This review concerns the potential for fisheries production from irrigation canals. Fish contributes significantly to the overall production of animal protein, especially in developing countries, e.g. Bangladesh, where fish protein accounts for 70% of dietary protein (Marr, 1986). Because of limited water resources new sources of fish are always under investigation. Irrigation schemes cover extensive, and continuously increasing, areas (Table 1) and the potential of their canals for fish production has so far been largely neglected, and there is consequently very little organised fish production in these systems. The aim of this paper is to bring the relevant information together and to outline some of the methods which may be applied to the production of fish in irrigation waters.
Production of foodfish from freshwaters can be achieved by various means. These all, however, fall into two broad categories - capture fisheries and aquaculture.
Capture fisheries constitute the exploitation of natural fish stocks, ideally at a level which allows a sustainable annual production from the resource. To prevent depletion of the fish stocks, in which case a consistent annual level of production could not be sustained, some form of regulation must be placed on the intensity of the fishing effort, either with respect to the number of fishermen (or their maximum allowable catch), or to that section of the fish population which can be exploited (to prevent depletion of the breeding adults, or the young recruits to the population). In addition, it may be desirable, or even necessary, to enhance the recruitment to the fishery by stocking the water body with artificially produced juvenile fish.
Levels of fish production from natural waters vary considerably. Influential factors include geographical location, size, fish species present, natural productivity of the water, degree of control over the fishery, fishing methods used, to name but a few. In reservoir and lake fisheries levels of fish production from capture fisheries can vary from 5 to almost 700 kg/ha/yr. The stock biomass, and therefore the productivity of fisheries in lentic systems, depends on the natural productivity of the system. This is influenced by a number of factors; principally, the nutrient levels in the water entering the system (a product of the natural fertility of the soil in the catchment, or the intensity of agricultural practices), the amount of allochthonous organic matter entering the system (which will slowly release nutrients to the system as it decomposes), and the residence time of the water within the system (a long residence time allows the accumulation of higher levels of nutrients in the water column, and thus greater levels of primary production).
| Year | |||||
|---|---|---|---|---|---|
| Continent | 1900 | 1950 | 1970 | 1985 | 2000 |
| Europe | 3.5 | 10 | 21 | 30 | 45 |
| Asia | 30 | 65 | 170 | 220 | 300 |
| Africa | 2.5 | 5 | 9 | 12 | 18 |
| North America | 4 | 13 | 25 | 32 | 35 |
| South America | 0.5 | 3 | 7 | 10 | 15 |
| Australia and | 0 | 0.5 | 1.6 | 2.2 | 3 |
| Oceania | |||||
* Areas given in millions of hectares
| Culture Method | Species | Yield kg/ha/yr |
|---|---|---|
| Extensive (enclosures)
| Mullet | 150 –ndash; ndash;300 |
| Shrimp (Singapore) | 1250 | |
| Semi- intensive
| Milkfish | 1000 |
| Carp | 125–700 | |
| Tilapia | 400–1200 | |
| Intensive
| Rainbow trout | 2million |
| Carp (Japan) | 1–4 million | |
| Shrimp (Japan) | 6000 |
Levels of fish production from rivers are considerably less, because of the much lower levels of primary production in these systems. This is related to the ephemeral nature of the floodplain habitat, on which most of the primary production within the river system is based. Additionally, in contrast to lentic systems, autochthonous production in rivers is constantly being swept downstream (in the form of plankton, decaying macrophytes, and nutrients released into the water column). Thus, the levels of nutrients at any one time in a lotic system, are much lower than in the more stable lentic systems. Fish production varies between different river systems, but the variation is not as marked as in lentic systems (the range being 11.08 –ndash; ndash;51.96 kg/ha/yr; Welcomme, 1985). This information relates to the floodplain fishery, and whilst the production per unit area is very low, the total production from some of the larger floodplains, such as Bangladesh and the lower Mekong, is considerable.
In aquaculture systems the culturist has a certain degree of control over the environment in which the fish are grown. Depending on the type of system employed, the farmer can vary the flow rate of water through the farm, to accommodate changes in stock biomass or water quality; by controlling stocking rates the most efficient use of the available water can be made; food can be provided for the stock to supplement or completely replace natural supplies; size at harvesting and time of harvest can be manipulated in response to market requirements; and much more.
Aquaculture systems can be classified broadly on the basis of increasing technical, financial and managerial inputs, into extensive, semi-intensive and intensive systems. Extensive systems usually comprise large earth ponds or penned-off areas of some larger water body. Intensive systems differ from extensive ones in the provision of feeds for the stock, and high stocking densities. Systems are characterised by short residence times of water in the system, whilst holding facilities consist of earth or lined ponds, concrete tanks or raceways, fibreglass tanks, or fixed or floating cages set within a larger water body. Semi-intensive systems lie somewhere between these. Typical levels of production from these various systems are; extensive - 250 kg/ha/yr; semi-intensive - 600 kg/ha/yr; intensive - generally over 1 000 kg/ha/yr, but up to 4 million kg/ha/yr (Bardach et al., 1973). Table 2 provides a little more detail on the potential levels of production from different systems of aquaculture.
With the increasing pressure on world water resources, especially in arid areas, there is a movement towards integration of water uses. This is especially true in the case of agriculture and fish production (particularly aquacultural fish production; Phillips, Beveridge and Clarke, 1988), which are in many ways complementary. There are, however, significant areas of conflict, particularly where pesticides and other agro-chemicals are concerned.
In recent years ‘Integrated Aquaculture’ has received considerable attention from aquaculturists and farming systems specialists alike. In some areas of the world integrated farming systems are highly developed and often make considerable use of the available water resources for fish production. In China for example a highly complex pond/dyke farming system has been developed (Ruddle, 1982; Ruddle and Zhong, 1988) in which pond culture of fish plays a significant part. In Indonesia, techniques of integrated rice/fish farming are highly developed and fish production from rice fields accounts for a considerable proportion of the country's total fish production from inland waters.
On the other hand, fish production in irrigation canals, which are often a conspicuous feature of agricultural systems throughout the world, has largely been neglected, and there is consequently very little information regarding such activities.
The reasons for this neglect of fish production in irrigation canals, its present status and its future potential form the basis of this review. Although reservoirs and other impoundments, and rice fields, technically form part of irrigation systems, fish production in these areas is extensively covered in other publications. This is, therefore, not considered in this review, which concentrates its attention upon fish production in the canals of irrigation systems.
