Despite the probable reduction in habitat diversity in irrigation canals, they are able to support various levels of fish production. Fish undoubtedly enter the irrigation system from the source waters and some fish species may form natural populations in the canals, although there is little data to support this. In fact, entry from the source waters may account for the greater part of existing fish stocks in many irrigation schemes, but these fish will only form self sustaining populations if the correct breeding stimuli are present, and if sufficient food is available to support the population. In most cases it is not known whether fish in canals are actively reproducing or whether they are only periodically recruited from the natural waterways.
Fisheries production from canals is poorly documented. In Egypt it is estimated that almost 16% of the freshwater fishery production comes from the Nile and its irrigation canals (Sadek, 1988). It is not clear what proportion comes from the Nile and what proportion from the canals. The total lengths of main irrigation and drainage canals is 3 532 km which represents considerable potential for a managed fishery or for aquaculture operations assuming the canals were suitable (Table 15). One constraint to the culture of fish in irrigation canals in Egypt, and in other countries, is the reluctance of the government to allow any type of facility in the supply canals for fear of affecting the quality of the irrigation water, and reducing flow rates.
In Sudan the fish biomass in the minor canals of the Gezira irrigation system ranged from 50 kg/ha to 2 786 kg/ha, with an average of 660 kg/ha. In terms of standing stock, this is greater than the biomass commonly found in many riverine habitats (Coates, 1984). Welcomme (1979a) indicated that the average yield of floodplain rivers during the dry season varies between 60 and 1 000 kg/ha. However the population density of fish will tend to vary during the year depending on the level of water in the river. There is usually an increase during the dry season when the fish are concentrated in a smaller area. This was observed during studies on the Kafue River in Africa (Welcomme, 1985).
Chizhik (1969) noted that in the USSR the potential production of fish in irrigation canals may reach 50–100 kg/ha, but this was in areas where the species were macrophyte feeders and there was an abundant source of food. These figures were based on a managed fishery with grass carp, silver carp, common carp and bighead carp being produced.
In Bangladesh, government road and rail side irrigation canals could potentially produce 500 kg/ha/yr which would add another 10 000 tonnes per year to the national inland fish production (Marr, 1986). Many of these canals are static (borrow pits) for much of the year and can therefore be managed in the same way as ponds.
The simple release of Oreochromis spp., Channa striata and Puntius gonionotus into irrigation canals supplying orchards in Thailand was estimated to have resulted in a crop of 350 kg/ha/yr, without supplementary feeding (Swingle 1972).
It is not known how fish species from floodplain rivers, non-floodplain rivers, and reservoirs compare in terms of their adaptability to a canal system. Neither is there much data on the broader ecological relationships between natural waters and associated irrigation canals. Coates (1984) found that the average length of individuals in the canals of the Gezira irrigation system, Sudan, was generally smaller than those in the nearby River Nile. This observation is supported by Daget (1976) for irrigation canals in other countries. This may suggest that the smaller-sized fish in the canals were juveniles using the vegetation cover as protection, or that these canals are important areas for breeding or to escape seasonal changes in the hydraulic or trophic regime of the natural waterways. Were this the case, a sustained fishing effort may seriously affect fish populations in the adjoining natural waters.
|Area||Length of freshwater irrigation canals||Length of drainage canals||Total|
For many years the management of fisheries in natural rivers received little attention, the continued production from the fisheries being somewhat taken for granted (Welcomme, 1979). However with the increasing competition for use of water resources for agriculture, industry and domestic purposes riverine fisheries resources are being affected and in many cases production is falling. Accompanying this is the increased pressure on the fish stocks to provide food for an ever expanding human population.
Management of the fish stock may involve restocking, introduction of new species, and the imposition of regulations to control fishing effort. In Egypt, grass carp stocked for weed control in the canals are harvested regularly and replaced with juveniles, thus controlling the weeds and producing a substantial fish crop. To prevent the possibility of the removal of the fish before weeds had been effectively controlled, a number of restrictions were placed on the fishermen. These included allowing the removal of fish only after the growth season of weeds, the prevention of harvesting more than the previous season's production, and the planned restocking of juveniles. Fishermen also replaced carp smaller than 30–40 cm in length and guarded their own fishing areas (Van Zon, 1984).
Fish introductions have to be viewed with some caution since there are numerous examples of introduced species totally altering the structure of the natural fish community. If there are vacant niches to exploit within the canal system then introductions may be beneficial to the fishery.
