Integrated Aquaculture and Monoculture of Low-Trophic Species

Aquaculture is undoubtedly a critical sector for satisfying the needs of a growing human population and meeting the Sustainable Development Goals of Agenda 2030. Nevertheless, some current paradigms should be changed to match the sustainable production of aquatic organisms. To improve sustainability, the linear economy model should move to a circular economy, and low-trophic-level species should replace high-trophic-level species monoculture.

Combining species with complementary ecosystemic functions and requirements in the same culture allows for more efficient production systems. In this way, developing integrated multi-spatial, multi-niche, or multi-trophic aquaculture (IMTA) may be a solution. Innovative systems associating autotrophic organisms, microbes, and suspension and deposit feeders with manufactured-diet-fed species may improve the efficiency of using natural resources and the circularity. In integrated systems, the co-products, frequently classified as worthless wastes in monoculture, are used as supplies to produce other species instead of being discarded into the environment.

On the other hand, farming low-trophic species (LTS) in monoculture and integrated culture is more environmentally efficient than farming high-trophic-level ones. Low-trophic species in aquaculture are defined as the primary producers (algae and aquatic plants), primary consumers (herbivorous and detritus feeders), and other animals that feed mainly on plankton and small benthic invertebrates. Many detritivorous species use more nutrients from bacteria, fungi, algae, and invertebrates than from dead particulate organic matter.

This Special Issue introduces different insights into how aquaculture can benefit from integrated systems, as well as the culture of low-trophic species. Freshwater prawns and bivalve filtering mollusks have emerged as a great alternative to produce human food using natural or cheap feed sources. However, the availability of seeds may be a constraint. Vetorelli et al. (article 1) evaluated the intensification of hatchery systems for the Amazon river prawn, Macrobrachium amazonicum, which has shown great potential for monoculture or IMTA systems. The results showed that M. amazonicum tolerates high intensification in the recirculating hatchery, which can boost seed production. Sühnel et al. (article 2) evaluated different stocking densities and different algal diets in a fluidized bed bottle nursery system for the oysters Crassostrea gasar and Crassostrea gigas. The results demonstrated that the production costs and time to oyster growth can be improved in this type of system. In addition, regarding seed supplies, Crisóstomo et al. (article 3) evaluated the conditioning of Argopecten purpuratus broodstock; here, the seed quality indicators used by the authors were slightly lower compared to the broodstock kept in the natural environment. However, the study showed the importance of further optimizing the broodstock conditioning aspects, allowing more predictable and sustainable production to ensure permanent seed supplies.

The biofloc technology (BFT) used for LTS in both monoculture and integrated culture was primarily investigated. In their first paper, Carvalho et al. (article 4) explored how the integration of different species in BFT can lead to more sustainable aquaculture; they evaluated the growth and nutrient absorption of the macroalgae Ulva lactuca when cultivated in an integrated system with Penaeus vannamei and Oreochromis niloticus. The results revealed an increase in macroalgae biomass, increased nitrate and phosphate removal, and superior water-quality parameter maintenance, demonstrating the viability of macroalgae cultivation integrated into BFT systems. These results were confirmed in a second study conducted by the same team (Carvalho et al., article 5), in which they determined that the concentrations of total suspended solids in an integrated culture of U. lactuta and P. vannamei did not affect the macroalgae growth or shrimp performance, showing that macroalgae may be used for nitrate absorption. In another study, Rind et al. (article 6) evaluated using different carbon sources in BFT for Nile tilapia and determined that tapioca flour improved the water quality, fish growth, the status of hematology, immunity, and the antioxidants in fish juveniles. In addition, in a BFT system, Costa et al. (article 7) demonstrated that the oyster Crassostrea gasar can act as a suspensivorous feeder; however, the suspended solids must be kept at concentrations below 200 mg/L to achieve the best results. All these studies demonstrated that even in intensive systems, such as BFT, combining species with different functions can bring benefits and improve production sustainability.

Accessing circular economy practices, Checa et al. (article 8) used bioremediation and efficiency indicators to evaluate the circularity performance of four IMTA trials in three aquaculture facilities established in Ireland, Brazil, and South Africa. Salmon, white shrimp, tilapia, abalone, and sea urchins were studied and cultivated together in various combinations with several low-trophic species in these IMTA trials to evaluate the improvement in circularity compared with the corresponding monoculture conditions. The results showed an increase in the circularity of up to 90% in terms of water recirculation, as well as bioremediation, which was improved by 80–90%, providing evidence for the potential role of IMTA in the circularity transition. To control biofouling in the cages of IMTA systems, Montgomery et al. (article 9) examined the use of California sea cucumbers (Apostichopus californicus) on cages containing adult Chinook salmon (Oncorhynchus tshawytscha) at a commercial farming operation. The results showed that the sea cucumbers actively fed on the biofouling but preferred to consume uneaten feed/feces at the bottom of the cages. Biofouling control in cages would likely be possible with a higher density of sea cucumbers. These results may contribute to developing a management framework for sea cucumber/salmon integrated multi-tropic aquaculture. Studying another bioremediator species in integrated systems, Hou et al. (article 10) investigated the effects of farming the snail Bellamya purificata at different stocking densities on the algal and fungal communities in sediment. The results showed that B. purificata at a low stocking density might enhance the resource utilization efficiency and minimize the environmental pollution.

Most of the freshwater fish farmed worldwide have a low-trophic status. Lima et al. (article 11) explored the use of fertilization to supplement the diet of tambaqui (Colossoma macropomum), the principal native fish farmed in South America. Their results showed that tambaqui can thrive by alternating between natural food sources and commercial feed without any growth change. This suggests that tambaqui is well-suited for farming in restorative and integrated aquaculture systems as well as in intensive systems that rely on commercial feed.

The culture of filtering mollusks is expanding in the West Hemisphere. Iitembu et al. (article 12) explored regional variations in oyster production techniques, market dynamics, and consumption patterns across seven Atlantic regions. The results showed that Crassostrea gigas is farmed in most Atlantic regions, except the USA, and that the financial challenges for small businesses, the ecological implications of seed production techniques, the biosecurity risks, and public health considerations are critical areas for attention. This study introduces valuable information for policymakers, aquaculture practitioners, and stakeholders in optimizing global shellfish industry strategies.

We are sure that the contents of this Special Issue represent an essential contribution to the culture of low-trophic species, mainly in integrated systems. This entails changes in the current paradigms, moving aquaculture toward a more conservative and restorative production process. In addition, it allows aquaculture development according to the circular economy model. In this way, aquaculture can contribute to reaching the Sustainable Development Goals of Agenda 2030.