Water Planning Institute
1. SITE SELECTION
2. PREPARATORY WORK
3. PURPOSE AND DIMENSIONING OF FISH FARM STRUCTURES
4. FISH POND ARRANGEMENTS
5. EARTH STRUCTURES
6. DIKE PROTECTION
1.1 Considerations in Site Selection
1.2 Basic Principles of Arrangement
The site for a fish farm should be selected with great circumspection and care, since the site and general arrangement will control the economics of operation.
- Sites should be selected for fish farms only where water of the required volume and quality is available at the times needed for operating the farm.
- Preference should be given to sites where a gravity water supply to the farm is possible.
- The quality of the water available must be such that the desired fish can be raised, e.g. fresh, brackish or salt water.
- Gravity drainage of the ponds should be possible.
- The fish farm should be sited primarily in areas unsuited to other agricultural uses.
- The soil in the area selected should, if possible, be impervious.
- For low construction costs, plain areas with slope less than one percent should be selected.
- The site should be in the vicinity of transportation routes, or where the access road can be constructed economically.
- In the proximity of inhabited areas, considerations of public health and the necessity of guarding against poachers should be kept in mind.
- Fish farms need electrical power, so the possibility and cost of connection should be considered.
- Any existing electric power line must be excluded from the area envisaged for the fish farm.
- Skilled operators are essential for operating major fish farms efficiently. An important consideration in site selection is, therefore, to provide an attractive environment and facilities for both professionals and operators.
Factors to be considered in site selection are described in more detail in Chapter 1 "Considerations in the Selection of Sites for Aquaculture".
Fish farms consist of many ponds performing different functions in fish production. Their relative positions, as well as their connection to the water supply and drainage facilities, power supply and transport roads, in fact the general arrangement of the fish farm, has a major influence on the operating costs. In deciding on the general arrangement, the following should be considered:
- The farm centre, consisting of operating buildings and living quarters, should have good road access.
- The facilities requiring frequent attendance, such as the hatchery, rearing and nursery ponds, holding ponds and pumping station, should be as near as possible to the centre.
- Separate filling and drainage possibilities should be provided, if possible, for each pond.
- Transport of feed from the grain storage to the ponds and of the fish harvested to the holding ponds should involve short hauling distances.
- The holding ponds should be close to the common external cropping pits serving several ponds.
The foregoing principles can be realized best by an arrangement in which the operating buildings, the holding ponds and the water control structures are in the vicinity of the geometrical centre of the area. Wherever the terrain conditions permit, this type of arrangement should be adopted.
2.1 Technological Requirements
2.2 General Technical Data
2.3 Geodetical Data
2.4 Hydrological and Meteorological Data
2.5 Geotechnical Data
2.6 Water Quality Data
The layout of the site is governed by the combined requirements of operation and the particular site conditions. Prior to designing work, all information should be collected regarding technological requirements and the data particular to the site.
- Species of fish to be produced.
- Sequence of operations envisaged: production from hatching to market fish, or production of market fish alone.
- Method of fry production.
- Production quantities envisaged.
- Methods and possibilities of nutrient supply to the ponds, such as organic manure, duck farming, fertilizer application.
- Feed distribution (fish feed, grain feed, etc.).
- Transport management - method and means of in-farm and external transportation.
- Buildings required (operation, social-cultural amenities, grain store, equipment shed, repair shop, housing, etc.).
- Data on existing water uses affected.
- Data affecting water supply and drainage.
- Future development plans for the area.
- Data on other facilities (roads, railways, etc.)
- Property conditions and data.
Topographic surveys to the scale 1:500 to 1:5000 and with contour lines of 20-25 cm (perhaps 1 ft) vertical spacing are needed for the entire fish farm area, in order to permit designing complete pond drainage and earthwork volume estimates of the required accuracy.
On any existing earth structures (embankments, canals, etc.) cross sections should be taken at 50-100 m spacing, with points spaced sufficiently close to each other to plot the actual terrain with ± 20 cm accuracy on the cross sections of 1:100 scale.
Cross sections should be plotted at the sites of major structures (intakes, road crossings, etc.)
Cross sections at not more than 500 m intervals and continuous profiles are needed on the connecting stream or canal extending for the distance affected by the fish farm (e.g. to the backwater limit).
