oxygenation in aquaculture
TRANSCRIPT
International Aquafeed is published five times a year by Perendale Publishers Ltd of the United Kingdom.All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2012 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1464-0058
January | February 2012
Feature title: Oxygenation in aquaculture
The International magazine for the aquaculture feed industry
D ry air consists of approxi-mately 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.
In comparison with air, oxygen content in water bodies, which are in equilibrium with the atmosphere, is considerably lower.
There is a variability of solubility of atmospheric gases in water. Oxygen is about twice as soluble as nitrogen, but carbon dioxide is in its turn 30 times more soluble than oxygen. The concentration of oxygen in water and air is 0.007 liter/liter and 0.209 liter/liter, respectively. This means that the oxygen content in water is approximately 30 times lower than oxygen in an equal volume of air.
Besides oxygen concentration, two more factors are physiologically important in rela-tion to physical constraints of gases and ultimately the way land-living animals and aquatic animals have evolved to obtain oxygen in their respective environments: density of medium and diffusion. Air is the medium for land-living animals and it is about 800 times lighter than water. In addi-tion oxygen diffuses much faster from air to living tissues in comparison with oxygen dissolved in water.
Analysis of all these factors makes it clear that respiration is a much easier task for land-living animals than for aquatic animals. The only disadvantage of air-breathing animals in relation to breathing is the loss of water during breathing, which is not an issue of course in the case of aquatic animals.
Oxygen content in the water is influenced by temperature, salinity
Temperature has a major impact in relation to oxygenation of fish or other animals: on one hand the metabolic rate of the animals increases (as long as the increase in temperature is within the tolerance limits of the cultured animal), and on the other hand the solubility of oxygen in water gets lower. In other words, at higher temperature, the demand for oxygen gets higher, while the oxygen available decreases.
Another factor that reduces the solubility of oxygen in water in addition to increased temperature is the presence of dissolved salts. The presence of salt ions reduces the ability of gases to dissolve in water. Oxygen is therefore less soluble in seawater compared with freshwater. As shown in Table 1, temperature has a much stronger effect on oxygen solubil-ity than salinity, as at high tem-perature solubility decreases by more than 30 percent, whereas at high salinity solubility decreases
by 16-18 percent. We noted before that solubility of gases is influenced by the solids dissolved in it.
It is important to underline here, that the solubility of each gas is not influenced by the other gases dissolved in the water within physiological limits. This means that for exam-ple the solubility of oxygen is not directly influenced by the amount of carbon dioxide dissolved in it.
Respiration in fishIn fish, gills are the respiratory organs. The
gills are highly perforated with thin blood capillaries, which get loaded with as much oxygen as possible from the water. The gills are enclosed in the gill cavity. The anatomical arrangement of the gills is such that blood flows in the gill lamellae in the opposite direc-tion than the flow water. The counter-current principle is therefore applied which results in
the pattern of flow of the blood, such that blood just before it leaves the gill lamellae is in contact with highly oxygenated water (see figure), and it is possible to increase further its oxygen content.
There are two basic mechanisms to achieve a flow of water over the gill surface. The first mecha-nism is the respiratory pump composed by the mouth cavity and the opercular cavity. The mode of action of the respiratory pump in fish is not continuous, but takes place in pulses composed of two phases. The respiratory pump consists of two compartments: the mouth cavity (buccal cavity) and the gill cavity (opercular cavity).
The gills separate these two compartments. So water passing from mouth to gill cavity has to pass through the gills.
Oxygen requirements are influenced by species cultured, temperature, fish size and feeding regime. Fish, like other animals, consume food and break it down to more simple compounds. The dual purpose of metabolism is thus the gain of energy (catabolism) and the build-up of tissues (anabolism) by polymerisation of more simple compounds, which becomes visible in the form of growth.
The anabolic processes besides building stones require as well energy. Production of energy takes place through oxidation and requires in the case of fish the presence of oxygen which is extracted from the water surrounding the fish, and acquired through the gills as described earlier. If a substrate
Oxygenation in aquacultureby Pavlos Makridis, Nils Hovden and Martin Gausen, Storvic Ltd, Scotland, Uk
Figure 1. Schematic diagram showing the blood flow in secondary gill lamellae which are the actual site of gas exchange in fish. Water flows in the opposite direction than blood optimizing the extraction of oxygen from water to the blood in this counter current pattern of flow.
Table 1: Solubility coefficient of oxygen in water expressed as mL per liter per mm Hg as a function of salinity (ppt) and temperature at extreme temperature and salinity to demonstrate the effect of the two factors.