Established methods of stock assessment and management for riverine fisheries could be used in irrigation systems, but the reservations about estimates of productivity (see section 6.1) should be borne in mind. The assessment of fish stocks and the prediction of catch is difficult in an unstable ecosystem, especially when such systems are affected by the activities of man. Holcik (1979) suggested several arbitrary methods for assessing fish stocks in rivers and indicated that the choice of method depends on the flow rate, the type of bottom and banks, the obstacles to the flow and the degree of navigation operating in the river.
A degree of management of fish stocks in irrigation canals will be essential in most systems in order to maintain a sustainable fishery. However, irrigation schemes often cover vast areas and consist of thousands of kilometres of canals, making effective fishery management a difficult task.
In many situations aquaculture may prove more appropriate than a managed fishery, because of the greater control possible over production and the confined nature of the production systems. Far more fish per unit area can be produced through aquaculture than through fisheries. The average production of fish from the oceans is less than 2 kg/ha/yr and from lakes 200– 250 kg/ha/yr (Hepher, 1985), whereas the potential production from intensive aquaculture, even in low technology culture systems, can be as high as 1 500–2 000 t/ha/yr (Hepher and Pruginin, 1981). Pantulu (1980), reviewing the potential for aquacultural fish production in the irrigation systems of the Lower Mekong Basin, predicts yields of 6 t/ha/yr (this, however, is from conventional pond culture in areas unsuitable for crop production, rather than in the irrigation canals).
Aquaculture systems are broadly classified as extensive, semi-intensive and intensive on the basis of the financial, technological and managerial inputs to the system. Extensive aquaculture relies on natural food sources, primarily plankton and algae, as food for the fish. Natural populations of fish may be confined, or the numbers of a particular species enhanced by low-level stocking of the water body, and growth of the food source may also be encouraged by limited fertilization of the water. Management and financial inputs are small.
In semi-intensive culture, enclosures are stocked with seed fish, and other species are discouraged. Food is provided, in the form of low protein (10–30%) feedstuffs, to supplement the natural production of food stimulated by the use of fertilizers. Management inputs consist of fertilization, stocking seed fish and regular supplementary feeding. Financial inputs mainly consist of seed fish and low-grade feed. The costs are not high.
Intensive culture systems represent complete control by the farmer over all aspects of the production cycle. Other fish species are eliminated from the enclosure prior to high density stocking with the culture species. Complete, premanufactured feeds are provided. These are high (30–50%) protein diets, usually based on fishmeal. Additional management inputs often include grading of the fish during the production cycle, into groupings of similar sized fish, to enhance the efficient management of the stock. Financial inputs are high, feed usually being the greatest drain on resources (up to 60% of production costs), and, depending on the species being cultured, seed costs can also be significant (Beveridge, 1984).
For each method of production there are various advantages and constraints which should be borne in mind before deciding on the most suitable system. In the context of irrigation canals, cages, pens or larger enclosures are the most appropriate forms of aquaculture system. Enclosures could be extensively or more intensively managed, but for cages and pens only semi-intensive or intensive management practices could be considered in most situations. These options are examined in more detail in the following sections.
Cage culture of fish may have been practised in Kampuchea as long ago as the nineteenth century, when fishermen constructed rattan cages or baskets to hold Clarias spp. awaiting transport to market (Pantulu, 1979). This practice spread to Vietnam, and Thailand, and cage culture of fish, and more recently prawns, is now practised throughout the world (Beveridge, 1987). Now, more than 70 species are cultured in cages of various shapes and sizes in both flowing and still waters (Coche, 1978) (see Table 16). Cages throughout the world are constructed from various materials depending on the availability and cost. The life expectancies of a range of materials are shown in Table 17.
The advantages of using cages in aquaculture can be grouped under two headings, resources and management. In terms of the former, cages are attractive because they make use of existingbodies of water, and may be complimentary to existing activities. For example, in Malaysia and Singapore cages are now being used for cleaning up eutrophic waters through the culture of planktivorous fish species in cages (Beveridge, 1984), and in Indonesia floating cages are used to produce carp at high densities in many of the country's reservoirs. In the same country, cages, placed in canals, are used for fish production, for example, in central Java.
Cages also act as fish aggregating/attracting devices (FAD's) which could have a beneficial effect on the natural populations of fish in irrigation canals, and so complement any concurrent fishery operations. There are several consequences of the presence of FAD's which are beneficial to fish populations. These include:
creation of shadow areas where zooplankton become visible,
shelter from predators for small fish,
presence of smaller fish attracts larger fish,
provides a substrate for plant and animal growth and eggs.
