Oxygen is a fundamental prerequisite for the life of the vast majority of organisms. Respiratory gas plays a part in many processes that take place in the cells and tissues of the body. These processes involve complex chemical reaction chains whose individual steps often require oxygen. A key process that supplies the body with vital energy for muscle and nerve activity, growth, and other tasks, is "biological oxidation", also called "internal respiration". During this process the substances from the food that an organism has absorbed gradually "burn" within enzymatically controlled reaction chains and store the released energy in a chemical compound (ATP). This process is similar to charging a battery that can release its energy later on as required. To keep biological oxidation going, however, there must always be enough oxygen available in the body cells. Because the cells inside the body do not have direct access to the oxygen in the air, all higher organisms develop special respiratory organs (e.g. lungs, trachea or in the case of fish, gills), via which respiratory air can enter the body and be transported and distributed by the blood stream. In contrast to the internal respiration of the body's cells, this oxygen intake, which occurs in fish mainly via the gills, is referred to as "external respiration”.
While terrestrial animals hardly have problems to gain sufficient oxygen from the air (the oxygen content in the atmosphere is about 20.95%), fishes have to expend much more energy on breathing. Water is 700 times denser and contains 30 times less oxygen than air. In order to ensure that there is a constant water current flowing past the gill filaments the fish has to use one third of its resting metabolic rate or, during strong swimming activity, even up to half, for breathing. Human beings seldom use more than 3%. Thus, a considerable part of the energy generated by the fish with the help of the oxygen it has absorbed is ultimately again used for breathing itself. In air breathers the ratio of ventilated air volume to the amount of blood that the heart pumps through the lungs is about 1 to 1. In fishes, it is approximately 16 to 1, however! The ventilation rate of the gill cover depends mainly on the species, its way of life, and the oxygen content of the water. When resting, a salmon moves its gill cover 60 to 70 times per minute but when swimming the frequency can increase to 150 / min. Flatfish that usually lie motionless on the seabed breathe only 30 to 40 times per minute. Fish that swim continuously such as mackerel and tuna have a comparatively small gill chamber. For them, the ventilation of the gill cover alone is not sufficient to supply their bodies with enough oxygen. If a mackerel is ecclosed in a small tank that does not have room for it to swim it will suffocate because even in 100% oxygen saturated water it can only enrich its blood with 11% oxygen. These species use the water current that flows past their gills for breathing. Other pelagic species, too, make use of this energy-saving technique as soon as their swimming velocity exceeds 35 cm / s.
The oxygen requirements of individual fish species differ significantly from one another because during the course of their evolution fishes have adapted to the conditions of their habitat. In general, one can assume that species such as salmon that live in fast-flowing, well-oxygenated waters also have high oxygen requirements. On the other hand, fishes living in sluggish or stagnant waters, such as cyprinids, usually get by with significantly less oxygen. But as often in the case of fishes there are exceptions, mainly in the tropics, where some species not only use their gills, but also other body structures with which they can absorb oxygen from the atmosphere, for breathing. They breathe through their skin surface, for example, or with the intestines, the swim bladder or special organs that are similar in structure and function to simple lungs.
Oxygen intake from the atmosphere and through photosynthesis
Some scientists suspect that fish perceive a decline in oxygen concentration in the water before it causes serious breathing problems. This offers them the chance to seek out water areas with more favourable oxygen conditions. If that is not possible, they try to compensate for the oxygen deficit through an increased stroke rate of the gill cover. However, that uses up more energy, reduces growth, weakens the fish and makes it more susceptible to diseases. That is why a needs-based supply of oxygen is one of the basic prerequisites for successful aquaculture. The oxygen consumption of fish is usually given in mg / kg body weight and hour. Since the value can vary greatly depending on the water temperature, the activity of the fish, feeding and other factors, one refers here to the basic needs of the resting fish, i.e. that amount of oxygen that the fish requires for maintaining its basic body functions.