The topographic survey should be connected, especially as regards elevations, to the national survey network.
Where water is obtained from a natural stream, data must be acquired on the stages and flow rates to be anticipated at the diversion point in the periods of pond filling and for compensation of water losses. Water supply should be designed for a flow rate of 80 percent probability.
In the case of ponds through which floods must be conveyed, or the dikes which are required to retain floods on the stream, the designer will require also data on design flood levels and discharges. The probability of occurrence of the design flood is normally specified by the competent water agency. In the absence of such specification, the flood of 1 percent probability of occurrence (once in a hundred years) should be adopted as the design flood. In the case of minor ponds, where a dam failure would cause no other losses, a flood of say 3 percent probability might be adopted as the design flood. The retention capacity of upstream ponds is taken into account in estimating the design flood.
Data on the peak values of monthly evaporation and rainfall are needed for estimating the water demand.
Data on the monthly average and extreme temperatures are needed for selecting the species of fish for farming, for planning the necessary feed supply rates and for designing the holding and storage facilities of live fish.
The annual volume of sediment entering the ponds should be estimated.
Data are needed on the direction and highest speed of wind prevailing in the area in order to design wave protection.
The geotechnical explorations should be extended to the entire area of the fish farm, to provide data on the soil stratification in the pond area, under the dikes, along the canal traces and at the sites of structures.
The data obtained by soil explorations should be suited to estimate:
- seepage losses
- underseepage conditions and the hazard of piping failure
- stability of the dikes
- the required degree of compaction
- the allowable flow velocity in the supply canals, and
- the foundation of the structures.
The methods of exploration and laboratory testing, as well as the interpretation of the results are described in more detail in Chapter 5 on soil characteristics for aquaculture farms.
The water supplied to the fish ponds must not contain pollutants and toxic substances detrimental to fish life. The composition of the feed water should be subject to quality analysis, including the following:
- oxygen content
- pH value
- total salts content
- ammonia content
- free CO2 content
- phenols, oil and tar content.
The water quality analyses should be such as to enable prediction of the interactions between the soil and the feed water.
3.2 Fry Rearing Ponds and Basins
3.3 Nursery Ponds
3.4 Production Ponds
3.5 Fish Holding Facilities
The most important structure in the technological process of artificial reproduction is the hatching house, where the following operations are carried out:
- Preparation of the breeders prior to stripping.
- Treatment of the breeders (hypothysis treatment, tranquilization, stripping, etc.).
- treatment, fertilization, spawning of the eggs.
- Treatment of the larvae.
- First feeding of the fry.
- Laboratory functions related to artificial reproduction (water analysis, egg analysis, detection of fish pests, chemical and medicament dosage, etc.).
- Packaging of fry.
The equipment needed in the hatchery includes breeder tanks, hypophizing and stripping table, hatching jars, fry holding vessels.
The small fry are reared from the age of 4-5 days to 3-4 weeks in the fry rearing ponds and basins. These should be arranged in places sheltered from wind, on impervious soil and close to the hatching house and road.
The preferable size of the fry rearing ponds ranges from 100 to 1 000 m2 . Within this range the actual size of the fry rearing pond depends on the number of fry which can be introduced within 1-2 days. The pond size should therefore be adjusted to the capacity of the hatching house.
The water supplied to the fry rearing ponds should be rich in oxygen and must contain no chemical or biological pollution. Water containing much suspended sediment must be allowed to settle first. The supply system should be dimensioned to permit the water in the pond to be exchanged twice daily. In densely populated ponds, a distribution network of perforated pipes should be provided at the bottom for the uniform distribution of inflow water. The fry rearing ponds are lined with stone or concrete.
Circular basins are made with diameters from 4 to 6 m and about 1 m depth. The freshwater is introduced tangentially through nozzle pipes to maintain the water in permanent circulation. The effluent is withdrawn from the centre of the basin.
Rectangular basins should have a ratio of short to long sides from 1:2 to 1:4. The water is introduced to, and withdrawn from, these basins along the short sides. The drainage system of the fry rearing ponds should be dimensioned hydraulically to permit each basin or pond to be drained in four hours. Sluices equipped with box traps should be used to collect the fry. For a guideline to determine the total area of all fry rearing ponds, as a general rule 100 to 200 feeding larvae require 1 m2 of water surface area.
