Viewed from space Earth really is a blue planet: more than two thirds of its surface are covered by water. The oceans’ huge dimensions should not, however, hide the fact that the amount of drinkable freshwater is in comparison negligible, or that most of this quantity is still held in the ice of the glaciers and polar caps. Rivers, lakes and groundwater together account for only 0.032 per cent of the total global water volume! And: this limited resource is not only needed for drinking, cooking and hygiene purposes but also during agricultural and industrial production processes. If the requirements of all sectors are added together to form “virtual water consumption” every European uses an average of at least 4,000 litres of water – every day. The global average “water footprint” per capita and year is 1,240 cubic metres although, as is to be expected, there are large differences between the different regions of the world and, indeed, often even between comparable industrialized nations. While water consumption in Germany averages 1,545 cubic metres per capita and year (which corresponds to more than 10,000 bathtubs full) it is about twice as high in the USA. Even very water-rich nations such as Brazil can have problems with water supply in some regions, especially since the available drinking water resources are often polluted with pesticides and other contaminants. There is thus an urgent need to save water worldwide. The pressure to use water-saving technologies also affects aquaculture, which accounts for a growing share of high-quality animal protein supply to mankind. According to the FAO, 82.1 million tonnes of fish, shellfish and crustaceans were produced worldwide in aquaculture in 2018. 51.3 million tonnes of this total were produced in freshwater and only 30.8 million tonnes in brackish water and marine environments. Although this imbalance has long been known and criticised the share of production in inland waters has actually increased over the last thirty years. From 1986 to 1995 an average of 57.7% of production was produced in freshwater, and today this figure is almost five percentage points higher. Although advances in engineering and technology would easily allow more fish to be produced in brackish and marine waters inland aquaculture is tending to boom because it is more accessible and cheaper. Until this changes fundamentally strict regulations will have to be in place to ensure that the natural aquatic resources of inland waters are used as sparingly and sustainably as possible.
Nutrient input to water bodies is a problem
A major source of water pollution from aquaculture is inputs of nitrogen compounds, especially nitrates. Nitrogen is an important nutrient for all living beings but too much nitrogen places a burden on both terrestrial and aquatic ecosystems. Excessive nitrate input to surface waters (eutrophication) can lead to mass reproduction of algae and cause fish mortality. Nitrate is just as harmful in groundwater. If drinking water is obtained from groundwater nitrate has to be removed during treatment at great expense. Otherwise there is a risk that it will be converted in the human body into potentially harmful substances such as nitrite and nitrosamines. The actual water requirements of aquaculture facilities vary depending on their location and the production method used. Most aquaculture is carried out in pond or flow-through systems but also partially in open net enclosures and closed systems, some of which make use of integrated water purification technologies. The water comes from a variety of sources, but mainly surface waters such as rivers and canals or precipitation. In many places groundwater is pumped to the surface, but it often requires extensive treatment before it can be used because its quality does not meet the requirements of aquaculture. High iron, sulphur and CO2 levels, in particular, cause problems. Water withdrawal from running waters, or temporary storage in fish ponds, affects the water balance of natural waters and can have an effect on their ecosystems as well as on other economic usage forms.
Evaporation and seepage lead to water loss
In the context of aquaculture, the term “water consumption” is not very appropriate because, strictly speaking, the water is not “consumed” but only used temporarily as a habitat and medium for the fish. Although contamination of the water with organic and inorganic nutrients could be significantly reduced by regular water exchange many operators go to great effort to save water and reduce water pollution. However, as a farm’s water requirements decrease, the technical effort and energy costs increase to the same extent. Compared to agricultural production, aquaculture is a very water-efficient method of producing animal protein. Nevertheless, some water loss is unavoidable. With every tonne of fish harvested about 760 litres of water are removed from the system. And indirect losses due to evaporation and seepage of water in the soil, as well as the amount of water needed to produce the fish feed, have to be added to this sum. Evaporation losses depend on the pond surface and temperatures, wind movement and the topographical conditions of the terrain. In extreme cases they can be as high as 6.3 mm per day which corresponds to daily losses of 63 cubic metres per hectare of pond surface. Water losses through seepage are mainly determined by soil properties, with clay soils performing much better than silt and sandy ground. Concrete or foil linings reduce seepage losses from ponds but they are comparatively expensive, which probably makes such effort more appropriate in small ponds. Water consumption in aquaculture can already be reduced through site selection. Hilly terrain and dense tree population reduce evaporation losses due to sun and wind. However, the trees must not be too close to the water, otherwise their roots will absorb additional water and evapotranspiration will occur. This also applies to reed and cane growth in the shore area. On the one hand, reed belts are helpful and useful because they shade the water and thus reduce evaporation and also provide fish and water birds with ecologically valuable habitats. On the other hand, vegetation increases water loss through transpiration and makes many pond management tasks more difficult and so must always be kept short to prevent the ponds from becoming overgrown. Another way to limit evaporation losses is to deepen the culture ponds which changes the ratio of the surface area to the pond volume. On average, ponds are about 1 to 1.5 metres deep. Due to the low water level the ponds warm up more quickly in spring which prolongs the growth period of the fish kept there. However, if the pond is deepened there is a risk that stable stratifications will form in the water in summer and that oxygen will be lacking below a thermocline at the bottom. This situation is particularly likely to occur under arid climatic conditions and in regions with rare rainfall. This means that ponds there must be somewhat deeper so that there is enough water available for the fish to survive in the dry, highly evaporative period. In order to limit evaporation losses farm operators sometimes try to cover the surface of smaller ponds with mats or films made of floating materials such as polystyrene, plastic or lightweight concrete. This can limit evaporation by 80 to 90 per cent but it has some disadvantages. The lack of evaporation heats up the water considerably, which can damage sensitive water organisms and also reduce the oxygen concentration. If the cover does not allow much light to pass through, photosynthesis in the aquatic plants is correspondingly weaker and this further increases the lack of oxygen in the water. The same effect can also be the result of algae blooms in the pond which mainly occur in nutrient rich water. The microalgae absorb sunlight, which heats up the water and increases evaporation. For this reason alone, many aquaculture companies try to remove the nutrients produced as effectively as possible from the water, especially nitrogen compounds. Within the framework of the EUfunded DeammRecirc project, African and European partners are, for example, developing new water purification technologies in order to be able to reuse the water on fish farms and reduce the costs of recirculation aquaculture systems (RAS). The project participants have adapted a “deammonification” technology from general wastewater treatment to the special needs of RAS aquaculture and in this way convert environmentally harmful nitrates into harmless nitrogen gas that is released into the atmosphere.
