In many irrigation systems there are a number of inherent problems which act as constraints to fish culture, and fisheries production. These include water losses, salinisation, and sedimentation. In addition, normal water management practices in irrigation systems create problems for fish production. These mainly relate to fluctuating water flow, and canal drainage. There are also problems relating to prevailing farming practices - the use of biocides and fertilizers, and their effect on water quality. These factors are illustrated in Figure 7 and are discussed in the following sections.
Water loss from irrigation systems, apart from scheduled abstraction, is by seepage or evaporation. The rate of loss is affected by climate, soil type and the length of the canal. In unlined canals seepage will also be affected by the depth of the water table, and the velocity and discharge volume of the water.
Seepage from the main irrigation canals may constitute a considerable part of the overall water losses especially where the canals are unlined and in areas where soil percolation rates are high (White, 1978; Michael, 1978). For example, the losses due to seepage are reported to be as high as 30% in the alluvial plains of the Uttar Pradesh and Punjab in India (ICID, 1967), and in Australia losses of up to 9 – 18 mm of irrigation water per day are common in sandy sub-soil areas (Carruthers and Clarke, 1981). In the USSR losses from the Kara-Kum canal in the first year of operation were reported to be as high as 43% of the overall discharge at the head (Holy, 1976).
Figure 7 Schematic representation of nonpoint source pollution rom irrigated agriculture
Seepage rates from canals, ditches and farms in the Grand Valley, USA, were estimated to be the cause of a large percentage of the salt loading in the valley. Of this, 23% originated from the canals and ditches, 32% from the lateral (field) canals, and 45% from on farm seepage (Hyatt et al., 1970). Total seepage from the main canals is estimated at 0.932 m3/m2/day with an average seepage of 0.071 m3/m2/day. The rate of seepage from the smaller ditches and laterals is lower but the total amount lost per year is almost three times higher due to the large area covered by the smaller canals (Skogerboe et al., 1983). The relation of irrigation to the overall hydrology of the area is shown in Figure 8.
Conventional methods of reducing seepage are often of a chemical nature. These include colmation, used for sandy, porous soils. In this process the system is flushed with water containing small soil particles in suspension. These seal the interstitial soil spaces, reducing the porosity of the soil. Alkalinisation, a chemical lining technique, involves the addition of sodium salts to the soil. The soil swells once saturated by exchangeable sodium, so reducing its porosity (Michael, 1978). These methods are costly and beyond the means of many irrigation management bodies. Physical methods include lining the canals with plastic sheeting or concrete, which is also very costly.
The problems of evaporation in many arid countries are becoming acute, with the result that crop production suffers through shortages of water. In Sudan, for example, the daily evaporation rates in the vicinity of Khartoum were estimated to be 7.5 mm and 7.9 mm at Wadi Halfa. In Egypt the annual evaporation rate from Lake Nasser is 2 500 mm and evaporative water losses of 1 140 mm are reported from Burma, 1 597 mm from Guyana, and losses as high as 3 000 mm are reported from some regions of India (ICID, 1967; Holy, 1976). In Egypt it is estimated that the main canals draw on 15 000 million m3 of water during the summer months and that seepage losses at this time are in the region of 1 500 million m3 or 10%. As the water evaporates the concentration of salts in the remaining water increases and may cause stress to fish in the canals especially if being held at high densities.
In the smaller distribution canals the loss of water by evaporation and seepage could be reduced by use of a closed conveyancing system. This is costly and, therefore, uneconomic in many countries. However, as the intensity of agricultural practices, and the value of the crops increases, so closed water conveyancing systems become increasingly popular. Thus in many of the industrialized countries, irrigation systems are largely comprised of piped water distribution networks, which are unsuitable for fish production.
Figure 8 Mean annual flow diagram of the Grand Valley hydrology (Skogerboe et al., 1983)
An alternative method for reducing evaporation is to design deeper and narrower canals, and screen them with on-bank vegetation. Plants provide shade and so reduce surface evaporation (Carruthers and Clarke, 1981). Overhanging vegetation also improves the environment of the canal for fish, and allows the development of a humid micro-climate over the water surface, further reducing evaporative water loss.
High rates of water loss from irrigation systems, be they from seepage or evaporation, create an unpredictable and unstable environment, which will have implications for fish production. Intensive aquaculture would suffer more than a natural fishery in this situation because of the dependence of the stock on the maintenance of a consistently high flow rate through the nets or pens. In some situations there is a risk of the irrigation system drying out at certain times of the year, particularly when seepage rates are high and flow rates low. In addition, the increase in temperature (if the water levels fall) would create quite severe physiological stress to some less tolerant species of fish. Fish production should be planned in coordination with such events, by having, for example, short production cycles.
