blood were treated by adding sodium chloride to the entire system. Proliferative gill disease (PGD) has affected the winter carryover fish in the PAS. Mortalities ranging from 25 to 100 percent of over-wintered fish have limited production to 17,000 pounds per acre, as opposed to the 20,000 to 23,000 pounds per acre expected from successful grow out of carryover stock. Lowering the pH of sediment by applying hydrated lime has reduced the incidence of PGD in carryover fish in the early spring months.

Summary
Aquaculture is growing at about 20 percent per year and will, within the next two decades, produce more than natural fisheries. The future development of aquaculture will be severely constrained by the availability of water and environmental concerns. The PAS has the potential to significantly increase fish production per unit area over conventional aquaculture techniques while reducing water requirements by 90 percent per pound of fish produced. This makes it possible to install PAS facilities at sites not suitable for conventional aquaculture production.

Calculating Surface Areas
Iowa State University, University Extension