Aquaculture production has risen over the last decades, and it is expected to continue to grow which means the proportional increment amount of waste. ‘Circular economy’ has been defined as “an economic system that replaces the ‘end-of-life’ concept with reducing, alternatively reusing, recycling, and recovering materials in production, distribution and consumption processes. Circular economy and bioeconomy converge in the use of biological resources, and particularly when this biomass is a by-product that represents an input for another industrial sector.

As fish are mostly carnivores, aquaculture diets contain high amounts of animal protein to yield optimal products. This requirement reduces its sustainability, therefore, in order to become the source of future food products it has to reduce food waste and stimulate a change towards sustainable and healthy diets for humans.

“Galicia, a regional part of Spain, provides more than 20% of the aquaculture products generated in Europe and represents more than the 80% of the Spanish productivity.”

Three groups of fishing activities can be found along the Galician coastline: aquaculture, shellfish harvesting, and fishing. Bivalves are by far the main species produced in Galicia, led by the Mediterranean mussel (Mytilus galloprovincialis).

Also, Galicia was a pioneer region in the aquaculture production of turbot (Scophthalmus maximus) and currently the first European producer. This City is also the first Spanish producer of Senegalese sole (Solea senegalesnsis), and the only European producer of blackspot sea bream Pagellus bogaraveo.

The features of Galician marine waters, such as temperature, upwelling and tides also determine the development of the characteristic vegetation of marine macroalgae, or seaweeds, mainly composed by fucoids, kelps and carragenophytes.

In Galicia, seaweeds are traditionally used to obtain agar, jelly or as an agricultural fertilizer. There is also an important number of seaweeds washed ashore, which are not covered by a specific legislation and are freely harvested for agar and alginate extraction, or as soil conditioner.

Aquaculture waste and by-products

In quantitative terms, the percentage of primary products, by-products and wastes generated from aquaculture depends on the selected species. For fish grown in aquaculture facilities it has been estimated that 45% is directly transformed while the remaining 55% is considered subproduct.

Similar efficiencies have been determined for crustaceans, where the shell, including the head, represents 60% of the total body weight while the portion directly destined to consumption is about 40%. Molluscs are the most efficient organisms since flesh constitutes 70% and shell 30% of total body weight.

“Regarding waste classifications, aquaculture wastes can be divided into four groups: solid wastes as particles in suspension; dissolved organic substances; dissolved chemical compounds; and pathogens. “

Although, more recent works classify wastes from RAS in just two classes: biological (fecal solids, uneaten food, and detached bacterial flocs) and effluents (mostly containing organic matter and nitrate).

“Finally, it is important to underline another two types of wastes derived from aquaculture: chemical and pathological.”

The presence of the first one, also called residues, is due to veterinary treatments that animals require to minimize the presence of diseases and mortality rates. The second one is the pathogenic load that may be found in waters.

Integrated multi-trophic aquaculture: reutilization of aquaculture wastes

To magnify the productivity of aquaculture systems and reduce their environmental impact, derived wastes, such as metabolic products or uneaten food, have to be considered potential source of minerals, vitamins, proteins and lipids, for their further use.

“Integrated multi-trophic aquaculture (IMTA) is considered a model to fulfil all these requirements.”

This production design implies the culture of few species from different trophic levels, so wastes produced by those from higher levels are inputs for species from lower levels, similar as occurs in natural ecosystems.

First level usually includes fish, crustacean and cephalopods. The second one involves filtering invertebrates (e.g. filter molluscs, anemones, sea cucumbers, etc.), which feed on organic matter generated by first level, such as feed remains or sub-products.

In the third level, marine macroalgae use inorganic compounds, like those from excretory products released by previous levels. IMTA ARTICLE systems allow the production of different valuable species with less amount of consumable and reduce the negative environmental impact.

Therefore, production systems based on IMTA model favors the responsible use of natural resources and a sustainable productivity.

Innovative application of aquaculture sub-products

Even when an IMTA model is applied, aquaculture still generates different types of sub-products which may be re-used in diverse ways depending on their characteristics.

“To maximize the throughput of this industry, a hierarchic model shall be applied to all sub-products.”

Primary aquaculture products are those obtained as part of the main production process while sub-products are those secondarily obtained and can be directly utilized if they comply legal requirements.

The reutilization of aquaculture sub-products permits to recover ingredients with high economical value for other industries.

Human food ingredients

Several compounds widely used in food industry can be obtained from sub-products, for example, fish flour, chitosan, proteins concentrated, collagen, gelatin, and astaxanthin.

These by-products may represent a sustainable and innovative source of high-quality type I collagen for its further use in biomedical, cosmetic, or nutraceutical fields. Indeed, protein represents a useful ingredient for many industries since it is responsible for providing textural properties.

“Protein can be incorporated into food products in order to improve their organoleptic features.”

Regarding freshwater production, rainbow trout (Oncorhynchus mykiss) frames have been treated with a separation technology named electro-dialysis with filtration membrane.

“This technique allowed fractionating active peptides from complex hydrolysates yielding enriched fractions with peptides that showed antioxidant properties.”

