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NUTRITION AND EFFLUENTS IN AQUACULTURE

By: Dr Mustapha ABA, Aquaculture Researcher, Fish Nutrition. Morocco.

The environmental impact caused by effluents generated by aquaculture production is variable and depends on the type of production system, the species used, the production density, and the type of feed. Therefore, the efficiency of feed use and input supply in aquaculture is among the most important factors determining profitability and environmental economic impacts, and is one of the major challenges for fish nutrition researchers.

The formulation of low-pollutant feeds has long been the subject of discussion within the scientific community and aquaculture companies. Their composition must allow the fish to meet their nutritional requirements and the degree of impact of these nutritional compounds on the environment must be as low as possible. This article outlines the importance of nutrition in aquaculture in order to reduce environmental impact.

In aquaculture, excessive excretion of phosphorus and nitrogen has direct negative consequences because, as excretion takes place in water, the elimination of these nutrients is practically impossible, unlike land animals whose excretion of faeces and urine from the environment is already sufficient to alleviate problems caused by waste. In this form, measures to reduce the discharge of nutrients into fish ponds have a greater impact on production, since poor water quality is one of the main factors that can lead to significant production losses and reduced fish growth, in addition to generating an effluent that will lead to eutrophication of the natural environment.

Reducing the volume of effluent is the most effective mechanism for saving water resources, not only by reducing water consumption, but also by reducing the polluting potential of aquaculture. Many aquaculture stakeholders believe that the application of good practices, which are used to prevent water pollution and its negative environmental impacts, could be a reasonable and affordable way to improve the quality and reduce the volume of aquaculture effluents. However, the most beneficial approach would probably be better environmental education of aquaculture producers, showing that the environmental benefits of Good Practices translate into higher profits.

In addition, aquaculture is also hampered by the contamination of water bodies with toxic wastes from agriculture, livestock, illegal sewage, domestic or industrial pollution and other forms of pollution due to anthropogenic factors.

Thus, in addition to providing year-round fish supply and income for producers, aquaculture has the potential to reduce pressure on wild fish populations in extractive fisheries, can provide environmental services in the form of wastewater treatment, bioremediation, and even rebuilding wild populations through specific reintroductions of endangered aquatic animals.

Intensive systems normally tend to generate a greater amount of more concentrated effluent, resulting in a greater impact, compared to extensive or semi-intensive systems. This is because they use high population densities, high nutrient densities complete feeds, constant feeds and high water turnover rates and quantities.

The main components, which generate an environmental impact, are solid particles, composed of food remains, excrement and dead animals, and water-soluble inorganic and organic compounds, mainly phosphorus and nitrogen, which are discharged directly into the natural environment.

In general, the ingredients used in aquaculture feeds are the same as those used to make up the feeds of other animals. Most ingredients are refined and processed products, co-products or by-products of extractive fishing, slaughterhouses or agriculture. The choice of an ingredient should be based not only on its nutritional efficiency, digestibility and cost, but also on other criteria such as sustainability, the environmental impact of production, and its “fish-in – fish-out” ratio, which is a metric unit used to show how many units of wild fish are needed to produce one unit of captive fish.

In addition, aquaculture is also hampered by the contamination of water bodies with toxic wastes from agriculture, livestock, illegal sewage, domestic or industrial pollution and other forms of pollution due to anthropogenic factors.

Thus, in addition to providing year-round fish supply and income for producers, aquaculture has the potential to reduce pressure on wild fish populations in extractive fisheries, can provide environmental services in the form of wastewater treatment, bioremediation, and even rebuilding wild populations through specific reintroductions of endangered aquatic animals.

Intensive systems normally tend to generate a greater amount of more concentrated effluent, resulting in a greater impact, compared to extensive or semi-intensive systems. This is because they use high population densities, high nutrient densities complete feeds, constant feeds and high water turnover rates and quantities.

The main components, which generate an environmental impact, are solid particles, composed of food remains, excrement and dead animals, and water-soluble inorganic and organic compounds, mainly phosphorus and nitrogen, which are discharged directly into the natural environment.

In general, the ingredients used in aquaculture feeds are the same as those used to make up the feeds of other animals. Most ingredients are refined and processed products, co-products or by-products of extractive fishing, slaughterhouses or agriculture. The choice of an ingredient should be based not only on its nutritional efficiency, digestibility and cost, but also on other criteria such as sustainability, the environmental impact of production, and its “fish-in – fish-out” ratio, which is a metric unit used to show how many units of wild fish are needed to produce one unit of captive fish.

Fishmeal and fish oil, mainly from extractive fisheries, are the two main ingredients and the main aquatic sources of protein and lipids available on the feed market. Fishmeal is the main source of protein used in the feeding of aquatic organisms, up to 50% of the total feed. It is used in animal feed because of its high protein concentration and excellent profile in amino acids, calcium, phosphorus and other minerals, its high digestibility and normally free of anti-nutritional factors.

Although most fish oil is used in the aquaculture industry (75%), it is increasingly being used for direct human consumption, mainly as a substitute for mineral oil or for the treatment of diseases. In addition, stagnating rates of extractive fishing, combined with the growth in global fish consumption and the growth of the feed industry to support the increased demand for aquaculture, is leading to the search for alternative sources of protein and energy that do not limit the growth of the aquaculture sector.

For example, the replacement of fishmeal with sustainable and environmentally friendly vegetable protein sources is a strong trend in aquaculture feed that has been adopted for different species. Furthermore, it is a way to reduce feed costs, since protein is the nutrient of increasing diet costs.

Another important point to consider is that the low use of phytic phosphorus from fish may have an impact on the environment, as additional inorganic phosphorus may be required to meet the needs of the species used, resulting in increased excretion of phosphorus, in the form of phytic acid, from faeces. This excess nutrient can harm the environment if not treated properly. The main measures adopted to improve the use of phosphorus from plant foods by monogastric animals are :

(a) the inclusion of specific enzymes, which hydrolyse the phosphate group of the Inositol molecule, making it available to animals

b) the use of genetically modified plants, which store most of the phosphorus in their seeds in inorganic form, i.e. readily available to monogastric animals.

In this context, particular attention should be paid to the importance of using highly digestible ingredients in the formulation of feedingstuffs, which is essential in order to know not only their true nutritional value, but also the contribution of faeces to the environment resulting from their inclusion, which is therefore fundamental for the development of nutritionally, economically and environmentally healthy food.

Other nutrients, nutraceuticals and additives may contribute to a better utilisation of the nutritional fractions of the diet and, consequently, to the health and hygiene of the animals reared, as well as to the quality of the product after slaughter. These are: vitamins and minerals; antioxidants; binders; pigments; enzymes; organic acids; attractants, palatabilisers and appetite stimulants; immunostimulants; pre and probiotics.

In this complex scenario, dominated by the interdependence of biotic and abiotic factors, nutrition in aquaculture becomes a valuable tool in the search for sustainable aquaculture production, combining high productive performance with lower environmental impact. The use of “ecological” rations, highly digestible and meeting the requirements of animal production and water recycling, without excess organic or inorganic phosphorus, makes it possible to reduce the concentration of effluents and limit subsequent changes to the environment. In addition, the use of an aquaculture effluent treatment system is an alternative to reduce the concentration of these nutrients in the effluent.

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