Cages can be made very cheaply, using widely available materials, such as bamboo for the main structure and plastic chemical drums as floats. Certain types of netting material may also be purchased at comparatively little cost. Thus, cage farming is very appropriate to the small-scale farmer.
|Species||Countries||Climate||Type of feeding||Lotic/Lentic||Cage/Pen|
|Salmonids||Rainbow trout||Europe, North America, Japan, high altitude tropics (eg Colombia, Bolivia, Papua New Guinea)||Temperate||Intensive. High protein (40%)||Lentic||Floating cage|
|Salmon (various species) smolts||Europe, North America, South America, Japan||Temperate||Intensive. High protein ( 45%)||Lentic||Floating cage|
|Carps||Chinese carps (silver carp, grass carp, bighead carp)||Asia, Europe, North America||Temperate - tropical||Mainly semi-intensive, although also extensive (Asia) and intensive (Europe, North America)||Lotic and lentic||Cages and pens|
|Indian major carps (Labeo rohita)||Asia||Sub-tropical - tropical||Semi-intensive||Mainly lentic||Mainly cages|
|Common Carp||Asia, Europe, North America, South America||Temperate - tropical||Mainly semi-intensive, although also intensive||Mainly lentic||Mainly cages|
|Tilapias||(O mossambicus, O. niloticus etc)||Asia, Africa, North America, South America||Sub-tropical - tropical||Mainly semi-intensive, although also intensive||Mainly lentic||Mainly cages|
|Catfishes||Channel catfish||North America||Temperate - sub-tropical||Intensive||Lentic||Floating cages|
|Clarias spp.||Southeast Asia, Africa||Tropical||Semi-intensive||Lotic and lentic||Floating cages|
|Snakeheads||Channa spp. Ophicephalus spp.||Southeast Asia||Tropical||Semi-intensive/ intensive||Lotic and lentic||Floating cages|
|Pangasius spp.||Southeast Asia||Tropical||Semi-intensive||Lentic||Floating cages|
|Material||Life expectancy in fresh waters (years)|
|Bamboo and logs||1 – 2|
|Metal drums||0.5 – 3|
|Used plastic drums||1.2+|
|Styrofoam - covered||5+|
|- not covered||2+|
|Spherical buoys - aluminium||10+|
1. Polystyrene filled
On the management side, cages characteristically have good water exchange allowing high stocking densities. This enhances the low capital costs per production unit, making them very cost effective. Having a large number of fish in a small enclosure has a number of management advantages. One, obviously, is that only a small area, relative to many other culture systems, is required to produce a unit volume of fish. Investment costs are often related to land/water area (land purchase, licenses etc.), so this is a positive advantage. The stock in cages can be much more easily managed than in a pond or lake system - feeding, grading for size, disease identification and treatment, and harvesting can be much more efficiently carried out.
Cages are also much more flexible than land-based facilities. For example, they can be easily moved to another location, either within the same water body or to a different one, to provide protection from pollutants, predators etc.
There are, of course, dissadvantages to fish culture in cages, and Balarin and Haller (1982) recognised several limitations which must be considered before fish cages are introduced to any water body.
These limitations include the need for adequate water exchange through the cages to remove metabolites and maintain oxygen levels. Different species of fish will consume oxygen at different rates depending on their lifestyle. Carnivorous species are active feeders and use up a greater amount of oxygen compared with slower moving benthic or filter feeders. In some canals the need for sufficient water exchange through the cages will restrict their number. This type of system will also preclude the culture of benthic feeders since it is necessary to raise the cages off the substrate to allow for the current to remove waste metabolites from the site.
In many canals water flow varies according to the season, and this may affect food conversion ratio's (FCR), survival rates and stocking densities. Beveridge (1987) suggests an optimum flow of water through marine cages to be 0.1 – 0.6 m/s and not exceeding 1 m/s. In higher currents there is a tendency towards deformation of the net sides of the cage which will reduce the volume of the cage to unacceptable proportions and fish will have to expend more energy in maintaining their position (Okabayashi, 1958; Beveridge, 1987).
Cages may create a reduction in the flow in the canals which in turn affects the efficiency of the irrigation system itself. In Indonesia this problem has been overcome by setting the cages within the floor of the drainage canals thus reducing the effects of the cages on the flow yet still obtaining a sufficient flow of nutrient rich water through the structure (Costa-Pierce and Effendi, 1988) (Table 18).
|Stocking density Fish/ha||Low (2–5 000)||Moderate (30–100 000)||High (200–500 000)|
|Feed types||Natural food only||Supplementary low cost (bran, manure etc)||Complete commercial 20–25% protein|
|Management||Stocking, harvesting||Stocking, feeding, harvesting||Stocking, feeding, water quality and quantity, stock manipula- tion, harvesting|
|(Rp/ha)||(20–50 000)||(100 000–5mill)||(500–700mill)|
|(Rp/ha/crop)||(40–100 000)||(500 000–2mill)||(150–200mill)|
|(Kg/ha/yr)||(100–300)||(1 000–5 000)||(100–250 000)|
|Net profit||Profitable Established, growing||Variable According to season||Unproven Many bankrupt in 1987|
1 100Rp = US$1
Other potential problems involve the possible spread of disease from natural fish populations which are attracted to the cages and the transferal of disease organisms from cage to cage. In Malaysia the occurrence of metazoan parasitic infections on Puntius binotatus in irrigation canals was found to be highest during the wet season. This period unfortunately coincides with the period of peak stock density, and therefore disease problems are intensified (Leong, 1986).