The fry reared in the fry ponds are transferred for further growing into the nursery ponds at the rate of about 100 000 per hectare. The optimal size of the nursery ponds ranges from 1 to 10 ha, but the maximum size should not surpass 30 ha. The water depth should be 1.0 to 1.5 m. The nursery ponds should be arranged in the vicinity of the fry ponds, preferably in a manner to permit direct transfer of the fry to the nursery pond, together with the water from the fry rearing ponds.
The water supplied to the nursery ponds should be both chemically and biologically clean. In general no flow through the ponds is necessary, but provision must be made to compensate for any water losses.
Complete and rapid pond drainage is an essential requirement. For this reason the bottom of the ponds should slope towards the outlet and should be so graded as part of the construction work on the ponds. The body weight of around 100 g is attained in these ponds during 160 to 220 days. Fishing should preferably be made possible from external cropping pits.
These ponds serve to grow yearlings to full grown fish. The ponds are designed according to the following specifications:
The ponds vary from 20 to 100 ha in size, but the largest pond size should be determined by taking the total fish production envisaged into consideration, so that the total fish production in a single pond should not surpass 100 t. For example, for attaining a total unit yield of 2.5 t/ha, the ponds should not be larger than 40 ha.
The ponds should be from 1.2 to 1.8 m deep, with pond depth of 1.8 m especially in regions with elevated temperatures. Water depths less then 0.6 m will result in reduced production.
Filling the ponds should not take more than 2 to 10 days, depending on pond size. No continuous flow through the ponds is required, but the losses due to evaporation and seepage must be compensated for during the growing season. Wherever possible, the production ponds should have filling and drainage systems permitting them to fill and drain independently of the other ponds.
Complete drainage is an essential prerequisition of intensive fish production. In dimensioning the drainage system, an essential criterion is that the number of days required to drain a pond should equal the square root of the pond area in hectares. For example, the draining sluice of a 30 ha growing pond should be dimensioned so as to make possible drainage in or 5.5 days. Another requirement is that the drainage period of the entire fish farm must not surpass twice the square root of the total pond area. For example, the drainage canal of a 500 ha fish farm should be capable of removing within or 45 days the water from all ponds of the fish farm.
The ponds are stocked with fish of 100 g body weight at the rate of 3 000/3 500/ha and the weight gain to 1 500 g/unit is attained in about 300 days.
For advanced fishing, external cropping pits should be provided for the ponds. The feed, manure and fertilizers should be stored in the vicinity of the ponds. For transporting feed, as well as the fish, a sound road network is essential.
The fish harvested from the ponds must be stored in holding ponds until they are marketed or processed. The capacity of the holding ponds is governed by the composition of the production according to species, the yield envisaged, the time schedule of fishing and marketing or processing. The breeding stock should be stored, if possible, in the production ponds, rather than in the holding ponds.
For holding purposes small 500 to 2 000 m2 basins or ponds of 0.2 to 10 ha area should be provided. Under climatic conditions similar to those of Hungary, the basins are termed "wintering" ponds or basins. The average depth of the holding ponds should be from 1.8 to 2.2 m.
The most important requirement to be observed in connection with the holding ponds is that the dissolved oxygen content of the water in them should never sink below the 2.0 mg/l level. Oxygen can be supplied by introducing a feed water saturated with oxygen, by allowing water to flow through the pond, or by injecting oxygen or air into the holding ponds. Air injection must be accompanied by passing flow through the pond in order to remove the products of metabolism. At high rates of air injection, the rate of flow through can be reduced to around one-third of that without air injection.
To estimate the flow rate through, or air injection rate into the ponds, information is required on the oxygen consumption of the fish stored, the oxygen content of the feed water and the efficiency of the air injection system.
The oxygen consumption rate of fish depends on the water temperature, or on their body temperature which is identical. The rate of oxygen consumption increases with temperature. Conversely, cold water is capable of absorbing more oxygen than warm water. As an example, the hourly oxygen consumption of carp of 1 kg average body weight is;
Each litre of saturated water contains the following amounts of oxygen:
When estimating the necessary flow rate, it should be remembered that the fish do not consume the entire oxygen content of the water for breathing, so that the residual oxygen content of the effluent from the ponds is still around 1 cm3/l.
