Can biofloc—converting waste into protein—play a key role?
As the demand for shrimp—the world’s most popular seafood—continues to grow, production methods are evolving to keep pace. Shrimp farms have advanced from traditional, extensive, and semi-intensive systems to more productive intensive and super-intensive systems. To reduce the negative impacts of these systems, however, farming practices must also evolve—ultimately to regenerative production systems that negate carbon emissions, restore ecosystems, and fortify local communities. Only then can shrimp farming achieve social acceptance while expanding to meet growing demand.
One promising solution involves intensive biofloc growout ponds. These ponds offer a pathway to regenerative systems based on their ability to absorb toxic ammonia, reduce the need for water exchange, enhance biosecurity, generate nutritious microbial biomass with growth-promoting properties, and reduce the protein requirement of feeds. However, biofloc ponds come with steep challenges: high energy costs for continuous vigorous aeration, complex protocols for managing water quality and biofloc density, and the risk of catastrophic loss in the event of equipment failure or mismanagement.
Intensive shrimp pond design: A brief history
While shrimp farming has witnessed many advances over the last 25 years in breeding, disease diagnostics, larval rearing, feeds, and processing, the basic design of intensive ponds has changed little since they were first developed in Taiwan during the 1970s. Intensive ponds are typically small, flat-bottomed rectangular basins with water inlets and outlets at opposite ends and paddlewheel aerators positioned to create circular currents. Recent additions such as plastic liners, automatic feeders, and central drain sumps have improved performance, but fundamental changes are needed to increase the efficiency of feed, water, and energy resources and to minimize disease incidence and upcycle wastes.
Management of intensive shrimp ponds
Feeds and wastes
As with animal feeds in general, only 20-30% of the nutrients in shrimp feeds is assimilated as shrimp flesh. The other 70-80% becomes waste, which impacts shrimp production. Conventional management practices are geared toward flushing these solid and dissolved wastes out of the pond. Solid wastes are bits of uneaten feed, feces, and other particulate organic matter that absorb oxygen as they decompose. As they settle to the bottom and become smothered with other sediments, they become anaerobic, producing toxic hydrogen sulfide and nitrite. Sludge is also a repository for pathogens such as AHPND and EHP.
To reduce the negative effects of solid waste, horizontal currents from paddlewheel aerators and other circulation devices are used to sweep solids toward a center drain for discharge. As radial currents approach the center of the pond, their horizontal velocity decreases, causing solids to begin dropping out of suspension before reaching the center drain. Some ponds are equipped with siphons to remove solids that accumulate around the center of the pond, but manual siphons are inefficient and time consuming, given the depth, poor visibility, large area, and cleaning requirements. Mechanical cleaning devices are not yet market-ready.
A more popular solution for solids removal is a conical central sump (“shrimp toilet”) which uses a steep slope beginning at the point of sedimentation to carry particles the final distance to the drain. Most central sumps are empirically designed and are only partially effective, due to insufficient size and depth.
Shrimp excrete ammonia as a byproduct of protein digestion. In intensive systems, ammonia and its oxidized derivative, nitrite, can accumulate to toxic levels. Toxic concentrations can be reduced by flushing the pond with new water which is low in ammonia and nitrite. Replacing water entails significant costs for pumping, filtration, and in some cases, disinfection. Filtration to remove shrimp predators and competitors also causes mortality of native aquatic organisms through impingement on screens and entrainment in pumps. The receiving water body is also degraded by the eutrophication burden of pond effluent. Ultimately, solutions are needed to eliminate the need for water exchange. A wide variety of intensive ponds with recycling water systems have been attempted, but design criteria and operating performance have been poorly understood.
In ponds with circular water flow, solids often deposit near the center without going out the drain (left). In earthen ponds, this can require excavating the deposits between cycles (center). Shrimp toilets, if designed properly, can help transport solids to the center drain (right). (Pat et al., 2022, Aquaculture Engineering).
Dissolved waste
Shrimp excrete ammonia as a byproduct of protein digestion. In intensive systems, ammonia and its oxidized derivative, nitrite, can accumulate to toxic levels. Toxic concentrations can be reduced by flushing the pond with new water which is low in ammonia and nitrite. Replacing water entails significant costs for pumping, filtration, and in some cases, disinfection. Filtration to remove shrimp predators and competitors also causes mortality of native aquatic organisms through impingement on screens and entrainment in pumps. The receiving water body is also degraded by the eutrophication burden of pond effluent. Ultimately, solutions are needed to eliminate the need for water exchange. A wide variety of intensive ponds with recycling water systems have been attempted, but design criteria and operating performance have been poorly understood.
PART II
The promise of biofloc systems
Biofloc is a collection of suspended, snow-like particles made up of organic detritus, bacteria, algae, fungi, protozoans, and other microbial organisms that form naturally when intensive aquaculture ponds are heavily aerated with limited water exchange. Aerator currents suspend organic matter within the pond. Bacteria colonize the particles and release a sticky extracellular polysaccharide matrix that binds other suspended particles to form biofloc aggregates. The diverse bacteria present in aquaculture biofloc ponds are ubiquitous, hardy, and resilient to a wide range of environmental parameters. Similar bioflocs are also present in municipal, agricultural, and industrial treatment systems around the world.
