Sustainable Aquaculture Development .. Theory and practice

Different methods are being used to reduce the environmental impact of aquaculture practices, but further research is still needed.

For example, the use of selective breeding to reduce the use of chemicals for disease treatment may diminish the population genetic difference and possibly increase the virulence of the pathogenic organisms that may specialize in particular organisms. Also, the use of vaccines represents a sustainable method but can cause significant stress for the fish during the vaccination which itself can increase the disease risk.

Land-based aquaculture with recirculation systems seems to be the best alternative option in terms of wastewater reduction and exploitation, prevention of disease outbreaks, escapees, monitoring costs and efficiency. However, there is a competition for land space that can be complemented by well-managed offshore polyculture facilities integrated with solar and wind farms. Despite those, numerous other sustainable measures were proposed .

Aquaculture sustainability is an on-going process that requires integration of all stakeholders. Government, farmers, ecologists, and consumers should drive aquaculture practices under a risk assessment approach to reduce wastes, disease outbreaks, and operational costs enhancing sustainability potential.

Integrated Multi-Trophic Aquaculture (IMTA)

An important step towards sustainable aquaculture is to consider excess food and fecal matter not as a waste product, but as a resource that contains high amounts of nutrients and essential fatty acids that should be recycled and not discarded . Based on this idea the concept of IMTA was created, which applies a simplified food web structure to a farming system of fed-species, such as fish and shrimp, together with extractive organisms, such as molluscs and seaweed that take up particles and nutrients from the environment .

Integrated aquaculture also produces higher yields than mono-species systems in addition to satisfying rising consumer demands for environmental standards . The practice of IMTA aims to perfect this principle by combining species at different trophic levels for a balanced-ecosystem approach.

Reducing the load of nutrients and organic matter released by IMTA systems, preserves the quality of the receiving ecosystem, a secondary economic benefit is obtained and the social image of aquaculture is improved.

Macroalgae are a popular component of IMTA setups and have a number of advantages over conventional mechanical or microbial filtration systems. Common nitrifications filters use up dissolved oxygen and require additional equipment and monitoring . Contrary to this, integrating algae into an aquaculture system counterbalances nutrients, CO2 levels, acidity, and increases dissolved oxygen while producing valuable biomass. Macroalgae are fast and easy to grow, highly productive, with a higher yield of cultivation than land plants, no need of pesticides and with great potential for various economic applications .

Rearing shellfish may have positive effects on the environment and promote biodiversity. Oysters and mussels cultivated close to fish farms benefit from the suspended organic matter, show improved growth and reduce organic nutrient load in the water and integrating mussel longline cultivation to existing operations should be economically viable. Their culture is, however, not a panacea as particle absorption is restricted to appropriate size ranges and the benefits of bivalve culture sometimes fail to be achieved at all . Moreover, bivalves also produce particulate waste in the form of pseudofeces, creating a need for further remediation of impacted sediment .

Particulate waste of fed species or shellfish cultivation can be used as a source of feed for detritivorous animals such as polychaetes and sea cucumbers. Some sea cucumber species have been found to thrive on the debris of fish farms. They can constitute an additional crop and have beneficial effects on sediments by re-working upper sediment layers and influencing the development of microbial communities. In closed aquaculture systems, such bioturbating detritivores can also be integrated into sand filters where oxic and anoxic conditions in the sediment become the site of nitrogen conversion and removal.

Lagooning/Artificial Wetlands

Wastewater lagooning is a highly effective, low-cost solution (initial installation and maintenance) for purifying wastewater from land-based farms. The treatment of wastewater consists of a series of physical, chemical, and biological processes to remove contaminants and separate clean or at least reusable water and solid waste, which can be used for a number of industrial or agricultural purposes. Such types of artificial wetlands are already widely used for the treatment of municipal waste and are especially effective at removing excess nitrogen and storing excess phosphorus in the soil. This type of phytotreatment has already been tested with wastewater from fish ponds as means of algal ponds and wetlands and has shown to be an efficient system by reducing nutrient contents and modifying physico-chemical parameters of water.

However, lagooning systems require large surface areas, thus, competing for land space with other sectors. The installation of aeration systems makes artificial wetlands more efficient by enhancing the oxidation rate of pollutants and allows reducing the space needed. Using the area for additional production makes this method more profitable. Various halophytes in different types of natural and constructed wetlands are already successfully used as down-stream filters of municipal, industrial, or agricultural wastewater. Their use can have great economic benefits when compared to conventional water treatment and they are being investigated as possible extractive species in integrated aquaculture as they are able to utilize high levels of ammonia, which can be toxic to other plants . Different species of halophytes have been successfully cultivated in the effluents of European sea bass (Dicentrarchus labrax) in RAS and can be integrated in artificial wetlands or cultivated hydroponically. While still a niche market, they are gaining popularity as a delicacy vegetable.