In the absence of more accurate computations, the flow rate through the holding ponds can be estimated at 1.0 l/sec for each ton of fish stored.
The allowable fish density in the holding ponds ranges from 5 to 15 kg/m2 pond area.
4.1 Barrage Ponds
4.2 Contour Ponds
4.3 Paddy Ponds
The arrangement of the fish ponds at any particular fish farm site is normally dictated by topographic conditions. The following types of pond arrangement are usual:
- Barrage ponds (Figure 1)
- Contour ponds (Figure 2)
- Paddy ponds (Figure 3).
The first two types of pond system are typical in rolling terrain, while paddy ponds are adopted in flat land. These types of arrangement differ from each other in their method of construction, structures and operation.
Figure 1. Barrage pond systems
Figure 2. Contour pond system
Such ponds are established in a valley of flat to medium longitudinal slope, by closing the valley with a low dam at a suitable site. A site is considered suitable if the shortest possible dam will create the largest possible pond. Thus, dams can be constructed most economically at a narrow point in the valley situated at the downstream end of a flat, wide section.
The floods on the stream will pass through the ponds and must be released without causing damage to the dams, dikes and structures, and without allowing the fish to escape. For this purpose a spillway must be provided.
The ponds established behind a longitudinal dike in the valley will not be affected by floods, these being passed along one of the valley sides. In other words, fish ponds are created on one side of the valley alone. To create gravitational supply to the ponds, the stream is blocked by a weir, the water being diverted through a gated intake to the supply canal. From the latter, each pond can be filled separately, while separate drainage is possible to the original stream bed except during flood conditions. The relocated stream bed must be dimensioned to carry the design flood safely, while the crest of the longitudinal dike running parallel to the stream must be raised above the design flood level. This arrangement is only possible on wide valley floors.
Each pond of fish farms in flat plain areas is surrounded by a dike. The flat terrain offers wider opportunities for the favourable arrangement of ponds, in which each pond can be filled and drained independently, thus creating the possibility of intensive fish farming.
5.1 Dams and Dikes
5.2 Feeder Canals
5.3 Drainage canals
5.4 Drain Ditch
5.5 Internal Pond Drains
5.6 Borrow Pits
5.7 Internal Harvesting Pits
Earth structures form an important part of any fish farm project. The dams and dikes are constructed of earth, while the supply and drainage canals, the seepage drain ditches, the pond drains and the harvesting pits are excavated in earth.
The dams and dikes (Figure 4) are normally constructed of the soil material available at the site. The dimensions and cross sectional shape of the dikes are governed by the purpose they are intended to serve and the material available for construction. The main principles to be observed are:
- The crest width should not be less than 2 m.
- The dike crests serving as unpaved roadways for internal transportation should be at least 3 m wide.
- Paved roads on the dike crests should be made with at least 1 m wide unpaved shoulders.
- The centre line of the dike crest should be elevated by 15-20 cm relative to the shoulders.
- The crest elevation above the highest water level, the safety freeboard, is controlled by the maximum wave height anticipated. In the absence of calculations, a freeboard of 50 cm should be provided.
- The slope inclinations should be selected as outlined in the notes on soils engineering.
- Where a reed belt is envisaged for wave action control (Figure 5) either a horizontal 3 m wide reed berm should be envisaged with the surface not more than 0.5 m below the normal pond level, or a slope inclined at 1:4 to 1:8 should be formed starting 0.3 m below the normal water level.
Figure 3. Paddy pond systems
Figure 4. Typical dam and dike cross sections
The feeder, or supply canals (Figure 6), carry water from the intake to the individual ponds, and are used occasionally as waterways for barges.
The feeder canals should be dimensioned to allow the various types of ponds to be filled within the times mentioned. Moreover, they should enable all ponds of the fish farm to be filled in less than 50 days.
The longitudinal profile of the feeder canal should be designed to ensure a canal water level at least 0.1 m higher than the normal water level at the intakes to the individual ponds. In estimating the water level in the feeder canal, the backwater caused by the structures should normally be taken into consideration.
Depending on the seepage losses anticipated from the feeder canal, some kind of lining, or other method of seepage control, may be necessary.