Brief history
The evolution of biofloc systems in aquaculture occurred concurrently in diverse systems during the 1970s. To name just a few, Steven Serfling and Dominic Mendola of Solar Aquafarms utilized biofloc systems for commercial tilapia production in California. Gerard Cuzon and coworkers at the Ifremer-COP (Institut français de recherche pour l’exploitation de la mer, Centre Océanologique du Pacifique) research facility in Tahiti utilized bioflocs for intensive shrimp production. Harvey Persyn and co-workers developed biofloc tank systems at Ralston Purina’s shrimp research facility in Florida. It was not until the late 1980s that the mechanism of action of bioflocs became more widely known and understood.
A biofloc system at HomeGrown Shrimp in Florida.
The science of biofloc
Bacterial communities in bioflocs are typically dominated by fast growing heterotrophic bacteria which rely on organic matter as their primary source of energy. They metabolize particulate organic carbon (such as feces or uneaten feed) and dissolved inorganic nitrogen (such as ammonia) to grow bacterial biomass. This transformation is remarkable considering that animal feed is normally eaten once and then wastes are discarded; in this case, wastes are converted into bacterial protein, which can be directly consumed a second time.
A fundamental characteristic of heterotrophic bacteria in biofloc systems is their improved performance in the presence of substrates with a balanced carbon to nitrogen (C:N) ratio. Researchers have demonstrated that substrates with excess carbon could be balanced by addition of inorganic nitrogen and substrates with excess nitrogen could be balanced by addition of organic carbon.1
Avnimelech et al. (1992) demonstrated that growth-inhibiting ammonia in super-intensive tilapia ponds with limited water exchange could be eliminated by adding organic carbon to balance the C:N ratio and stimulate bacterial growth. They found that supplemental organic carbon (wheat flour) could be added directly to the water or added indirectly to the feed to dilute the protein level from 30% to 20%. In both cases, bacteria within the pond utilized supplemental carbon to metabolize the excess ammonia for growth. Tilapia then consumed the microbial biomass, which spared their dietary protein requirements.
Concurrently, Hopkins et al. (1994) reported similar benefits of low-protein feed with Litopenaeus vannamei stocked at varying densities in ponds with zero water exchange and variable aeration rates. Production rates in all treatments were comparable, with no difference produced by different feed protein levels.
Contribution to shrimp nutrition
The ability to filter and consume biofloc aggregates varies among species. L. vannamei is more efficient than P. monodon or F. brasiliensis, but not as efficient as tilapia. Consumption efficiency also depends on the concentration and size of aggregates.
Growth-promoting effects
In natural food webs, detritus is an important food source in penaeid stomachs followed by copepod and animal remains (Bombeo-Tuburan et al., 1993). Aside from its value as a source of macronutrients such as amino acids, lipids, and carbohydrates, detritus from intensive shrimp ponds also has a strong growth-enhancing effect. This effect was first reported by Leber and Pruder (1988). They compared growth of shrimp in clear seawater pumped from a seawater well versus growth in the same water after it had passed through an intensive shrimp round. Effluent from the intensive pond resulted in an 89% growth improvement as compared to the clear well water.
Glencross et al. (2013; 2014) demonstrated in multiple trials that a commercially produced microbial-biomass product (Novacq™, CSIRO, Dutton Park, QLD, Australia) exhibited strong growth-promoting effects in shrimp. This patented product was also found to be effective as a replacement for fishmeal and fish oil.
Commercial biofloc systems
During the mid 1990’s, the potential of achieving high production and low FCR using low protein feed and minimal water exchange looked promising. The concept involved production of microbial biomass from nutrients that would otherwise be lost during water exchange. The microbial biomass would be consumed in situ, without associated costs of separating, drying, and processing.
Belize Aquaculture Ltd.
In 1999, Belize Aquaculture Ltd., built a pioneering, state-of-the-art shrimp farm using biofloc technology to produce high yields (~15 mt/ha/cycle) of L. vannamei in highly aerated, plastic-lined ponds with low protein feeds and zero water exchange (McIntosh, 2000, 2001). The results greatly advanced the prospect of intensive shrimp production in biofloc ponds, but they also illuminated a number of challenges and complexities. The high density of biofloc required continuous vigorous aeration, meticulous management of water and sediment quality, and vigilance to prevent catastrophic losses in the event of aeration failure. High levels of suspended solids concentrated in a hypoxic zone at the center of the pond, which caused stress and chronic mortality.
Broodstock growout and nursery systems
While high biofloc densities are challenging to manage for commercial growout, lower-density systems have proven useful for broodstock growout and nursery systems. These enable reduced reliance on water exchange, improved biosecurity, enhanced nutrition and egg production, and use of raceway configurations with greenhouse covers to facilitate temperature control.