IRRIGATION

Case studies - cage culture in irrigation canals

Indonesia/Java

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.

Table 20 Economic Analysis of Sewage Cages at Cianjur. (Costa-Pierce and Effendi, 1988)

Economic variable	Unit	Cost per unit	Quantity per yr	Total Cost
Gross receipts
a. Fish prodn.	kg	1600	60	96000
b. Sand	m3	2000	48	96000
Total gross receipts				192000
Variable costs
a. Fingerlings (100–125g)	kg	1800	20	36000
b. Feed	kg	480	0	0
c. Labour	Man-days	2000	3	6000
d. Repair and Maintenance				36350
@ 50% Fixed costs
Total variable costs				78350
Fixed Costs
a. Bamboo	piece	1500	40	60000
b. Nails	kg	1500	2	3000
c. Wood 5×7×5m	Board	1700	1	1700
d. Labour	Man-days	2000	4	8000
Total Fixed costs				72700
Total Costs				151050
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)

Egypt

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).

Thailand

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.

7.2.4 Pen Culture

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.

Table 21 Survival and production of the freshwater prawn in the three culture systems. (Menasveta and Piyatiratitivokul, 1982)

Culture systems mnths	Initial stocking	Survival	Survival (%)	Production (kg)	Production in k g / r a i 1 / 6
Pond	12000	5760	48.0	315	210
Cage	120	63	52.5	1.55	138
Ditch (Canal)	600	211	35.2	5.55	74

Notes:
^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.