Cages are also prone to rapid fouling by algae, especially in nutrient rich waters. Fouling will reduce the water exchange and may also add to the degree of net deformation in stronger currents. Both these factors are detrimental to the health of the fish. Excessive fouling increases labour demands, and reduces the carrying capacity of cages.
Extensive and semi intensive culture in cages
Extensive and semi-intensive culture is better suited to the tropical fresh water areas of the world, because of the high levels of primary production in these waters (Brylinsky, 1980), and it has been shown that there is a positive correlation between fish yield and primary productivity (Oglesby, 1977, 1982; Marten and Polovina, 1982). Extensive and semi-intensive culture is also limited to benthic, planktivorous, detrital and drift feeders, since carnivorous species have high protein requirements which cannot be met by primary production alone and other species have specialised mouthparts not suited to this type of food (Beveridge, 1984).
Waters with a high organic loading are also suitable for this type of culture (Costa-Pierce and Effendi, 1988). Since the productivity of the water body is dependent on the availability of essential nutrients (particularly phosphorus), light and temperature, the greater the nutrient loading the greater the productivity of the water, and the higher the allowable stock density (unless a high level of supplementary feeding is planned).
Suitable species for extensive culture would be planktivores, omnivores, and detritivores. Macrophyte feeders would have to be provided with supplementary feed since cages are free of the substrate and aquatic macrophytes would therefore be unavailable to fish grown in them. Particularly high production (for a non-intensive system) can be achieved with some species when cultured in cages. Beveridge (1984) notes that production of tilapia in cages by extensive culture can be as high as 1.9 kg/m3/month (Table 19), although the same author does not foresee this level of production being sustainable for long periods of time. Calculations for appropriate fish stocking densities based on available oxygen and food supply are shown in appendices 2 and 3.
In Thailand, extensive culture of bighead carp, grass carp and Nile tilapia in irrigation canals used for vegetable crops has been fairly successful (Little and Muir, 1987), although the mortality rate of the carps was high at 40%. This could be attributed to pesticide residues and high temperatures in the canals (Edwards et al., 1986), which is likely to be a common problem for aquaculture in irrigation systems.
|Cage size (mm3)||250||150||4500||900||150–1000||2500|
|Stock density no/m3||10||50||50||1.6–2||4–8||4|
|Size stocked (g)||1||-||-||12–16||1||-|
|Culture time (mnth)||5||4||4||6–9||4–5||4|
|Size at harvest||200||160||100||225–300||100||250|
A - Oliva (1983)
B - Bisuna (in Beveridge 1984)
C - Guerrero (1983)
D - Guerrero (1983)
E - Mane (1979)
F - Alvarez (1981)
Intensive culture in cages
This type of culture relies almost entirely on the use of complete feeds, since the natural productivity of the water would not support the characteristically high stocking densities employed in intensive systems. There has been little interest, however, in the commercial production of complete feeds for low value fish (Beveridge, 1984). This is mainly attributable to the high cost of the food in relation to the low market value of such species. Feed costs can account for up to 60% of the total operating costs. Therefore, intensive culture in cages is often restricted, for economic reasons, to those species which command a higher market price.
The flow rate through a cage can be critical to the survival of the fish in the enclosure. Oxygen requirements may vary with species, size and stage of development and may well exceed that provided by canal water alone. The problems associated with insufficient supplies of oxygen include, lowered food conversion ratios, poor health and a decrease in the feeding behaviour. The amount of oxygen in the water decreases with increasing temperature and salinity. It is also affected by the amount of photosynthetic activity occurring during the day (producing oxygen) and the amount of oxygen required by plants for respiration (both day and night). It is possible to calculate the stocking density with knowledge of the flow rate, the oxygen content of the water, and the oxygen requirement of the fish. The calculation (Beveridge, 1984) is shown in Appendix 2.
