Dry air consists of approximately 21 percent oxygen, 78 percent nitrogen and one percent argon. In addition to these gases there is also carbon dioxide at a concentration of 0.04 percent, which despite its low level is physiologically important for all living organisms.

1. January | February 2012
Feature title: Oxygenation in aquaculture
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The International magazine for the aquaculture feed industry

2. FEATURE FEATURE
Monitoring of until a stable value is established. In the case of a
oxygenation sensor mounted at a stable point, the measure-
Measurement of ments are of little value, in the case of standing
oxygen concentration waters or a container of water with low current.
in water usually takes The Clark type of oxygen sensor requires a
place by use of an oxy- current of at least five cm/s to function properly.
gen electrode, which In the case of currents in cages as shown in Table
was developed by Prof. three, this value is not so easy to achieve, and
Leland Clark in 1956. stirring is necessary. To sum up, in the case of
This basically measures manual measurement, this type of electrode can
Oxygenation in aquaculture Figure 2. A schematic diagram showing the respiratory
an electric current, which
is based on the reduction
function well, whereas if the sensor is mounted at
a fixed point, the issue of current speed becomes
pump. Water entering the mouth is further led by suction of oxygen at the cathode: an important issue.
by Pavlos Makridis, Nils Hovden and Martin Gausen, Storvic Ltd, Scotland, Uk to the gill cavity and passes thereby through the gills. O2 + 2H+ + 4e- The Clark-type of oxygen sensor - electrode
Opening and closing of the mouth and the opercular valve 2 0H- requires frequent replacement of electrolyte and
ensure that water flows in one direction.
Whereas at the membrane, and frequent calibration. A relatively
D
is utilised for the production of energy, it is fully anode electrode sil-
ry air consists of approxi- Oxygen content in the the pattern of flow of the blood, such that blood oxidised and the final products are: energy, carbon ver fells out of solu-
mately 21 percent oxygen, water is influenced by just before it leaves the gill lamellae is in contact dioxide and water. tion:
78 percent nitrogen and one temperature, salinity with highly oxygenated water (see figure), and it All these processes are included in the term of Ag Ag+ + e-
percent argon. In addition to Temperature has a major impact in relation to is possible to increase further its oxygen content. metabolism. The rate of metabolism is influenced The sensitivity of
these gases there is also carbon dioxide oxygenation of fish or other animals: on one hand There are two basic mechanisms to achieve a by a large array of abiotic and biotic factors (see this type of oxygen
at a concentration of 0.04 percent, which the metabolic rate of the animals increases (as flow of water over the gill surface. The first mecha- Table 2). From all these factors it should be sensor depends on
despite its low level is physiologically long as the increase in temperature is within the nism is the respiratory pump composed by the underlined here that that activity is the most potent the area of the cath-
important for all living organisms. tolerance limits of the cultured animal), and on the mouth cavity and the opercular cavity. The mode factor. Oxygen consumption is proportional to ode and thickness
other hand the solubility of oxygen in water gets of action of the respiratory pump in fish is not the metabolic rate, and it is therefore a common of the membrane
In comparison with air, oxygen content lower. In other words, at higher temperature, the continuous, but takes place in pulses composed of approach to measure metabolic rate by measuring of the sensor, which
in water bodies, which are in equilibrium demand for oxygen gets higher, while the oxygen two phases. The respiratory pump consists of two oxygen consumption. may limit the diffu-
with the atmosphere, is considerably lower. available decreases. compartments: the mouth cavity (buccal cavity) Temperature has a strong impact on oxygen sion of oxygen to the
There is a variability of Another factor that reduces and the gill cavity (opercular cavity). requirements as it affects the activity of enzymatic cathode. It becomes
solubility of atmospheric the solubility of oxygen in water in The gills separate these two compartments. So processes. Besides the enzymatic processes, tem- evident from the
gases in water. Oxygen is Table 1: Solubility addition to increased temperature water passing from mouth to gill cavity has to pass perature has an effect on the ability of hemoglobin equations above
coefficient of oxygen in
about twice as soluble as water expressed as mL
is the presence of dissolved salts. through the gills. to bind oxygen and the solubility of membranes. that the sensor in
nitrogen, but carbon dioxide per liter per mm Hg as a The presence of salt ions reduces Oxygen requirements are influenced by species Another important effect of temperature on one way consumes
is in its turn 30 times more function of salinity (ppt) the ability of gases to dissolve in cultured, temperature, fish size and feeding regime. metabolism is related to the amount of water oxygen, which is the
soluble than oxygen. The and temperature at extreme water. Oxygen is therefore less Fish, like other animals, consume food and break bound by proteins. Water molecules are bound parameter it actually
concentration of oxygen in temperature and salinity to soluble in seawater compared it down to more simple compounds. The dual to polar groups in the protein molecule, and the measures.
demonstrate the effect of the
water and air is 0.007 liter/ two factors.