5 ppt 35 ppt
5oC 54.7 44.9
25oC 36.4 30.7
30 | InternatIonal AquAFeed | January-February 2012
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is utilised for the production of energy, it is fully oxidised and the final products are: energy, carbon dioxide and water.
All these processes are included in the term of metabolism. The rate of metabolism is influenced by a large array of abiotic and biotic factors (see Table 2). From all these factors it should be underlined here that that activity is the most potent factor. Oxygen consumption is proportional to the metabolic rate, and it is therefore a common approach to measure metabolic rate by measuring oxygen consumption.
Temperature has a strong impact on oxygen requirements as it affects the activity of enzymatic processes. Besides the enzymatic processes, tem-perature has an effect on the ability of hemoglobin to bind oxygen and the solubility of membranes. Another important effect of temperature on metabolism is related to the amount of water bound by proteins. Water molecules are bound to polar groups in the protein molecule, and the amount of water is influenced by temperature. The effect of temperature is normally described by a Q10 value, which expresses the multiplication factor when temperature is increased by 10oC. Q10 receives a value between two and three in most cases.
When calculating the need for oxygena-tion it is important to know the average size of the fish comprising the population in question. As a general rule, per kg of biomass, smaller fish require much higher quantities of oxygen than larger fish.
Monitoring of oxygenation
Measurement of oxygen concentration in water usually takes place by use of an oxy-gen electrode, which was developed by Prof. Leland Clark in 1956. This basically measures an electric current, which is based on the reduction of oxygen at the cathode:
O2 + 2H+ + 4e- 2 0H-
Whereas at the
anode electrode sil-ver fells out of solu-tion:
Ag Ag+ + e- The sensitivity of
this type of oxygen sensor depends on the area of the cath-ode and thickness of the membrane of the sensor, which may limit the diffu-sion of oxygen to the cathode. It becomes evident from the equations above that the sensor in one way consumes oxygen, which is the parameter it actually measures.
To circumvent this practical problem, the sensor should be in motion in relation to the water. In prac-tical terms, this means that if the measure-ment is taken manu-ally by a technician, this person should shake smoothly the sensor in the water
until a stable value is established. In the case of a sensor mounted at a stable point, the measure-ments are of little value, in the case of standing waters or a container of water with low current.
The Clark type of oxygen sensor requires a current of at least five cm/s to function properly. In the case of currents in cages as shown in Table three, this value is not so easy to achieve, and stirring is necessary. To sum up, in the case of manual measurement, this type of electrode can function well, whereas if the sensor is mounted at a fixed point, the issue of current speed becomes an important issue.
The Clark-type of oxygen sensor - electrode requires frequent replacement of electrolyte and membrane, and frequent calibration. A relatively
Figure 2. A schematic diagram showing the respiratory pump. Water entering the mouth is further led by suction to the gill cavity and passes thereby through the gills. Opening and closing of the mouth and the opercular valve ensure that water flows in one direction.
30 | InternatIonal AquAFeed | January-February 2012 January-February 2012 | InternatIonal AquAFeed | 31
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new technology has been developed based on the presence of a fluorescent compound in the sensor.
This methodology does circumvents several of the technical disadvantages of the previous method as it does not consume oxygen and stirring is therefore not necessary. These optic oxygen sen-sors are more expensive to purchase, but on the other hand have a lower maintenance cost.
Factors that may be influenced by insufficient oxygenation
It has been documented that the single most important factor for increased growth and productivity in aquaculture is to maintain sufficient oxygen saturation level over time in the water where the species grow. At saturation level below 85 percent, feed utilisation begins to fall and the fish is increasingly vulnerable to sickness and, in the end, mortality:- at 75% saturation reduced appetite starts to
appear- at 60% saturation increased mortality is shown- at 40% saturation there is no appetite among fish- at 30% saturation there is massive mortality.
Feed is composed of three main groups of food-stuffs: protein, fat, and carbo-hydrates. The amount of oxy-gen needed to metabolize a gram of food differs for these three groups of foodstuffs. Fat gives more than double the energy released during the catabolism of protein and car-bohydrates and at the same time requires a proportionally increased amount of oxygen to achieve this process.
It is logical to assume that fish consuming a fatty diet will have higher oxygen requirements compared with fish consuming a larger propor-
tion of carbohydrates. It has been postulated that decreased oxy-gen levels may have an impact on resistance of fish to infectious diseases (viral and bacterial dis-
eases), as in the case of channel catfish, Atlantic salmon and other species. Increased infestation of parasites has also been observed.
Basic principles in oxygenationInjection of a gas in aquaculture is governed
by certain principles which will be described here in order to make easy to perceive the limitations and possibilities related to oxygenation.