Consideration of the oxygen budget of the system would be particularly important if fish were to be cultured in a series of cages along the length of a canal. The first cage would receive fully oxygenated water, whilst those downstream would receive water with less oxygen, and more carbon dioxide and metabolic wastes. In a typical irrigation canal there would be very little opportunity for re-oxygenation of the water, and conditions would quickly become unsuitable for fish culture. Aeration equipment (e.g. paddle-wheel aerators) would therefore be required.
The culture of carp extensively in the drainage and sewage canals in Indonesia has been successful for many years. This system relies almost entirely on the natural food web to provide food for the carp. This practice is rather an exception to conventional cage culture which usually relates to a system relying on formulated feeds, high capital investment and little contribution from natural production.
Vaas and Saachlan (1956) reported the growth of carp in cages in canals running through the centre of Bandung, West Java. This practice was thought either to have originated either in Japan, or from the practice of holding fish in the canals before sale, during which time increases in weight were noticed. In this particular study, the canals were 4–5 m in width with a minimum water depth of 30–40 cm, and acted as an open sewer for human and kitchen wastes. The rectangular cages were stocked with common carp 8–12 cm long. These fish reached weights of 50– 75 kg in 2–3 months with little supplementary feeding, usually no more than some rice bran or stale bread. Feeding in the cages tended to be opportunistic with the smaller fish consuming species of chironomid larvae and Oligochaetae. These organisms are found in large numbers in areas of high organic deposits in both tropical and temperate waters (Pearson and Rosenberg, 1978; Gowen and Bradbury, 1987; Redding, 1988), and may form part of the natural diet of many omnivorous and detritivorous fish.
Later studies by Costa-Pierce and Effendi (1988) on cage culture in Cianjur, Indonesia, indicated that the present method of producing carps in cages in canals was both financially successful (Table 20) and did not interfere with the flow rates in the canals. Initially, however, the cages were floating, but by the late 1960s irrigation authorities noted that not only were the flow rates of water to the paddy fields affected but also the number of cages in the system was so large that they were causing flooding during the rainy season. From the ban on floating cages in the early 1970s evolved the in-bottom bamboo cages which are used today.
Cages of 3 x 4 m are dug into the bottom of the canal and left for a period of six months, during which time accumulated debris is occasionally swept off the top of the cage (Figure 9). It was found that the cage served three functions; the production of carp, collection of sand (which has a commercial value), and the direct processing of sewage. Fish were stocked at 1 kg/m3 at a size of 8–10 fish per kg. In six months they had reached a size of 1 kg, and thus two crops a year can be grown. One cage can produce 60 kg of fish per year without addition of food, and also produces 1.5m3 of sand a week during the rains. The economic analysis is shown in Table 20 with a return over total costs of I.RP. 40 950. It should be noted, however, that this system operates in an environment subject to exceptionally high organic loading, which largely accounts for its success.
|Economic variable||Unit||Cost per unit||Quantity per yr||Total Cost|
|a. Fish prodn.||kg||1600||60||96000|
|Total gross receipts||192000|
|a. Fingerlings (100–125g)||kg||1800||20||36000|
|d. Repair and Maintenance||36350|
|@ 50% Fixed costs|
|Total variable costs||78350|
|c. Wood 5×7×5m||Board||1700||1||1700|
|Total Fixed costs||72700|
|Return above variable costs||113650|
|Return above total costs||40950|
Costs are in Indonesian Rupees (Rp)
Figure 9 Submerged cage used in Cota Cianjur, Indonesia (Costa-Pierce and Effendi, 1988)
Much of the information relating to fish production in irrigation canals from aquaculture, little as it is, comes from recent experimental work in Egypt. An interesting example are investigations carried out in the Nile river and associated irrigation canals.
In 1983 it was estimated that 13% of the protein intake of the population of Egypt was fish protein (Jauncey and Stewart 1987). With a population of 50 million (1986), rising to 70 million in the year 2000 (a population growth rate of 2.6%), and a per capita consumption of fish of between 4.7 – 6.2 kg/yr, by the year 2000 there is likely to be a shortfall in fish production from inland and maritime fisheries of 450 000 tonnes per year (Jauncey and Stewart, 1987).
Fish production from aquaculture in 1986 was estimated to contribute 106 000 tonnes to the total Egyptian fish production (Sadek 1988). Traditional areas for production include extensive ‘howash’ enclosures, seepage ponds, irrigation systems and village ponds. The average fish yield varies between 760 – 2 500 kg/ha depending on the intensity of the operation. In addition, cage culture of mullet and tilapia in Lake Quarun and the governmental fish farm near Lake Manzala are expected to produce 1 300 kg/ha/yr (Sadek, 1988). However if demand for fish and the rate of population increase continues there will be increasing pressure to increase aquaculture production, since the maximum sustainable yield (MSY) of the fisheries sector has already been exceeded.