The feeder canals are made with trapezoidal, or mixed, cross-sections. The bottom width and water depth are found from the hydraulic calculations. The dikes along the feeder canals should have at least 2.0 m wide crests. The slope inclinations depend, just as in the case of the pond dikes, on the quality of soils, but should not be steeper than 1:1.5. The safety freeboard along the feeder canals should normally be 0.5 m, but can be reduced sometimes to 0.3 m over the terminal sections.
Canals to be operated at freezing temperatures should be designed for water depths of more than 1.0 m.
The drainage canals (Figure 6) serve to convey the water from the ponds to the recipient stream when the ponds are drained.
The lowest water level in the canal at the outlet of the drain sluices should be at least 20 cm deeper than the lowest bottom elevation in the pond, or than the bottom of the harvesting pit. Where this is impossible to achieve, pumps should be considered to ensure complete drainage of the pond.
The conveying capacity of the drainage canals should be estimated starting from the drainage times and periods mentioned for the various types of pond.
These ditches (Figure 6) serve to drain any seepage from the ponds and the feeder canals, and the external runoff that is prevented by the pond dikes from draining towards the recipient stream and, thus, to protect the surrounding areas against the potential damages caused by such waters.
The bottom of the ditch should nowhere be less than 0.3 m below the terrain level and the water should not be allowed to overflow, even when carrying the design discharge.
The drain ditch follows largely the trace of the extreme dike. The distance from the dike depends on the soil conditions and stratification (see notes on soils engineering, "piping control"). The distance between the outer toe of the dike slope and the edge of the drain ditch should not be less than 2.0 m.
Figure 5. Temporary wave protection
Figure 6. Canal cross sections
The water from undrained depressions within the pond area is conveyed to the outlet sluice, from where the fish are carried to the cropping pit. The slope of these drains should be steeper than 0.1 percent, while they should be cut deeper than 0.2 m. The minimal bottom width desirable is 3.0 m and the sides should be sloped at 1:3, or flatter. The earth excavated from these drains might possibly be used for constructing the dikes or for filling any depressions. Any material that cannot be used for such purposes should be deposited at a distance of not less than 5 m from the drain, so as not to interfere with draining the pond, and to prevent it from sliding back into the drains.
Where it is impossible to secure the earth needed for constructing the dikes from the cuts envisaged, separate borrow pits must be opened. Material may be obtained from areas outside the ponds, but also from within the pond as part of the bottom levelling work, or as special borrow pits. Attempts should be made to obtain the dike material from levelling the pond bottom, which should slope towards the outlet sluice. The bottom should be levelled to an accuracy of ± 15 cm. Any internal borrow pit should be designed by observing the principles outlined in connection with the pond drain network.
For harvesting, cropping pits should be placed in the vicinity of the outlet sluice, either within or outside the pond. An internal pit serves one pond only, while two or more ponds can be connected to an external pit.
Internal harvesting pits may be arranged parallel or perpendicular to the dike. The bottom area required for the cropping pit is around 40 m2/ha. To accommodate the length of the nets in current use, the pits should be 10 to 25 m wide. The differential elevation between the two ends of the bottom of the pit should be 20 cm. The preferable depth of the pits ranges from 0.6 to 1.0 m. The internal borrow pits and pond drains should be connected to the harvesting pit, so that their bottom at the entrance should be at least 20 cm above the pit bottom.
External harvesting pits are connected to the pond outlet sluices over drainage canals of varying length. Fresh water should always be introduced into the external harvesting pits during the period of harvesting. External harvesting pits need not be dimensioned for the entire fish population in the ponds, since the fish are admitted intermittently to the pit. The preferable area in plan varies from 15×50 to 20×70 m. The bottom of the external harvesting pits should be at least 30 cm deeper than the deepest point within the pond and an additional differential elevation of 20 cm is necessary between the two ends of the harvesting pit. The crest of the dike around the external harvesting pit should be designed so as to have a pit depth of 0.8 to 1.0 m below it.
The slopes of the harvesting pit should not be steeper than 1:2, while over the section where the net is drawn out they should be inclined at 1:4.
6.1 Wave Action
6.2 Biological Protection
6.3 Wave Control Linings
The dikes require effective protection against the destroying effects of waves. Even the surfaces of earth structures not exposed directly to pond water may often require protection against rain and wind erosion, depending on the prevailing climatic conditions.