In waters suffering from a high rate of evaporation the water quality will suffer through the concentration of salts, heavy metals, and other pollutants. In severe cases this may prove restrictive to the growth and health of certain species of fish - once stressed fish are more likely to succumb to disease organisms in the water. There are species of fish, however, which are tolerant of such conditions. Tilapia are perhaps the best example, so although the choice of species is limited, fish production is still possible.
Aquaculture or fisheries operations in these situations are necessarily of a high risk nature, although fish production could be used to offset the cost incurred by the water loss, or to contribute towards the cost of reducing water losses.
Narrower canals may create problems for cage culture as the cages would be more likely to restrict water flow if they stretched across the width of the canal (a ‘narrow’ secondary canal may only be one metre in width). Conversely, narrow canals have to be deepened to carry the same volume of water and deeper canals are better for cage culture of fish (secondary canals are normally no more than one metre deep). Here, cages could be suspended above the bottom, reducing the potential effect of anoxic sediments on the cage environment.
Poor water management in irrigation schemes can often lead to salinisation of the soils. Waterlogging, through overwatering, is a common result of mismanagement, and can lead to a rise in the water table and a subsequent increase in salinity and alkalinity of the soil. Alkali or sodic soils, as badly affected soils are known, contain sufficient exchangeable sodium to interfere with plant growth (Donneen and Westcot, 1984). This, in turn, results in an increase in the alkalinity of the surface waters - often to a level restrictive to fish growth.
It has been estimated that about 50 million acres of the world's irrigated land is affected by salinisation, and in the USA 20–25% of all irrigated land suffers from salt-caused yield reduction (El Ashry, et al., 1985). In India an estimated 10 million hectares, of the 40 million hectares irrigated in 1978/9, were affected by waterlogging (Michael, 1987).
Saline soils are unproductive in terms of agriculture, and in time the land becomes unusable. This is true of many areas of India where such soils are called “Usar” meaning barren (Michael, 1978). In Rajasthan, for example, out of an area of 1.1 million hectares under well-irrigation, 57% is affected by salinity and akalinity (Paliwal, 1972). In the Grand Valley, USA, the estimated net salt loading due to irrigation seepage and on-farm percolation in the valley ranges from 450 000 to 800 000 tonnes annually (Hyatt et al., 1970). This is thought to be half the total quantity of salt entering the Colorado river system each year (Riggle and Kysar, 1985). It is estimated that in China the area of land now subject to salinisation is 2.7 x 106 hectares, in the provinces of Hebei, Henan and Shangdong, all of which are supplied by the Beijing-Hangzhou Grand Canal, which runs for 1 150 km. With seepage from the canal increasing it is estimated that up to 4.7 x 106 hectares could be affected (Dakang, 1987). Once land becomes unusable for agriculture it is usually left as derelict land and not supplied with water.
In addition to the severe effects of overwatering on agriculture, through salinisation, there is also a health aspect to the problem. For example, in India the increase in irrigation has been correlated with the increased incidence of malaria (Michael, 1987). This is thought to be related to the increased area of waterlogged land. Stagnant water provides a habitat for many vectors of disease, and irrigation systems often provide ideal habitats for these organisms. This is discussed further in section 5.
Overwatering is often the result of poor management of the available water resources, resulting in inefficient distribution of water. In such situations farmers tend to stockpile available water, in an attempt to overcome periodic shortages. An example of this practice occurred in the Maharashtra Scheme in India (Carruthers and Clarke, 1981). In Sudan the field canals were consistently found to contain water outwith the growing period which was either an attempt by farmers to stockpile water in an effort to increase yields or through poor maintenance of the sluice gates entering the fields (Redding-Coates and Coates, 1981).
This situation inevitably leads to irregular flows and water levels in the supply canals, which would be of concern to any aquaculture facility, i.e. pens or cages, which would be heavily stocked with fish, and thus reliant on a constant supply of water.
Part of the problem lies in the fact that the water requirements for individual crops have not been precisely determined, since so many variables need to be considered. These include crop mixtures, leaching, soil types, length of growing season, percentage bare soil and degree of evapotranspiration. At present, water requirements are calculated on the basis of assumed crop mixes, rough estimates of water use efficiency, arbitrary application of leaching and seepage rates, and insufficient data on levels of evapotranspiration. Carruthers and Clarke (1981) have proposed that planning water requirements should be flexible to allow for these variables.
The extent of salt accumulation and the degree of alkalinisation of the soil depends on i) the soil type, ii) its permeability, and iii) the quality of the irrigation water.
Light textured soils tend to be less salinised but the soils are more likely to be alkali if the irrigation water contains a higher proportion of sodium and bicarbonate than calcium or magnesium, or if there is a hard pan of lime or clay (Michael 1978).