Egypt possesses approximately 50 000 km of irrigation and drainage canals, of which approximately 3 532 km are of a suitable size for aquaculture. The potential for some form of aquaculture in these waters is considerable according to Jauncey and Stewart (1987).
However, a major constraint to large-scale development lies in the government ban of the use of agricultural land and fresh water from the Nile irrigation canals for commercial aquaculture. Brackish and saline drainage waters are available for aquaculture, but these waters have many associated risks, including the runoff of pesticides from agriculture and pollution from industrial and mining activities.
An experimental cage culture system has been set up in the Nile river and irrigation canals in the El Behera Governorate, approximately 100 km from Alexandria (Nour, pers.comm.). The project aims to produce tilapia, common carp and grey mullet in polyculture, and catfish in monoculture systems. The cages are set in the Nile river since the main supply canals are not available for use at this stage, due to concerns over the possible effect on flow rates.
The cages are made by farmers, from local materials such as bamboo, and are stocked with fingerlings donated by the University of Alexandria, who are involved in running the project. Fingerlings of 20–30 g are stocked at a density of approximately 10 kg/m3 in March/May (i.e. 500–700 fingerlings per 2 m × 4 m × 2 m cage). The species composition of each cage is: carp (60%), tilapia (10–20%), and grey mullet (10%). These are fed a pelleted diet containing 25% protein at a rate of 2% body weight weekly (at a cost of LE. 300 per tonne).
The fish reach market size (tilapia 200 g, carp 500 g, grey mullet 150 g) by October/November, at which time they are harvested. The main constraint to this project is the high mortality experienced in the capture and transportation of fingerlings from the earth-lined hatchery ponds.
The number of cages in this area has risen from 10 to 600 in three years and are very popular with the local farmers, who can expect to obtain an average profit of US$ 200 from an initial outlay of US$ 80.
Ishak (1982), and Ishak et al. (1986) described similar experimental culture of Oreochromis niloticus in the Nile irrigation canals. Cages with a volume of 3m3 were fixed in running and static water canals and stocked with 100 individual fish of approximately 30 g. After feeding at a rate of 5% body weight per day for 150 days, the fish in flowing canals had reached a weight of 152 g, whereas the fish in static water had only reached 76 g in weight. The same authors also showed that using 4m3 cages two crops of fish could be obtained per year with a growout period of 105 days. In this example a total production of 40 kg/m was obtained in a period of seven months (equivalent to 700 t/ha/yr). Approximately 100 tonnes could be produced per annum from an area of 1 ha of cages (each 12–16m2).
In Thailand experimental work has been carried out in producing the freshwater prawn Macrobrachium rosenbergii in different culture systems. Since the construction of dams for irrigation purposes the wild stocks have declined as the natural migration and spawning routes were obstructed, and the waters became increasingly polluted with both industrial and agricultural effluents (Menasveta & Piyatiratitivokul, 1982). The increasing market for prawns has stimulated interest in the possible use of alternative culture systems for ongrowing this species.
Nylon-screen net cages (2 m × 3 m × 1.8 m) (Figure 10), with a mesh size of 16/cm2 were submerged in an irrigation canal at a depth of 1.2 m in the Rangsit irrigation area. The other systems used for comparison were an earthen pond (30 m × 30 m) with a depth of 1.5 m, and a long ditch in an orchard (1.2 m × 100 m × 1.2 m, depth 0.9m).
Figure 10 Schematic illustration of a simple fixed cage used for prawn culture in canals in Thailand Stocking density at the start of the trial was 5 six-week old prawns/m2. They were fed a compounded diet at the rate of 5% body weight per day, containing 40% protein, 20% carbohydrate, 15% fat, 20% ash, and 5% moisture. The results showed that after 6 months, although the growth rate in the ponds was greatest, the survival rate was highest in the cages (Table 21). Production was highest in the earth ponds, at 210 kg/rai (0.16ha). The cages performed second best, with 138 kg/rai, and production from the ditch was 78 kg/rai. This suggested that the culture of prawns in irrigation canals is economically viable despite the high turbidity of the water. However, usually such canals are used for transport and the water level is not kept constant. This system warrants further economic analysis in terms of the possible increased production from canals which have already been constructed, as opposed to the high cost of excavating earthen ponds. In addition, if cooperation between the irrigation authorities and the aquaculturists could be attained, to remove the constraint to aquaculture of water level fluctuations in the canals, then the full production potential of the canals could be realised.
Hiranwat et al. (1985) stocked irrigation canals with grass carp and Puntius gonionotus, as a weed control measure. After four months the fish were harvested. The average individual weight of the grass carp had increased from 48 g to 631 g in this period, whilst that of the Puntius had risen from 23 g to 81 g.