An important factor that influences efficient injection of a gas in water is the size of bubbles as they exit the diffuser. Small size show several advantages in related to larger ones. If gas is divided to small bubbles the contact surface with water is much higher than in the case of large bubbles. In a sphere, as the diameter increases the
ration of volume to external sur-face decreases.
This means that the content of large bubbles has fewer chances to dissolve in the water that the same amount of gas in small bubbles. It is needless to point out here that both in the case of oxygenation in tanks and in cages oxygen that reaches the surface of water and burst is a loss for the farmer as it enters the atmos-phere and is of no use for the fish farmed. If you thereby oxygenate your farm and notice the water “boiling” due to gas injected in the water, you should take it as warning that large amounts of gas are getting wasted.
Another disadvantage of large bubbles is that they rise fast in the water column to reach the surface and thereby remain for a reduced time in the water reducing further the ability of oxygen to dissolve. A further disadvantage of large bubbles is that large
bubbles show a tendency to “merge” and thereby becoming even larger increasing the problem.
From the description above it becomes clear that ideal oxygenation involves the formation of small bubbles, which rise slowly in the water column, and result in efficient oxygena-tion of the water as a maxi-mal interface of gas-liquid is provided. These tiny gas
bubbles give a “milky” appearance to the water.A second important factor is the distribution
of the gas in the cage or the tank. In the case of circular tanks, these are not so deep so the bubbles have a short distance to cover before they get dissolved so the need for small bubbles is quite high. On the other hand, as the water is well mixed compared with other systems a few areas of gas injection are sufficient to provide fish the necessary oxygen.
In earth ponds or in raceways it is impor-tant to inject oxygen in the area close to the entrance of the raceway so oxygen has higher chances to be utilised by the fish population. In the case of fish cages, oxygen has to be distributed over a large area. The gas can be injected at a larger depth than is usual in the case of tanks or ponds, so there is more distance to be covered in the water column
by the bubbles and thereby more time to achieve oxygenation over the water masses.
CagesThere is a widespread belief among farmers
that oxygen demands of fish farmed in cages at sea are under all situations covered by the currents existing at sea. It is easy however to determine at first that oxygen concentration inside the cages is lower than the oxygen concentration a few meters outside the cage (Figure 3). This difference is powered by two factors: (a) the consumption of oxygen within the cage, and (b) the ability of the current to replace the depleted oxygen with the fresh supplied brought by water rich in oxygen. The current in the area of the cages is much lower than the current outside the cages (Table 3).
It is obvious that the work of the currents is hindered in the case of cages placed in the sea by the net surrounding the cage. This net in the case of most farms in the Mediterranean is double to hinder the escape of fish. In addi-tion, the size of fish is smaller than for example is the case in salmon farming, so mesh size is on average smaller than in salmon farming. An additional problem that arises is the fouling of nets with micro- and macroalgae which reduces considerably the renewal of water and causes further problem in the cages.
In the case of farming of gilthead seabream and seabass, production is such that there is a peak of the total biomass towards late summer and autumn. The large biomasses in the on-growing cages results in increased demand for oxygen, where addition of oxygen in the cages by natural currents may not be sufficient, as the temperature is still quite high in autumn.
This type of oxygenation may be applied either after manual registration of low oxygen concentration in the cage or after continuous monitoring by an automatic system. An auto-
matic system for monitoring of oxygen level in the cages ensures that low levels at any time of the day or night will result in an alarm procedure, able to result in addition of oxygen within a reasonable time period. ■
Figure 3. Dissolved oxygen concentration outside and within a cage. The difference in oxygen level indicated an exchange rate between about 3-6 times/hour.
Table 3: Parallel monitoring of current velocity at surface outside a farm and inside a cage.
Current outside farm (cm/s) 2-4 4-6 6-8 8-10 10-12
Current inside farm (cm/s) 1.7 1.7 2.1 2.2 2.2
Reduction of current (%) 44 67 70 76 80
Table 2: Factors that influence metabolic rate and consequently the oxygen requirements in fish.
Abiotic factors Biotic factors
Temperature Activity level
Salinity Weight
Oxygen Oxygen debt
Ammonia Stress
Acidity Starvation
Season Quality of feed
32 | InternatIonal AquAFeed | January-February 2012
FEATURE
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Volume 15 I s sue 1 2 012
the international magazine for the aquaculture feed industry
The effects of dissolved oxygen on fish growth in aquaculture
On-farm feed management practices– for three Indian major carp species in Andhra Pradesh, India
Oxygenation in aquaculture
Developing a plant-based diet- for Cobia Rachycentron canadum
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