The length and height of wind generated waves depend on wind speed, the fetch length and the depth of water. Of the large number of empirical wave height formulae, the one suggested by Dyakova is
h = 0.0186 W0.71 B0.24 H0.54
where h is the wave height in metres, W the wind speed in m/sec, B the fetch length in km and H the water depth in metres.
Biological protection consists of establishing a vegetation cover as a means of controlling both wave action and erosion, provided that such vegetation can be planted and grown under the particular conditions, and is compatible with the requirements of fish production.
Information on the plants potentially applicable and their biological requirements in any particular area should best be obtained at the site. Under the conditions prevailing in Europe, a reed belt has been found most effective for controlling wave action.
For effective wave action control, the reed belt should be 4 m wide for the first 1 000 m length of fetch, and should be increased by 2 m for every subsequent 1 000 m increment of fetch length. The density of the stand should be at least 70 reeds/m2 and each reed should have a diameter of not less than 8 mm above the water level.
The wave control reed belt is planted on the reed berm of the dike (Figure 5). Over the surface on which the reed is to be planted a topsoil or humus cover of at least 20 cm thickness should be spread, in which the reed is planted either by seeding, as a soil root mixture or as root or shoot cuttings. Except for propagation as shoot cuttings, reed planting operations are confined to the winter hibernation interval. The shoots should be planted at the time when the fresh shoots have developed 7 to 8 leaves.
Whichever method of reed propagation is adopted, the water demand of the growing plants must be satisfied. Without the possibility of irrigation, the result of reed planting will be questionable. In the first period of growth, up to the appearance of 3 to 4 leaves, the soil should be saturated, but a permanent water cover is definitely undesirable.
On the slope surfaces and dike crests exposed to rain and wind erosion, a sound grass cover is often the most economical method of control. For grass planting the surfaces should be covered by at least 10 cm of topsoil, but this can occasionally be replaced by manure and fertilizers. The grass cover is established on properly prepared surfaces most expediently by seeding. The desirable seed mixture should be determined with due consideration to the soil type and climatic conditions. The grass cover must be mowed regularly and in dry climates provision must be made also for irrigation. Where irrigation is impracticable, the costs of grass seeding will be wasted.
Where no biological protection is possible, the earth structures exposed to wave action must be protected by other types of lining. Guidance to designing such linings can be found in the notes on soils engineering.
7.1 Monk Sluices
7.2 Open Sluices
7.4 Fish Control Structures
The structures normally used for supplying water to, and draining water from a fish farm are of types familiar in hydraulic engineering. Those particular to fish farm projects will be dealt with here.
Monk sluices (Figure 7) are the structures generally used for supplying water to, and draining water from, fish ponds. By means of stop-log closures and fish screens or racks, the operator is free to decide whether the lower or the upper water layer from a pond should be released.
Monk sluices consist of two main parts, namely the shaft and the horizontal pipe. Three pairs of slots are incorporated in the shaft, one for the fish screen, the other two for the stop-log closure. The shaft is normally on the upstream side, thus inlet monks for filling have their shaft on the side of the supply canal, while outlet monks are on the pond side. The pipe crosses under the dike and serves also to dissipate the energy of the water falling over the stop-logs. The pipe may be built with circular, horseshoe, or rectangular cross sections. For higher conveying capacity twin pipes may be provided.
Access to the shaft is made possible by a walkway at least 0.5 m wide. For working on top of the shaft, a platform is cantilevered therefrom. A handrail must be provided along the walkway and the platform to prevent accidents.
Open sluices are built where the passage of barges is required, or where the discharges to be passed are higher than those which normal monk sluices are capable of carrying. Their application is also needed for the inlet sluices of external cropping pits, to facilitate the passage of fish. They are preferable also in the screen-box structure of nursery ponds where drainage or fishing of two nursery ponds can be handled by means of a single structure.
In the longitudinal section the structure follows the cross-sectional shape of the dike. The open cross section is U-shaped. To enable traffic to pass over the structure, the open U can be bridged by a slab.
With closing gates provided on both the upstream and the downstream ends, such open sluices can be designed as locks.