An alternative to cage culture, is to produce fish in a more extensive system, in which the canal is blocked off at intervals by barriers to form a series of pens. This system may best be utilized for polyculture, as this has been shown to be the best technique to make full use of the natural resources in an extensive or semi-intensive system (however, it is possible that an irrigation system may not exhibit the diversity of niches required for efficient polyculture). In such a system, inputs are minimal, comprising seed fish, regular monitoring of the site for water quality and predators, and probably some supplementary feeding (although this latter will be dependant on the natural productivity of the system, and the stocking densities employed). The carrying capacity can be calculated using the same equation as that for caged fish, ignoring the allowance for the transmission factor (see Appendices 2 and 3) if the pen stretches the whole width of the canal. However it must be stressed that every canal system is different, and the water quality and oxygen content should be monitored carefully before any decisions on species and stocking rates are made.
|Culture systems mnths||Initial stocking||Survival||Survival (%)||Production (kg)||Production in k g / r a i 1 / 6|
1. 1 rai = 0.16 ha
Whilst pens and cages are similar in that the sides of the enclosure are man-made, pens differ in that the base of the enclosure is the substrate itself, rather than an artificial structure such as a net or wooden mesh. Pens have certain advantages over cages, perhaps the principal one being access to benthic organisms, providing an additional food source (Beveridge, 1984). They do, however, suffer a major disadvantage in that they are difficult to harvest. Pens generally are larger than cages, and are less suited to intensive culture.
Low-cost materials such as bamboo stakes or woven rush mesh are used as barriers to prevent the escape of larger fish. However, to prevent the entry of predatory species it is necessary, in the earlier stages at least, to use small meshed netting. In fact, it is safer to use netting throughout the culture cycle, as bamboo walls etc are more likely to be breached.
In addition to the growout of fish, pen culture could be useful for nursing young fish as part of a stock enhancement programme. After raising them for a few months in an enclosed area of canal, the fish could be released to the rest of the system once large enough to escape potential predators.
Irrigation development began with the first 5-year plan following the 1949 revolution. This concentrated on reservoir construction, and by the time the cultural revolution of 1966– 1969 began, both agricultural and aquacultural development had expanded considerably (Tapiador and Coche, 1977).
The total inland water surfaces in China cover 17 million ha, approximately 1.8% of the territory, and there are 5 – 6 million hectares used in fish production, 60% of this being ponds and 40% reservoirs and lakes. Ten million hectares are dedicated to fish production in the paddy fields.
One of China's most productive agricultural areas lies in the flat delta of the Zhujiang (Pearl River) which covers an area of 12 000 km2 south of Canton. It is also one of the most populous areas having a density of 17 persons per hectare of cultivated land (Ruddle et al., 1983; Ruddle and Gungfu Zhong 1988). Since the reclamation of the swamplands almost six centuries ago, through a process of gradual reclamation from wetlands to ponds surrounded by dikes, the economy has been based on a fine balance between terrestrial and aquatic ecosystems.
The main part of the delta is crossed by numerous canals which serve primarily as transport systems. The integrated system of intensive agriculture and polyculture of carp, and other freshwater fishes, covers an area of 800 km2 and supports a population of 1.2 million people. In 1979 there was an estimated 2.74 million ha of inland waterways in this region, and the total output of freshwater fish was 1 115 900 t, of which 813 300 t was cultured.
The majority of cultured fish (60%) are produced in the ponds on the lowland Yangtse and Pearl river areas, although the greatest quantity of fish comes from the fisheries of the canals and lakes of the Yangtse delta (Solecki, 1966). Pond water, high in nutrients and plankton, is discharged into the irrigation canals and contributes towards the fertilization of crops (mainly mulberry trees, used in silkworm production). The water is then returned to the pond, enriched with cocoon waste and dead silkworm larvae which provide a nutritious fish food.
This is a very simplified version of the cycle of events in this integrated system. Other crops and livestock are also involved in the cycle, and the energy relationships operating in this mulberry-dike-fish system are very complex. Livestock and human excrement is also used as fertiliser for both crop and fish production. Because the techniques and technology used in such integrated systems are not well documented, the levels of productivity and the economic status of integrated farming systems is poorly understood (Ruddle and Zhong, 1988).
However it is known that many irrigation systems in China now produce 300–350 kg/ha/yr of fish from extensive aquaculture using pens in canals. This system is particularly refined in the Zhujiang delta region, where fish production can be as high as 1350 kg/ha/yr. The irrigation system consists mainly of gravity flow canals, with supplementary pumps. The canals are found, to a great extent, in the low-lying plains and have flow velocities of 0.1–0.3 m/s. They are used for irrigation, drainage and navigation. Rice is the main crop and water is available throughout the year, which is an advantage for fish culture in the canals.
