Mr. J. SELTZ
Marine fish rearing can be carried out using different methods extensive, semi-extensive or intensive (Annex 1)
Each method has it's own particular structure answering to the biotechnical and economical requirements of the type of rearing carried out.
The aim of this present document is to give the description of these diverse structures.
The raceway is a rectangular or trapezoidal section canal, through which flows a water current from one end (supply) to the other (evacuation).
is quite satisfactory if it's built in such a way so as to avoid the creation of dead zones. This can be done if the supply and the evacuation of water is carried out over the whole width of the tank, or if the evacuation is in the shape of a V (see annexes 3 and 4).
The length (L)/width (l) proportion is important when designing raceways. The stock must not be abble to create circular movements which would bring about the accumulation of dirt in the centre. Because of this, it is recommendable not to have a L/l proportion under 6. Also, the width must not be too large as it would give a great section and thus a feeble current speed. So, dimensions such as 20 m × 2 m × 30 m × or 50 m × 4 m give good results, the average depth is around 1 m. The slope of the bottom is generally at 1%.
With raceways, there is a feeble speed of the water which is incapable of ensuring the evacuation of wastes. In fact, it's the fish themselves, when there is a sufficient load, that keep the tank clean continuously stopping the sedimentation from settling (20 kg/m3 for trout). For the other cases, there is sedimentation and so accumulation of organic matter which causes determination in the zone. Frequent cleaning is necessary. This can be carried out easily by simply lowering the water level in the tank. This operation increases the current speed as well accumulating the fish together which brings the wastes to surface and evacuates them.
The outlet monk (annex 5) has three slides which permit the ensurance of a triple function :
- stop the fish from escaping,
- evacuate the water at the bottom containing organic matter (loose plank in the second groove),
- maintain an regulate the water level in the tank by means of a set of planks which are bevelled into the third groove.
The shape of the raceway allows easy partitioning and accumulation of fish.
The BURROWS type tanks are rectangular and have a central partition and guides (or deflectors) at both ends which permit a better hydraulic circuit.
The water arrives in gushes which are directed towards the axe of the tank.
The water is evacuated out through an opening which is located in the middle of the central axe of the tank. All these improvements, when compared to raceways, result in better water current and elimination of wastes.
These tanks are more efficient than raceways for automatic cleaning/admissible load. However, the circulation time of suspended particles in the tank is long, which can cause irrigation at gill level with delicat fish (especially fry). Manipulations (accumulation, fishing) in the tanks are difficult because there is the risk that partitions or deflectors might cause injury to the fish. Also, the periodical upkeep work is more difficult because of the different partitionings.
Burrows type tanks have an advantage from an hydraulic point of view but are harder to manoeuvre for upkeep and fish manipulations. Also, the cost price is clearly higher when compared to the classical raceway.
The circular tanks have a tangential supply. There is central evacuation, level regulation being done on the outside, using diverse systems such as the inclinable tube or double pipe (see Annex 8).
The current caused by the power of the water which flows in tangentially, carries the particles towards the central evacuation, and this very easily as the shape of the bottom is slightly conical (5% slope).
The principal advantage of these circular tanks is the automatic cleaning which is brought about by the circular current and the excellent oxygen distribution within the liquid mass which permits greater densities of fish (around 20% more) when compared to the rectangular tanks.
However, the great speed of the current does not only have advantages. The swimming effort that the fish have to make, greater than in rectangular tanks, can give a slight decrease in the growth performance.
One solution, is, to modify the movement of the fluid by directing the gush of water or even by periodically creating this gush. On the other hand, the resistance of fish will be better which is very interesting for restock fish.
The different manipulation that are carried out on fish are more difficult in large round tanks. These tanks can not be partitioned off.
The construction of a tank can call for different materials. The rearing characteristics and the cost of construction can vary a lot depending on the type of material used.
Raceways can be built quite easily in earth. No particular problem arises for the embankment. This is carried out by means of a bulldozer or excavator. The operations are reduced to a minimum: excavation, mound levelling, compaction. The ends of the tanks are often in concrete: at the intake, so as to avoid any excavation caused by the fail of the water and at the outlet so as to able to build correctly the work, or the fishery.
The floor must be impervious. The slope must ensure a proper stability to the whole unit.
It is important, when designing earth tanks, to plan adequate accesses for later mechanized upkeep (cleaning out, levelling of the banks, etc...). In this context hero, the water surface/total surface proportions less than the concrete option which gives a worse land return.
At zootechnical level, the cleaning and disinfecting of the earth tank is difficult. On the other hand, it has a beneficial effect on the ammonia rates which it limits, means of a purifying outflow from the bacterial flora which are caught into the substratum. As the cost price per square meter is cheap, the earth tank is mostly suited for fattening of certain size fish (> 50 g), which do not require special sanitary conditions, producing great quantities of ammoniac and needing great rearing volumes.