The closing mechanism in open sluices may be stop-logs or vertical-lift steel gates. Openings wider than 1.5 m are cumbersome to close by means of stop-logs and here steel gates with proper hoisting gear are the better solution. The hoisting gear of frequently operated gates should be driven by an electric motor where power connection is possible. The steel gates should be of the double-leaf type which can be released from either below or above the gate, as required by the operator.
Spillways are essential components for the safe operation of fish ponds formed by valley closure. These serve to pass the floods arriving at the ponds without damage, while preventing the water level in the ponds from sinking below the normal operating elevation and the fish from escaping from the pond system.
The spillways should be dimensioned to pass the design flood without damage. The structure must be capable of passing at least the 10 percent probability flood without any human interference. At higher flood discharges human control may be necessary at the closing mechanisms of the structure.
A wide variety of spillway designs have been developed, of which a few simple types will be described.
Simple unlined spillways are earth canals arranged at the junction of the dam and the valley side and having a sill level corresponding to the normal operating water level in the pond (Figure 8). Normally, no fish screen is provided. For stabilizing the sill, a recessed beam of concrete or hand-placed rock may be advisable. The depth of flow over the spillway, or the highest allowable flow velocity must be determined with due regard to the local soil conditions so as to prevent scouring. The surface of the spillway may be protected against scouring by a grass cover, reinforced if necessary by rows of buried brushwork or hand-placed stone ribs.
Improved unlined spillways are similar in design to the foregoing but with fish screens between piers and lined for some distance upstream and downstream of the screen (Figure 9). The piers have cantilevers at the top, on which a service walkway is supported to permit cleaning the screen.
Reinforced concrete spillways with recessed sill have the sill elevation below the normal operating water level in the pond, which is maintained by means of stop-logs (Figure 10). The fish are prevented from escaping by a fish screen. To pass major floods, the stop-logs must be lifted clear of the water. For ease of handling the stop-logs, the spans between the piers should not be longer than 1.5 m. The sill should not be lower than 0.6 m below the normal pond level. The piers should carry a walkway or a footbridge.
Figure 7. Monk sluices
Figure 8. Simple unlined spillway
Figure 9. Improved unlined spillway
Figure 10. Reinforced concrete spillway with recessed sill
Drop shaft spillways are normally combined with the outlet sluice. Where the subsoil offers good foundation conditions, such structures are capable of passing safely minor flood discharges. Behind the shaft of the monk sluice a drop shaft having a crest elevation at the normal pond level is also provided. The flood discharge is carried by the pipe of the sluice, so that the pipe must be dimensioned for the design flood flow. At the downstream end of the pipe, an energy dissipator is needed. Cleaning of the fish screen around the crest of the drop shaft can be accomplished from the access bridge of the structure.
At valley-dam fish ponds supplied by a perennial stream, measures must be taken to prevent fish from escaping from the upstream pond. A fish screen or an electrical fish deterrent are possible solutions.
The fish screen should extend at least up to the level if the 10 percent flood. The screen must be cleaned at the time of floods, so that a working platform must be provided for the operator. The mesh width of the screen or the spacing of the rack bars depend on the size of fish kept in the pond. In production ponds a bar spacing of 2 cm is sufficient.
Elekes, K. and T. Selmeczy, 1975, Mezögazdaságj; vizhaszmositás. Vol. 2, Halászat, edited by Gy Fóris. Budapest, VIZDOK és Mezögazdasági Könykiadó Vállalat
Huet, M. and J.A. Timmermans, 1972 , Textbook of fish culture: breeding and cultivation of fish. Farnham, Surrey, Fishing News Book Ltd., 436 p. 4th ed.
Pillay, T.V.R., 1979, The state of aquaculture. In Advances in aquaculture, edited by T.V.R. Pillay and W.A. Dill. Farnham, Surrey, Fishing News Books Ltd., for FAO, pp. 1-10
Tang, Y.A., 1979, Physical problems in fish farm construction. In Advances in aquaculture edited by T.V.R. Pillay and W.A. Dill. Farnham, Surrey, Fishing News Books Ltd., for FAO, pp. 99-107
Woynárovich, E. and L. Horváth, 1980, The artificial propagation of warm-water finfishes: a manual for extension. FAO Fish. Tech. Pap., (201):183 p. Issued also in French and Spanish