Maintaining the integrity and function of ecosystems is vital for the sustainable spread of aquaculture operations.

Sustainable Feed Management

Sourcing of aquaculture feed is one of the sustainability core challenges of marine finfish aquaculture. Intensified production and the cultivation of high value carnivorous fish largely depends upon the use of fish meal and fish oil as the main feed ingredients, making it a consumer of capture fisheries products, especially of nontargeted fisheries and small forage fish. This has caused environmental as well as economic concerns, with feed costs being a large part of total production expenses, and important progress has been made towards sustainability by improving feed efficiency, turning fish offal into useful silage or designing plant-based, polychaete-based, and insect-based protein feeds. The challenge has been to replace fish oil with other alternatives and ensure the high content of highly unsaturated fatty acids within the feed to maintain the nutritional quality of the fish for human consumption. While the use of land-based feed may reduce the pressure on fisheries, it can significantly increase the pressure on freshwater resources (water footprint), due to water consumption and pollution in crop production.

However, recently new approaches have been developed to reduce excess feed used and loss of food. In intensive fish farming where feeding is taking place by an automatic system, it is important to monitor the feeding activity of the fish and adjust the amount of feed to the feeding behavior. Such monitoring can be done by using, for example, an underwater camera technology or other similar methods that detect uneaten feed and stopping the feeding process.

More sustainable feed management can be achieved through IMTA in a number of ways. Dissolved nitrogen in wastewater used to promote the growth of micro- or macroalgae replaces the need for fertilizer or growth media. These extractive species can in turn be cultivated as a feed source, increasing their practical value. The macroalgae Ulva lactuca, which has low economic value when sold directly, provides high quality feed for abalone or sea urchins and microalgae serve as feed for mussels. Thus, algae production can be considered as a more sustainable industry than continuing to harvest fish for fishmeal. It is estimated that if microalgae were used as fishmeal replacement, the effect would be to remove 30% of the fishing pressure thereby helping to conserve marine ecosystems. Algae may substitute fish oil, be viable and even improve growth of farmed fish so it should be considered to reduce harvesting fish for fishmeal.

Effluents can be directly valorised by growing primary producers. Sludges and effluents from aquaculture activities may also be bioremediated by decomposers, detritivores, and biofilms, whose biomass in turn presents a useful resource for feed production. Solid wastes are an appropriate feed for polychaetes, which provide protein and important fatty acids as feed for fish, shrimp, or crab production, reducing or eliminating the need for manufactured feed based on farmed crops or fish meal. Fish feed can also be supplemented or even replaced through the application of biofloc technology. In this approach, microbial growth in the water column of fish tanks or ponds is stimulated through the addition of carbohydrates in the form of sugar, starch, or cellulose . Heterotrophic bacteria, using these as a substrate to build proteins for growth, require nitrogen, which they take from the surrounding water. Dissolved nitrogen species in the water are, thus, transformed into bacterial biomass, aggregating into bioflocs that can be consumed by shrimp or fish. Biofloc technology in tilapia ponds can provide 50% of the protein consumed by the fish and when used as a protein source in shrimp feed, microbial floc meal showed the same results or even outperformed soybean or fishmeal ingredients. Biofloc systems have been mainly used in freshwater ecosystem and for herbivorous organisms but recent studies included mullets and pacific shrimp. The successful application of biofloc technology, however, requires knowledge of the system, close monitoring to maintain appropriate C:N ratios and microbial densities and upscaling from laboratory to economic application.

Sustainable Use of Chemicals

There is an increasing tendency to develop methods with the aim of reducing extensive chemical substance use, and, therefore, minimizing environmental pollution. Such alternative methods can have other positive effects on production such as cost minimization for the producer and increased consumer acceptance.

Methods that have been used vary between several factors such as the fish species and disease in question as well as the production methods used and characteristics of the area where the facility is located. They can be divided as precautionary methods (e.g., limiting fish density), methods concerning disease prevention (e.g., immunostimulant feeds and herbal medicine additives), co-culturing of different marine species which both benefits from such a production model, different physical techniques used such as so called “lice skirts,” plankton nets around the fish cage, to avoid parasite infestations as well as other varying from above mentioned aspects.

The largest aquaculture producing countries have been using regulations to report disease occurrence and to limit the fish density according to those measures because high density of fish or cages highly increase the risk of disease transmission, e.g., infectious salmon anemia or infectious pancreatic necrosis.