There are two types of sheeted tanks:
a) tanks with rigid frameworks sheeted with a plastic film. This frame-work is often in plywood and of circular shape.
b) earth tanks, sealed off with a plastic film, (polyene, butyl, etc…) which have V shaped walls and are connected to the intake and outlet water structures (for example, the sheet is caught between a lath of wood which is screwed onto the concrete).
This technic permits having tanks which are cheap and easy to implant. It gives the opportunity of using pervious soils. However, when placing the plastic film, special attention must be taken (smooth support) and it can be easily perforated (rats, etc…)
All the usual rearing operations are to be carried out very carefully. The cleaning and disinfection, although better than in the earth tanks, are however not very easy as the plastic film is very fragile and at the corners and curves of the tank it can be creased.
The life length for a plastic film is variable depending on the type used and rarely lasts for more than five years.
Reinforced concrete or masonary are materials which guarantee the longest life length for the tanks, and all shapes are obtainable no matter how complex they be.
The tanks which are completely built of concrete have a better water tightness and the longest life length but they are also the most expensive. That is why an other solution has been found, by building a concrete slab onto which are connected hollow concrete blocks. The latter are cased with a vertical and horizontal chaining (annex 9).
The slab must lie on a soil which is correctly compacted and be cast in one piece. On sandy soil, a layer of gravel is spread before casting the slab.
With concrete or masonary tanks the cleaning an disinfection operations are easier (as the walls are smooth and well looked after). Therefore it is easier to control the rearing conditions. So, are very suitable for the first stage rearing (until they reach around 20 - 30 grammes).
The tanks can be built in polyester resin. However, the high cost price limits the use of this material to small circular or rectangular tanks (less than 5 m3).
The advantage of using these tanks in that in that they are mobile and have an easy upkeep (the walls are perfectly smooth).
This is the material that is used for reproduction and fry stocking structures (annex 10).
The circular shape is the most advantageous when talking about the pull distribution on the walls. Thus, the later are not as thick or reinforced as those for rectangular tanks. however, the sheeting is more soophisticated, which compensates the reduction of the cost of material.
The circular shape is also more adaptable for flexible structures (rigid frame work with plastic film or polyester resin).
For a raceway, the shearing pull caused by the pressure of the water on the walls call from a stronger construction. So, the framework and the thickness of the concrete must be calculated in consequence, leaving a good security margin. With masonary constructions, the presence of a horizontal and vertical chaining is necessary. For the structure in polyester, the reinforcements are sealed onto the walls,
As circular tanks leave spaces between them, the area is not put to full use. The raceways give a better performance in this domain ; the concrete or masonary tanks can be built side, one wall being shared by two basins.
The circular shape offers the principal advantage with a good hydraulic circuit. This brings about the accumulation and central evacuation of the wastes. The even distribution of the pulls on the walls permit flexible constructions. However, when a certain size has been reached, manipulations of the fish (accumulation, fishing, sorting out) are difficult. This type is recommendable for larval rearing or even pre-fattening. After these stages, it is unsuitable.
Raceways are more suitable for fattening of fish. The earth ones are kept for fattening. The concrete or masonary structures are suitable for all stages of growth. The rectangular polyester tanks are kept for fry stocking The tanks with a plastic film present too many risks of dammage. However, they permit the realization of cheap tanks in pervious soils.
Finally, the BURROWS type tanks, which were largely used for intensive pisciculture, are hardly ever used now.
The fish present in rearing varies in quantity (number of kilos) and quality (size rates) throughout the year (see annex 11 and 12).
It reaches the maximum depending on the following :
- The total yearly production
- The production divisions through the year
- The growth speed of the fish
This maximum, known as the maximum stock present, determines the total volume of the facilities as well as the quantity of water necessary for the production scheduled.
The total volume of the required facilities results from the maximum stock present, in dividing the quantities of fish by the corresponding acceptable loads (quantity of fish per cubic meter of the rearing volume).
The loads which are acceptable, depending on the species, and the size of the fish, all of the same species. Therefore, concerning trout, loads of 30 to 50 kg/m3 are quite normally reached at the end of rearing in industrial fishcultures. For troutlings, the normal loads are around 10 kg/m3.
Within marine species, such as sea-bass and sea-bream, they are at present lower (between 15 to 20 kg at maximum).
The reason for the renewal of water in the volumes is to :
- Bring oxygen, which is necessary for the fish to live.
- Eliminate dissolved toxic matter coming from food wastes and animal excretions.
The quantity of water necessary can be estimated by the oxygen needs of the fish. For this, it is sufficient to determine the quantity of oxygen which is really at the disposal of the fish and compare it with the consumed by the fish (annexes 13, 14, 15).