Fish culture is not carried out in the main supply canals, because of irregular flow rates and periodic maintenance work which involves draining. Other characteristics of the supply canals which would restrict fish culture are the high flow velocities, and the problems which may arise if there are many pens to impede the flow.
Culture of aquatic weeds such as water hyacinth, duck weed and azolla is carried out in some of the smaller canals, these products being used as fish foods. According to Tapiador and Coche (1977) the conversion ratio in the case of grass carp fed on aquatic weeds is in the region of 35:1.
Only in certain provinces, e.g., Zhujiang (Guangdong), are canals used extensively for polyculture of carps. The species used are listed below:
|Grass carp||Ctenopharyngodon idella|
|Silver carp||Hypophthalmichthys molitrix|
|Bighead carp||Aristichthys nobilis|
|Black or Snail carp||Mylopharyngodon piceus|
|Mud carp||Cirrhinus molitorella|
|Common carp||Cyprinus carpio|
In the Zhejiang (Shaoxing) province the culture of fish in the navigable irrigation canals is a traditional practice. The cultivated area served by the irrigation canals in this system is 12 693 ha. The canals (3 m deep) are blocked off every 40 – 50 ha by a bamboo and net fence, which can be moved to allow the passage of boats. Management practices include general maintenance, annual stocking, controlling predators by intensive fishing prior to stocking, regulating the minimum size of fish that can be taken from the stock, and harvesting the majority of the stock during winter.
The main species grown are bighead carp (60%) and silver carp (40%) and where there is a weed problem grass carp are also used. The fish are stocked in spring to summer at a density of 1 800 fish/ha at 15 g or 900 fish/ha at 250 g. The yield from this system ranges from approximately 300 kg/ha to 1 350 kg/ha per year. There is also potential in the canals for the culture of freshwater pearl mussels.
Almost 70% of fish produced in Shunde county are sold live to Hong Kong, Macao and Guangzhou and contribute the largest amount of income to the area. In total the Zhujiang Delta produces 80% of China's live fish exports, amounting to 90 000 tons/year (1979).
Daget (1976) suggested that, in Africa, a suitable species for culture in the slower flowing canals, which may be prone to low oxygen levels is Clarias lazera, as it is a facultative air breather. Semi-intensive culture in the tropical regions is based on species which feed on material low in the food chain such as planktivores, omnivores, detritivores, phytophagous and benthic feeders. These species include tilapias (Oreochromis niloticus, O. mossambicus, O. aureus), silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis) and common carp (Cyprinus carpio) (Beveridge 1987). Most work has been done using freshwater species although the potential for culture of siganids and mullets in brackish water should not be overlooked. In the Limcanal, Yugoslavia, mussels and finfish have been successfully reared and ongrown, although in this system good water quality was essential for mussel culture (Loix 1987).
Commercially cultured species in cages and pens include carps, tilapias, catfishes, snakeheads and pangasiids (Table 16), all of which may be suitable for culture in irrigation canals. They are all very tolerant of conditions of poor water quality (particularly high suspended solids and low dissolved oxygen), and their culture is well established in various parts of the world. Tilapias are perhaps the most versatile species, especially where derelict, high-salinity waters are concerned, although its escape into the canals and associated waters could be damaging. It is also, along with Chinese and Indian carps, more suited to extensive and semi-intensive production techniques. Snakeheads and the catfish are carnivorous and would, therefore, require more expensive, high protein feeds. Freshwater prawns (Macrobrachium spp.) should also be considered for pen culture.
The possibility of using indigenous fish, rather than the ubiquitous tilapias or other introductions, should never be overlooked.
There is considerable potential for fish production in irrigation canals which have suitable flow rates, water quality and depth, especially in tropical areas where primary productivity is higher (Brylinsky, 1980). Many water bodies such as dams, lakes and rivers destined to become part of irrigation schemes, have been extensively studied in terms of productivity and ecology. However the canal systems themselves have been given little attention in terms of their potential for fish production. There are already several examples of integrated fish production in natural and man-made water bodies (see Figure 11), but the potential of canals for fish production has received little investigation.
Capture fisheries may prove difficult to manage in some irrigation systems, and aquaculture is thought to be a more practical option. Since primary production is the basis for all successive energy flow in the aquatic food web, then extensive and semi- intensive culture in the tropics may be more feasible in canals with a high eutrophic status. Whilst intensive fish production does not rely on natural productivity, the cost of complete feeds often renders intensive systems uneconomic, except for high value species.