Closed recirculation systems reduce water requirements
A significant item in the water balance of aquaculture is the production of the formulated feed. The substitution of marine animal raw materials (fishmeal and fish oil) by vegetable raw materials from agriculture has further increased indirect water consumption in aquaculture. Fish or crustaceans require less than 2 kg of cereal concentrate to achieve one kilogram of growth, making them the most efficient animal products in terms of indirect water consumption. Nevertheless, water consumption remains high since about 1.2 m3 of water are needed worldwide to produce 1 kg of grain for animal feed. One way to reduce the relative water consumption per kg of product is to intensify aquaculture production. This development culminates in closed recirculating aquaculture systems (RAS). Within such systems the water is treated again and again for repeated use and so – with the exception of the indirect water requirement for feed – this is the most promising approach to the economical use of fresh water. Many experts believe that intensive aquaculture in general and RAS in particular have the greatest growth potential of all land-based aquaculture methods in terms of productivity, controllability and efficiency. It would be even more advantageous to shift aquaculture more towards the marine sector, especially to offshore locations because (compared to freshwater) seawater is available in sufficient quantities and many problems that can only be solved at high cost on land can be almost neglected in the sea. Nutrient inputs which are critical in rivers and lakes hardly cause any problems in offshore farms, already because of the high dilution. The water-saving technologies of closed systems are already used to varying degrees in pond management. An example of this can be found in Danish trout farms which are now almost always operated with partial recirculation systems and this has reduced water requirements significantly. The high fish densities typical of all recirculating systems present fish farmers with special challenges. The systems must guarantee sufficient oxygen supply for both the fish and the aerobic processes involved in water treatment (nitrification) and they must ensure that the ammonia/ ammonium compounds excreted by the fish do not exceed critical levels and are rapidly converted into non-toxic or less toxic substances. In addition to RAS as the “archetype” of water-saving production technologies, systems derived from RAS are also gaining in importance today with integrated multitrophic aquacultures (IMTA) and aquaponics systems. With these systems, an attempt is made to go beyond the water-saving approach and to make good use of the nutrients produced.
Integrated aquaculture systems offer certain advantages
In IMTA, production species of different trophic levels are combined with each other in order to use nutrients several times. Typical examples of these systems are fish farms surrounded by cultures with detritivorous and filtering species such as crabs and mussels which feed on the particulate organic substances that are found in food residues and fish excrements. Algae cultures are used to remove the dissolved inorganic compounds (nitrates) from the water and convert them into biomass. The attempt to partially bind the nutrients emitted from fish production, to reduce environmental pollution and to produce additional food is ecologically shrewd but, from an economic point of view, certainly controversial. This also applies to aquaponics facilities in which the dissolved nutrients excreted by the fish serve as fertiliser for plants. The combination of fish and plant production enables more nutrient- efficient fish and vegetable production but there are doubts as to whether this integration is actually more worthwhile than separate production that can be much more specifically targeted to the different needs of the target organisms. The water-saving effect of the plants is undisputed, however. In condensation traps even the water vapour transpiring from the leaves of plants can condense and be reused for aquaculture. In the “Tomato Fish” project, for example, scientists from the Berlin Leibniz Institute of Freshwater Ecology and Inland Fisheries have developed an aquaponics system that uses just 220 litres of water to produce 1 kg of fish and 1.6 kg of tomatoes. With conventional production systems, a good 600 to 1,000 litres of water would be needed to produce the same quantities. Basically, the idea of combining the production of organisms of different trophic levels is not new. Such integrated methods based on resource-efficient nutrient cycles have been used in Southeast Asia for several centuries. Integrated agrarian aquacultures (IAA) combine plant production in fields and gardens with agricultural animal husbandry and pond aquacultures. In many places IAA even represent an important livelihood for poorer populations because they guarantee regional food supply. However, under the conditions of a globalised world, climate change, limited water availability and growing dependence on industrially produced aquafeed traditional integrated systems will have to be permanently developed and adapted to the growing challenges. Water-saving production systems are an indispensable prerequisite for future economic success. This affects feed manufacturers, in particular, since the industry's dependence on high-quality feed will continue to increase in the future. Although alternative raw materials such as microalgae and seaweed, single cell proteins, insect meal, mussels, and waste from food processing are being used increasingly, the growing demand for aquafeed alone is likely to increase water requirements. The effects of climate change on aquaculture are already apparent and will continue to increase in the future, as will the pressure to reduce water consumption even further and make aquaculture production more resilient and at the same time less dependent on climatic influences. The timely implementation of water-saving technologies could thus prove to be an important strategy for adapting to the expected climate changes.