The quantity of water required varies depending on:
- the quantity and size of the fish
- the temperature
Therefore the water requirements of a rearing unit are not constant and can vary throughout the year. When pumping is employed to give the water supply, the number of pumps be taken into account.
When there is no new water available, different methods are employed to make up for this lack:
- The tanks are placed in line, one after the other, which have an intermediatry drop permitting re-oxgenation.
- water recycling employing pumps
- aeration systems
- insufflation of oxygen
All these methods permit having greater quantities of fish in the rearing unit, the limiting factors factor comes from the quantity of ammoniac produced.
Semi-Intensive or extensive rearing call for tanks, with large surface areas, from a few hundred square metres to many hectares. The cost of construction must be kept as low as possible. These are earth constructions: having very little concrete. This latter only being used for the water inlet and outlet and the fishery structures.
The control of the water quality and of the fish is more difficult to master. In fact, all intervention, because of the great rearing volume and of the feeble renewal rates, is limited and the environment react slowly.
Above all, these ponds allow for individual drainage.
There are many types of tanks (annex 11) :
- with flat bottoms, which have ditches and drains to allow complete drainage and avoid fish being trapped in puddles.
- with slightly sloping bottoms towards the central ditch allowing the accumulation of the fish.
In practice, these systems work quite well. The drains or ditches need constant upkeep so as to avoid clogging up.
It is preferable to have a tank with a sloped bottom, the deepest part being near the outlet structure (monk)
It must not be shallow so as to avoid the development of macrophytes which limit the light rays getting to the plants.
It must not be too deep either as there would be a too great volume of water, which should be different to renew.
The water height range, permitting good working conditions, is around 1 m to 1,70 m.
There must be a sufficient slope so the fish can down deep enough and not be left without water when the pond is drained for fishing. Values between 5 and 10 per thousand are acceptable.
So as to gather all the fish around the outlet monk, it would be interesting to place this monk in a corner of the tank and not at the side.
The two principal advantages of a dyke are :
Sturdiness and water tightness.
For the construction of a dyke, the soil in proximity is used when possible. The material most suitable is sandy clay. If sand is used, the width of the dyke must be doubled or the use of a clay core, linking the dyke to the impervious soil could be of help.
Normally, the width of the dyke at the surface must have the same height with a minimum of 1 m. A greater dimension can be used to allow the passage of vehicles.
The height level of the dyke must be, 30 cm for small ponds and 50 cm for larger ones, above the water level.
Generally, the slopes accepted, are, from 1 : 1 to 1 : 1, 5 for the outside wall and 1 : 2 for the inside one. the slope of the inside wall can be reduced down to 1 : 1 for small ponds or if the materials used permit so.
Ponds generally have a supply from a water intake canal. This latter is a simple pipe with or without a control system (monk, sluice gate or swivel pipe bend).
The evacuation is performed by means of a monk having three grooves which join up with the pipe. This monk has a grill and two plank systems permitting, either the evacuation of the water on the bottom or the perfect water tightness by blocking up, with clay, the space between the two rows of planks (annex 13).
It could be interesting to provide capture or fishery rooms, so to facilitate fishing.
There are two types of fishery :
- counter-current fisheries which are placed at the entrance of the pond,
- fisheries located at the outlet of the pond.
This method of collecting fish is employed when the level is sufficiently low enough to send a fresh counter-current into the fishery provided inside the pond. When the fish notices this water arrival, it swims into the capture room against the current where it is then caught (positive rheotactism).
Two types can be found :
- fisheries which are located in front the dyke, in other words inside the pond. The principal incovenience with this type, is that it clogs up the structure regularly, which hinders fishing considerably.
The fisheries located behind the dyke, where the outlet is located. The advantage with this type is the possibility of employing it for many tanks, which reduces costs.
Intensive rearing offish at sea can be carried out in two types of structure :
- floating cages,
- immersed cages.
This is a relatively simple structure having a rigid floating pontoon carrying a supple net pocket which contains the fish. The unit is anchored by means of ropes and weights.
The pontoon has a platform which allows access to the rearing. It is kept afloat by means of floats of different types (cylinder drums injected with polyurethane, PVC pipe framework injected with polyurethane).
The pontoon has pegs and hooks onto which is attached the net pocket, which is weighted by a PVC framework. The net mesh used depends on the size of the fish. For a given size, it is better to use the largest size possible mesh, so to ensure the best possible water renewal through the cage. however, fouling must be carefully looked after. This reduces the free space allowing the flow of the water and deteriorates the rearing conditions. Frequent changing is needed, especially during the summer period.
For each cage, a set of nets with different mesh sizes is required.
4 mm–10 mm : fry to 20 g
12 mm–16 mm : from 20 to 150 g
16 mm–24 mm : 150 g and bigger
It is recommended to use a net without knots which avoids injuring the fish.