Astaxanthin is the most common carotenoid obtained from aquaculture sub-products, particularly, salmon, trout, krill, shrimps, fresh water crabs and crustacean shells are the main sources for recovery.

Carotenoids and other colorants are useful food ingredients which are present in many products destined to human consume since they enhance their organoleptic characteristics by providing color but also additional properties.

Animal feeding

Different sub-products derived from aquaculture such as fish flour, ground shell, chitosan, astaxanthin, proteins concentrated and silage, that resulted from the liquefaction of the fish, can be incorporated to feed formulation for aquaculture animals, farm animals and pets.

Crushed shells represent an important calcium supplementation, very useful when introduced in hen feeding. The replacement of the calcium present in limestone with that from oyster shells has been proved to enhance egg production, strength weight and thickness.

“Chickens fed using oyster shells also showed a quicker increase of weight.”

For instance, the application of aquaculture by-products for designing animal feeding has several advantages, including the reduction of cost production and environmental impact of the aquaculture industries.

This synergy between aquaculture and livestock farming has special importance in Galicia since it reinforces two main economic activities in the region.

Agriculture

Although the re-use of aquaculture subproducts results more efficient in higher levels of the waste hierarchy, their employment in agriculture is also a potential destination to be considered.

“Historically, shells from mussel (Mytilus galloprovincialis) have been used as a liming agent or as mulches for soil amendment in farming in Galicia.”

In fact, the agricultural application of shells represents the second major shell market. The use of this natural product allows their application in ecological agriculture and represents a replacement for mined-CaCO3.

Industrial uses: food packaging, cosmetic and pharmaceutical

For food packaging, cosmetic and pharmaceutical industries, marine protein based products such as collagen or gelatin, lipids and pigments result very useful.

Moreover, food industry is boosting the development of biodegradable active packaging to reduce single-use plastics and improve shelf-life products.

“Freshwater cultured species, such as rainbow trout (Oncorhynchus mykiss), are a valuable source of active peptide hydrolysates and oils rich in PUFAs.”

Both beta-carotene and astaxanthin are very common marine carotenoids present in salmon, trout, krill, shrimps, and crustacean shells, but also in macro and microalgae. Pigments represent sources of colors that additionally provide bioactivities.

Astaxanthin has been reported to have antioxidant properties, stimulate immune system, prevents diabetes, cardiovascular and neurodegenerative diseases. In cosmetics, it has been used in skin care and anti-aging formulations.

“In addition to the pigments, macroalgae represents a sustainable source of biodegradable and non-toxic natural bioactive compounds.”

Many micro and macroalgae molecules have moisturizing, anti-aging, lightning and/or photoprotective properties and then have been applied for sunscreen creams, peeling, slimming, hair and dental care products.

Biodiesel and other uses

This combustible alternative represents a green source of energy for two reasons: in first place for the reduction of waste production and in second place because biodiesel is biodegradable, so it produces less air toxins and lower amounts of CO2 than other hydrocarbon-based fuel or diesel.

“Wastes capable of yielding oil, such as skin, fishbone or liver, are the most suitable ones for obtaining biodiesel.”

Aquaculture wastes and underused sub-products can provide an alternative substrate for producing single-cell protein (SCP). This sustainable production of SCP can return to the aquaculture company that provided wastes as a fishmeal ingredient.

Other potential applications of aquaculture sub-products include the use of shells as constructing materials.

Future trends and conclusions

In order to achieve a more efficient production system, different prototypes to integrate multiple trophic levels (IMTA) were implanted in Galicia. However, even this efficient model can generate wastes and sub-products.

“In this scenario, a circular bio-economy model should be adopted to re-utilize wastes and subproducts and maximize their throughput while reducing their negative environmental impact.”

Nevertheless, the recent implementation of ‘circular economy’, ‘bio-economy’ or ‘circular bio-economy’ strategies presents few drawbacks as the time and cost-consuming processes that delay the approval of new products derived from these production systems.

Therefore, even though these innovative and sustainable models have been demonstrated to be efficient, they still require visibility and stronger support.

This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “AQUACULTURE AS A CIRCULAR BIO-ECONOMY MODEL WITH GALICIA AS A STUDY CASE: HOW TO TRANSFORM WASTE INTO REVALORIZED BY-PRODUCTS”, developed by: M. FRAGA-CORRAL – Universidade de Vigo, P. RONZA – Universidad de Santiago de Compostela, P. GARCIA-OLIVEIRA – Universidade de Vigo – Centro de Investigaçaô de Montanha, A.G. PEREIRA – Universidade de Vigo – Centro de Investigaçaô de Montanha, A.P. LOSADA – Universidad de Santiago de Compostela, M.A. PRIETO – Universidade de Vigo – Centro de Investigaçaô de Montanha, M.I. QUIROGA – Universidad de Santiago de Compostela, J. SIMAL-GANDARA – Universidade de Vigo”.
The original article was published on NOVEMBER, 2021 through ELSEVIER. under the use of a creative commons open access license. The full version can be accessed freely online through this link: https://doi.org/10.1016/j.tifs.2021.11.026

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