Conventional fish farms with ponds or raceways are usually built in areas where large quantities of water of a suitable quality are available. This makes it possible to regulate the oxygen requirements of the fish via the water exchange, the influx of fresh and oxygenated water. This is relatively inexpensive and requires little technical effort but on the other hand it also means that the production capacity depends on the availability, quantity and quality of the water flowing through the facility. And even if sufficient water is available, the quantity flowing through a fish farm cannot simply be increased at will due to technical, economic and biological limitations. If the stocking rates are to be increased and more fish reared in the system it will be impossible to get around aerating the water, i.e. enriching the system with oxygen.
In natural waters, oxygen usually enters into the water in one of two ways. The first means is via the atmospheric air, about one fifth of which consists of oxygen, which weighs on the water surface. Depending on air pressure and temperature, salinity and the oxygen saturation level of the water and other factors different amounts of oxygen pass from the air into the water. In relatively warm water, for example, less oxygen is readily dissolved than in colder water. Shallow waters with a large surface area are normally better supplied with oxygen than deep ponds with a small surface area. It is also important that the contact between the water and the air is not too restricted by floating plants. The second important source of oxygen is aquatic plants that produce respiratory gas in the course of photosynthesis. Not only macrovegetation on the riverbed is of significance here but also phytoplankton. The roles played by these two photosynthetically active fractions in oxygen supply depend on the specifics of the water body (size, depth, stocking density), the light conditions (shade, water turbidity) and other factors such as the water temperature. Although aquatic plants can produce significant amounts of oxygen they are still of only limited use as a provider of oxygen for fish production. Their oxygen production varies during the day: it begins gradually at break of day, reaches its maximum in the early afternoon and then decreases again in the evening. When darkness falls photosynthesis stops completely. A similar rhythm is also evident as the year progresses. In the summer months, plenty of oxygen is produced, in the winter it falls to almost zero.
Technical aeration systems increase aquaculture production
The diffusion processes at the water surface depend on the partial pressure of the oxygen in the air and in the water. However, since the oxygen content of the air can be regarded as constant, the rate of diffusion at this interface is mainly determined by the oxygen saturation of the water. Even in still water, the upper layer of water is usually saturated quickly. The transport of oxygen into deeper water layers ("convection") is, however, much slower. The situation is much better in turbulent waters, in which the water is mixed to greater depths, for example, due to the effect of wind. Under such conditions oxygen input via diffusion from the atmosphere can reach about 1.5 g per square meter and day; in large ponds whose surfaces are exposed to wind it can be even twice as high.
Most mechanical aeration and oxygen supply systems used in aquaculture imitate this wind effect. They keep the water constantly in motion, prevent thermal stratification of the water body, and increase the respiratory active surface by swirling the water or breaking it into small droplets, throwing it into the air or trickling. Such effects are used for example in cascade aerators, Venturi nozzles, sprinklers, or devices which suck water up and then hurl it into the air as a fountain, or injectors. Paddlewheels are particularly common in fish farms for improving the oxygen balance. They are simple constructions whose efficiency can be controlled directly and they have two desirable effects: they aerate the water body and produce a current that provides substantially homogenous living conditions in all corners of the pond. Also popular are diffusers that emit compressed air into the water through fine pores in the form of very tiny bubbles. The smaller the bubbles, the more effective is the gas exchange as they rise in the water column because the ratio of surface area to volume is particularly favourable. These aeration systems are particularly suitable for deep ponds because the duration of water contact is longer there.
In intensively farmed aquaculture facilities aeration of the water is one of the basic prerequisites for trouble-free production without which the existing biological and economic opportunities cannot be satisfactorily exploited. In the same proportion as the intensity of production increases, oxygen requirements also increase because numerous biological processes and activities accelerate accordingly. In ponds with high stocking densities more feeding is necessary, for example, and so there is more fish excrement. Due to this fact alone the oxygen concentration in the water can fall by 20 to 30 percent during such phases. In these situations, a needs-based oxygen supply even has a life-sustaining function. Conventional aeration methods that are based on the mere addition of air quickly reach their limits because in addition to the 21% oxygen that the air contains it also contains other gases, in particular nitrogen. The absorptive capacity (solubility) of water for gases is, however, limited so that many intensive production facilities prefer to aerate their tanks, raceways or ponds not with air but with pure oxygen. When using pure oxygen the oxygen partial pressure and thus the natural saturation limit in the water is increased by a factor of 4.8 compared to aeration with mere air. This property is particularly advantageous in the case of high oxygen concentrations close to air saturation and at high water temperatures during the summer.