The anchorage is carried out by means of weights, often simple concrete blocks placed on the outside of the cage's four corners and linked to the latter with a metal chain and extended with a rope.
Floating cages are semi-rigid structures, easy to build, economic, and perfectly suited to well sheltered sites. They can also be used in more open areas providing that the construction of the pontoon is reinforced and that it is equipped with antiswell devices.
Floating cages can be grouped together on one floating structure, facilitating supervision and rearing operations. These seem to be limited to fry fattening, with fry already of a certain size (around 20–30 grammes).
For the more open sites at sea, the immersed cage technics are used, examples of them are given in Annex 16.
It is a structure which has a biconical shape net pocket supported by a central PVC mast. Three polyethlene hoops are placed around this mast (1 median and 2 summit hoops). The volume for these cages can vary between 20 and 40 m3 depending on the diameter of the median hoop, having a must of 6 meters in length.
This type of cage needs only one anchorage point. S it can revolve arond its own axe and move easily depending on the current.
The immersion and balancing at desired depth is carried out by partially filling the central mast.
It can also be used in a floating position. In this case, the emerged part of the net is automatically cleaned by solar drying.
This is a rigid cage model with a PVC framework and where the traditional net has been replaced by a metallic grill of copper and nickel alloyage particularly interesting for their anti-fouling characteristics.
The great pull caused by the rigidness of the structure make it obligatory, that it be uses in a immersed position.
Immersed cages permit rearing at sea, but the control of the rearing (observation, alimentation, measures counts, etc…) is much more difficult to carry out which limit them preferably to the final fattening of fish which are already quite big. (100 g).
Floating cages allow precise control and of the rearing when compared with immersed cages. However, it requires a sheltered site or the placing of efficient anti- swell protections.
Compared to rearings on land, they dont call for a lot of work, pumping is avoided but the follow up of the rearing is more difficult and only deals with fattening fish which have already reached a minimum size (from 10 to 30 g).
- “Aquacultural engineering” by F. WEATON - Ed. J. WILEY and sons.
- “Inland aquaculture engineering” A.D.C.P - FAO
- “Technologie des bassins en terra destinés à l'aquaculture marine” par Ph. LEQUENNE - Université de NANTES
- “Conception générale des petits ouvrages d'art” - Bulletin de vulgarisation forestière
- “Construction of linings for reservoirs, tanks and pollution - control facilities” by W. KAYS - Ed. J. WILEY and sons
- “Eléments sur la pisciculture d'étangs en France” par B. VIU - CEMAGREF - Div. ALA
- “La production d'anguilles en Italie” par J. GAULT et ph. LUCET - CEMAGREF - CEPRALMAR
- “Traité de pisciculture” par M. HUET -Ed. ch. de WYNGAERT
|50 to 100 kg/ha||- Natural fry production improvement|
|- artificial fry production|
|- predatory control|
|1 to 2 t/ha||- artificial fry production|
|- natural fry production improvement|
|- food distribution|
|20 to 25 kg/m3|
10 to 15 kg/m3
|- Hydraulic (tank)|
earth race way
concrete race way
SCHEMA DES MOINES DE SORTIE ET DU CANAL D'EVACUATION
A : concrete
B : Masonary
maximum instant stock: 24, 7 T (40% of the annual production)
|Pods Mayer (g)|
|individual weight (g)||10||0,1||0,1||0,1||0,2||0,3||0,4||0,6||0,4||0,3||0,2||0,1||0,1||0,1||0,1||0,1||0,2||0,3||0,4||0,6||0,4||0,3||0,2||0,1||0,1|
|Stock en place (T)||24,6||23,9||27,1||29,4||31,5||31,6||30,0||23,6||20,5||18,4||18,4||19,3|
maximum instant stock: 31, 6 T (52% of the annual production)
pour Salinité = O. Profondeur = O. Altitude = O. Humidité relative 100%.
D = Dose de saturation en mg.p.l.pour une salinité de 0 g.p.l.
b = Coefficient pour de calcul de la correction de salinité (valable jusqu'á45 g.p.l.).
D : oxygen saturation rate in mg O2/l for a salinity of 0‰
b : correction coefficient due to salinity
D' : oxygen saturation rate in mg/l for a salinity of S‰
D' = D - b.S‰
C1: Loads < 10kg/m3/h C: maximal load
C2: Loads > 10kg/m3/h
Text book of Fish culture
Construction and Layout of ponds
Text book of Fish Culture
Fig. 23 Preparation for new pounds. Construction of the dike using an excavator.
The monk. Above: Cross-section of a pond dike showing the monk and the outlet pipe:
Left: Monk with three grooves. One for the screen and the two others for the series of boards father Maier-hofman (Modified I).