with freshwater. As shown in purpose of metabolism is thus the gain of energy amount of water is influenced by temperature. To circumvent
liter and 0.209 liter/liter, Table 1, temperature has a much (catabolism) and the build-up of tissues (anabolism) The effect of temperature is normally described this practical problem,
5 ppt 35 ppt
respectively. This means stronger effect on oxygen solubil- by polymerisation of more simple compounds, by a Q10 value, which expresses the multiplication the sensor should be
that the oxygen content in ity than salinity, as at high tem- which becomes visible in the form of growth. factor when temperature is increased by 10oC. in motion in relation
water is approximately 30 5oC 54.7 44.9 perature solubility decreases by The anabolic processes besides building stones Q10 receives a value between two and three in to the water. In prac-
times lower than oxygen in 25oC 36.4 30.7 more than 30 percent, whereas require as well energy. Production of energy takes most cases. tical terms, this means
an equal volume of air. at high salinity solubility decreases place through oxidation and requires in the case When calculating the need for oxygena- that if the measure-
Besides oxygen concentration, two more by 16-18 percent. We noted before that solubility of fish the presence of oxygen which is extracted tion it is important to know the average ment is taken manu-
factors are physiologically important in rela- of gases is influenced by the solids dissolved in it. from the water surrounding the fish, and acquired size of the fish comprising the population in ally by a technician,
tion to physical constraints of gases and It is important to underline here, that the through the gills as described earlier. If a substrate question. As a general rule, per kg of biomass, this person should
ultimately the way land-living animals and solubility of each gas is not influenced by the smaller fish require much higher quantities of shake smoothly the
aquatic animals have evolved to obtain other gases dissolved in the water within oxygen than larger fish. sensor in the water
oxygen in their respective environments: physiological limits. This means that for exam-
density of medium and diffusion. Air is the ple the solubility of oxygen is not directly
medium for land-living animals and it is influenced by the amount of carbon dioxide
about 800 times lighter than water. In addi- dissolved in it.
Working together –
tion oxygen diffuses much faster from air
to living tissues in comparison with oxygen Respiration in fish
dissolved in water. In fish, gills are the respiratory organs. The
Analysis of all these factors makes it gills are highly perforated with thin blood
clear that respiration is a much easier task capillaries, which get loaded with as much
for land-living animals than for aquatic oxygen as possible from the water. The gills
Figure 1. Schematic diagram showing
the blood flow in secondary gill lamellae
which are the actual site of gas exchange
For a better tomorrow
animals. The only disadvantage of air- are enclosed in the gill cavity. The anatomical in fish. Water flows in the opposite
breathing animals in relation to breathing arrangement of the gills is such that blood direction than blood optimizing the
extraction of oxygen from water to the Storvik Aqua AS Storvik LTD Storvik SA
is the loss of water during breathing, which flows in the gill lamellae in the opposite direc- blood in this counter current pattern of 6600 Sunndalsøra, Norway Lochgilphead, Scotland Puerto Montt, Chile
is not an issue of course in the case of tion than the flow water. The counter-current flow. Tel: +47 71699500 Tel: +44 (0) 1546603989 Tel: +56 65290305 Certified according to
storvik@storvik.no info@storvik.co.uk gerencia@storvik.cl NS-EN ISO 9001
aquatic animals. principle is therefore applied which results in
30 | InternatIonal AquAFeed | January-February 2012 January-February 2012 | InternatIonal AquAFeed | 31

3. FEATURE
new technology has been developed based on the ration of volume to external sur-
Fatten up your bottom line. Bühler high-performance animal and aqua feed production
presence of a fluorescent compound in the sensor. face decreases.
This methodology does circumvents several of This means that the content of systems are used by leading companies around the world. These producers know they
the technical disadvantages of the previous method large bubbles has fewer chances
as it does not consume oxygen and stirring is to dissolve in the water that the can rely not just on the technology itself, but also on the support that accompanies it. A
therefore not necessary. These optic oxygen sen- same amount of gas in small
sors are more expensive to purchase, but on the bubbles. It is needless to point service combining local presence with global expertise both lowers feed mill operating
other hand have a lower maintenance cost. out here that both in the case of
oxygenation in tanks and in cages
costs and increases capacity utilization. To find out more, visit www.buhlergroup.com
Factors that may be influenced oxygen that reaches the surface
by insufficient oxygenation of water and burst is a loss for Figure 3. Dissolved oxygen concentration outside
It has been documented that the single the farmer as it enters the atmos- and within a cage. The difference in oxygen level
most important factor for increased growth and phere and is of no use for the fish indicated an exchange rate between about 3-6
times/hour.
productivity in aquaculture is to maintain sufficient farmed. If you thereby oxygenate Visit us at Victam Asia 2012 in Bangkok, Thailand,
oxygen saturation level over time in the water your farm and notice the water
where the species grow. At saturation level below “boiling” due to gas injected in the water, you by the bubbles and thereby more time to booth A071 (15 - 17 February 2012)
85 percent, feed utilisation begins to fall and the should take it as warning that large amounts of achieve oxygenation over the water masses.