Pure oxygen optimizes feed conversion and growth rates
The addition of pure oxygen enables the production of larger quantities of fish in a comparatively small volume of water in both cold and warm water systems. The realizable fish density is only limited to a slight extent by the supply of oxygen. This allows even small companies with low water supply to keep considerably more biomass in the facility and makes it easier for them to assert themselves in the economic environment. Already by improving oxygen saturation in the incoming water from 90 to 100% allows an increase of aquaculture production by nearly one-third! The oxygen content also affects the feed conversion and growth rates; under optimal oxygen conditions the fish grow significantly better with the same feed consumption. However, the expected higher yields are offset by the necessary investments for oxygen storage, equipment and aeration devices as well as the ongoing costs of pure oxygen. Of course, conventional aeration systems that add simply air to the water also lead to costs for their acquisition and running expenses. Here it is in particular the operating costs that are significant because the pumps and drive motors for air compression and water circulation require a lot of energy, irrespective of whether they are electric, powered by diesel or other fuels. In comparison, the energy requirements for the addition of pure oxygen are usually much lower because, due to the strong concentration differences, the transfer of oxygen into the water is possible without too much pressure or other energy-intensive physical means. Pure oxygen is an industrial product today and it is commercially available as a gas or in liquid form. It is mostly liquid oxygen that is used in aquaculture and it is produced from air according to the Linde method in a very pure form by fractional distillation. The liquid oxygen is stored in tanks with vacuum insulation and "evaporated" again before use, whereby one litre of liquid oxygen gives approximately 853 litres of gaseous oxygen (at 15° C and 1 bar). Another possibility for oxygen supply is to use oxygen generators, which produce the respiratory gas directly on site with a molecular filter which removes the nitrogen from the air. These devices are only suitable for smaller plants however because their capacity is limited and the purity of the oxygen produced is rarely higher than 90%.
Special care must be taken when working with liquid oxygen because it is a very strong oxidizing agent, especially for organic materials. Oils and fats may ignite if they come into direct contact with it, and the health and life of the fish would be threatened. That is why liquid oxygen is first vaporized before application and then introduced into the oxygen-depleted water with the help of metering devices, thereby preventing the toxic effects of oversaturation. Oxygen supply is precisely controlled by a computer program during the day to make sure that enough oxygen is always there when it is needed, for example when demand peaks after feeding. Pure oxygen is also used in other areas, for example in emergency aeration systems to supply the fish with oxygen during a power failure or during live transport of animals.
In principle, the entry systems for pure oxygen work just the same as aerators for "normal" air. The longer the contact time between the water and gas, the higher the pressure in the water, and the smaller the oxygen bubbles, the more efficient the oxygen transfer. This principle can be found, for example, in Venturi-like tapered tubes in which pure oxygen is fed laterally dosed into the water stream. During passage through the ever narrower tubes the flow rate and the pressure in the water increase so that the oxygen is mixed evenly into it. The U-shaped tubes that often extend 20 or 30 metres deep into the bed work in a similar way. In these systems, too, the introduced oxygen gas bubbles are entrained by the water current and can mix well with the water during their passage through the U-tube. The key factor here is the depth of the tube, because the pressure of the water rises every 10 meters by one bar. In contrast, oxygen injectors often operate on the counter flow principle, i.e. water and oxygen flow towards each other from opposite directions within a closed system, and this causes the vigorous circulation of the two media. Although most injectors are relatively small the oxygen is optimally mixed into the water.
Particularly economical and popular are tent-like structures which float on the water and are aerated internally with pure oxygen. No oxygen is lost and it can diffuse directly into the water and is mostly additionally introduced into the water body by paddles.
The use of pure oxygen can reduce production risks of aquaculture facilities and increase their profitability. This requires, however, that appropriate technologies and proven metering devices are used. For pure oxygen is not a substitute for water, and "too much" can be harmful in this area, too.