Other methods that have been used include functional feed additives, which can be administered via feed. They are non-nutritive ingredients that affect fish performance concerning, for example, feed utilization, survival, and growth rate. These additives promote fish growth and can replace antibiotics in those countries where it is still used as preventing and growth promotional measures, and thereby avoiding associated negative effects. Most widely researched options include probiotics, prebiotics, synbiotics, acidifiers, plant extracts, nucleotides, and such immunostimulants as β-glucan and lactoferrin. Chinese herbal medicines can be used as an alternative in disease prevention in aquaculture because of antibacterial, antiviral, immunostimulant, and growth promoting substances.

Co-culturing of different fish species can be used to limit pathogens during production.

Genetic improvement through selective breeding has been used in aquaculture mostly to improve growth performance of fish. In Europe, around 80% of aquaculture originates from selective breeding mainly because of this reason. However, it is possible to select fish for other traits such as disease susceptibility . Also, vaccines are being used to minimize the need for antibiotic treatments. However, the method can be associated with some negative effects such as stress caused to fish during the treatment when handling of fish is involved.

Lytic bacteriophages can be used as therapeutic agents against marine bacterial diseases such as those associated with vibrios. However, more research is still needed to apply these viruses under field conditions and to avoid dispersal of unwanted genes and effects on fish microbiota.

Offshore

Generally, mariculture facilities are located close to human settlements, in protected coasts or inland, with shallow waters and low hydrodynamic energy. Recently, more aquaculture facilities are being placed in semi-exposed areas with a potential of offshore aquaculture development. To have a regular supply of oxygen and temperature, and to avoid diseases, parasites, algal blooms, and user conflicts, the aquaculture industry has been developing new technologies for offshore facilities. Offshore systems are not environmentally sustainable in the long term (there is no reduction on contaminant outputs) but are a more respectful alternative with the environment since the load capacity increases with the depth and the higher dispersion of the waste. From a spatial perspective, offshore production has a lot of potential; it may be expanded to many countries, move away from sensitive marine biocenosis areas, be combined with wind, oyster, and mussel farms, increase biodiversity by creating an artificial reef, etc.. However, the open sea is considerably rougher than coastal waters and strong waves can break structures or detach organisms, worldwide distribution of species is uncertain, and there are political, technical, and cost-distance limitations. Free-floating and propelled installations may be too expensive and improvements on mooring systems are required to expand farms at deeper areas.

Some finfish farms are already located at open sea, most of them are round and submersible net cages to dissipate water currents and to avoid waves. Seaweed and bivalve become profitable more quickly because they do not need to be fed and operational costs are lower. Several farms including IMTA have been developed, mainly in Asian countries, but all at shallow waters and close to the coast. Moreover, it is still difficult to avoid the detachment of seaweed and mussels with traditional longline systems.

Wind-powered water pumps and solar-powered water heating systems are proposed to be integrated with offshore systems because will reduce long-term operating costs and environmental implications and increase competitiveness and profitability.

For aquaculture to expand, greater social acceptance, adapted regulations and long-term sustainability are necessary. However, offshore production development is sometimes limited because few countries have regulations that explicitly mentions offshore or open-ocean aquaculture, and fewer still have codified definitions of this type of culture. Sometimes governance of offshore industries is ambiguous or undefined, presenting obstacles to allow their activities (e.g., United States and Australia).

Copper Net Cages

Biofouling development reduces the water flow inside the fish cage forcing farmers to clean and change polymer nets frequently, or use antifouling coatings, thus, increasing operational costs. Antifouling coatings are discarded every year producing toxic effects because of the release of their main active ingredients to the environment (copper oxide, cadmium, and zinc). Organisms grown inside and outside of trials using copper nets were safe for human consumption and no accumulation of copper was detected in sediment, but assessment and environmental monitoring is needed because copper alloy released more copper to the surrounding environment and affect the biota more compared to the conventional net.

Precision Fish Farming (PFF)

There is a recent sustainable framework of fish farming called Precision Fish Farming (PFF), which developed from the concept of Precision Livestock Farming (PLF) to pisciculture. PLF and PFF use hardware (e.g., sensors), observers, and intelligent software to improve animal health and welfare while increasing productivity, yield, and environmental sustainability. In contrast to livestock production, intensive fish farming methods are more recent, crop is largely determined by environmental conditions and monitoring methods are more expensive and complex at aquatic ecosystems.

In Conclusion...

The key to affordable and sustainable aquaculture practices lies in the independence from natural resources through recycling or remediation of fish production wastes.

Many countries with important aquaculture production have some environmental regulations but lack clear frameworks for emerging technologies such as offshore farming. Every farmer may develop sustainable principles, methodologies, or practices. Therefore, it is recommended to promote certifications of good practices.

Environmental monitoring costs may be too expensive and onerous, hindering its application or interpretation and make sustainable development more difficult.