fish is increasingly vulnerable to sickness and, in gas are getting wasted.
the end, mortality: Another disadvantage of large bubbles Cages
- at 75% saturation reduced appetite starts to is that they rise fast in the water column to There is a widespread belief among farmers Bühler AG, Feed & Biomass, CH-9240 Uzwil, Switzerland, T +41 71 955 11 11, F +41 71 955 28 96
appear reach the surface and thereby remain for a that oxygen demands of fish farmed in cages fu.buz@buhlergroup.com, www.buhlergroup.com
- at 60% saturation increased mortality is shown reduced time in the water reducing further at sea are under all situations covered by the
- at 40% saturation there is no appetite among fish the ability of oxygen to dissolve. A further currents existing at sea. It is easy however to
- at 30% saturation there is massive mortality. disadvantage of large bubbles is that large determine at first that oxygen concentration
Feed is composed of bubbles show a tendency inside the cages is lower than the oxygen
three main groups of food- Table 2: Factors that influence to “merge” and thereby concentration a few meters outside the cage
stuffs: protein, fat, and carbo- metabolic rate and consequently becoming even larger (Figure 3). This difference is powered by two
the oxygen requirements in fish.
hydrates. The amount of oxy- increasing the problem. factors: (a) the consumption of oxygen within
gen needed to metabolize a From the description the cage, and (b) the ability of the current to
gram of food differs for these Abiotic factors Biotic factors above it becomes clear that replace the depleted oxygen with the fresh
three groups of foodstuffs. Fat Temperature Activity level ideal oxygenation involves supplied brought by water rich in oxygen. The
gives more than double the Salinity Weight
the formation of small current in the area of the cages is much lower
energy released during the bubbles, which rise slowly than the current outside the cages (Table 3).
Oxygen Oxygen debt
catabolism of protein and car- in the water column, and It is obvious that the work of the currents
bohydrates and at the same Ammonia Stress result in efficient oxygena- is hindered in the case of cages placed in the
time requires a proportionally Acidity Starvation tion of the water as a maxi- sea by the net surrounding the cage. This net
increased amount of oxygen Season Quality of feed mal interface of gas-liquid in the case of most farms in the Mediterranean
to achieve this process. is provided. These tiny gas is double to hinder the escape of fish. In addi-
It is logical to assume that fish consuming a bubbles give a “milky” appearance to the water. tion, the size of fish is smaller than for example
fatty diet will have higher oxygen requirements A second important factor is the distribution is the case in salmon farming, so mesh size is
compared with fish consuming a larger propor- of the gas in the cage or the tank. In the case on average smaller than in salmon farming. An
tion of carbohydrates. It has been of circular tanks, these are not so deep so the additional problem that arises is the fouling
postulated that decreased oxy- bubbles have a short distance to cover before of nets with micro- and macroalgae which
gen levels may have an impact they get dissolved so the need for small bubbles reduces considerably the renewal of water and
on resistance of fish to infectious is quite high. On the other hand, as the water is causes further problem in the cages.
diseases (viral and bacterial dis- well mixed compared with other systems a few In the case of farming of gilthead seabream
eases), as in the case of channel catfish, Atlantic areas of gas injection are sufficient to provide fish and seabass, production is such that there is a
salmon and other species. Increased infestation of the necessary oxygen. peak of the total biomass towards late summer
parasites has also been observed. In earth ponds or in raceways it is impor- and autumn. The large biomasses in the on-
tant to inject oxygen in the area close to the growing cages results in increased demand for
Basic principles in oxygenation entrance of the raceway so oxygen has higher oxygen, where addition of oxygen in the cages
Injection of a gas in aquaculture is governed chances to be utilised by the fish population. by natural currents may not be sufficient, as the
by certain principles which will be described here In the case of fish cages, oxygen has to be temperature is still quite high in autumn.
in order to make easy to perceive the limitations distributed over a large area. The gas can This type of oxygenation may be applied
and possibilities related to oxygenation. be injected at a larger depth than is usual in either after manual registration of low oxygen
An important factor that influences efficient the case of tanks or ponds, so there is more concentration in the cage or after continuous
injection of a gas in water is the size of bubbles distance to be covered in the water column monitoring by an automatic system. An auto-
as they exit the diffuser. Small matic system for monitoring
size show several advantages Table 3: Parallel monitoring of current velocity at surface outside a farm and of oxygen level in the cages
in related to larger ones. If gas inside a cage. ensures that low levels at any
is divided to small bubbles the time of the day or night will
contact surface with water is result in an alarm procedure,
Current outside farm (cm/s) 2-4 4-6 6-8 8-10 10-12
much higher than in the case
of large bubbles. In a sphere,
Current inside farm (cm/s) 1.7 1.7 2.1 2.2 2.2
able to result in addition of
oxygen within a reasonable
Innovations for a better world.
as the diameter increases the Reduction of current (%) 44 67 70 76 80 time period. ■
32 | InternatIonal AquAFeed | January-February 2012

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