In the Chambal Irrigation Project in India, for example, the irrigation of an area with soils of a high clay content and with poor drainage, has caused almost 25% of the land to become affected by increasing salinity after only 10 years of irrigation (Michael, 1987). In situations like this, the water table often rises to the root zone, and through capillary action this water is drawn to the surface, where it evaporates leaving a surface layer of salts (Michael, 1987; Dakang, 1987; Skogerboe and Walker, 1972). This, of course, increases the salinity of the soil environment and reduces its suitability for agriculture.
Permeable soils result in high seepage losses from canals and lateral ditches. This leads to excessive surface runoff in downstream areas, which contributes to the process of salinisation. Seepage water tends to increase in salinity and hardness downstream, by the solution of minerals from the soil through which the water passes. In the Sugar Creek watershed in Oklahoma, for example, the total hardness of the water below a series of dam sites before their construction was 220–340 mg/1, whereas after construction this rose to 720 mg/1 (Yost and Naney, 1975). The usual range of hardness for fish culture is between 50 and 300 mg/1. The same authors found an increase of 150–660 % in total hardness in the waters collected from nine different reservoirs and their seepage waters. The increase seems to be greater in shale than sandstone, since in the latter most of the leachable ions have already been lost.
If poor quality, salt-polluted water is reused for irrigation downstream, then the rate of salinisation of areas so irrigated will be significantly increased. This situation is of particular concern in the Murray River basin in Australia. This catchment covers one-seventh of the continent, and supports a very important agricultural region. The water here is used several times over as it flows towards the sea, and concern over salinisation in the downstream areas has prompted the designation of areas, remote from the main farming land, for the dumping of saline drainage water (Framji et al., 1981).
Evaporation is also a significant factor in salinisation in arid climates, by causing an increase in the relative concentration of salts in the irrigation water, and so reducing its quality (see section 3.1.2).
In some areas an intrusion of sea water into the water table can contribute to salinisation if this water is used for irrigation. In the Changjiang estuary, China, for example, increases in chloride and hardness can be found 120 km upstream from the delta (Dakang 1987). This problem is more common where coastal aquifers are being heavily used as a source of irrigation water.
Saline and alkali soils, being less fertile will often have high applications of fertilisers, this in turn will affect the quality of the drainage waters making them less suitable for fish culture.
Despite the effects on agricultural production, saline waters could still be used for aquaculture production, if salinity control measures for agriculture proved to be uneconomic. In river basins with salinity problems the construction of hydro salinity models to represent the salt and water regimes has been incorporated into a planning framework for developing management strategies to control this problem. If this type of management scheme were to incorporate aquaculture into the general plan then water generally too saline for agricultural crops could be utilised for aquaculture. This would release some of the money being used to control the salt concentration in the waters. Some brackish water species (e.g Epinephelus tauvina), can be cultured fairly intensively and may prove to be more economic than desalinisation methods to improve irrigation water for agriculture in some areas.
In Israel the development of crops which could be grown in brackish water areas previously used for aquaculture created severe competition for this resource. However many fish farmers have been able to integrate into the water management system by restructuring fish ponds to act as irrigation reservoirs. Substantial production levels are still attained, however, from a polyculture system using these reservoirs which are periodically dried out at the end of the crop growing season (Milstein, pers. comm.; Sarig, 1984; Hepher, 1985). Although this type of culture is practised in reservoirs, it indicates the possibility of integrating successfully the water management regimes of both aquaculture and irrigated agriculture.
Unlined canals work efficiently only at, or close to a specific discharge rate. Most are designed to accommodate a maximum flow of only 1.6 times the minimum flow. Excess flow rates will lead to erosion, and lower flow rates will lead to the deposition of silt within the canals. The inclusion of a dam as part of the irrigation scheme will result in a smaller amount of silt entering the canals if the reservoir is deep, as it will act as a silt trap.
If the reservoir, however, is shallow enough for water to be overturned and mixed regularly, this could result in an overloading of the canals with silt. The silt load can also be high when the water source is a river. This will depend a great deal on the nature of the catchment. For example, catchments suffering from deforestation, or supporting large mining activities, will be characterised by high suspended solids in the river system.
Uncontrolled forestry or mining activities, in particular, often result in high silt loadings in a river system, and a consequent increase in sedimentation in rivers, reservoirs and irrigation systems downstream. The Agno River Irrigation system (ARIS), in the Philippines, is a significant example of this situation, suffering, through sedimentation, from lowered watering efficiencies and severe heavy metal pollution (Baluyut, 1985).
This scheme uses water from the Agno river to irrigate approximately 40 000 hectares of agricultural land. However, since its inception in 1957 there has been a constant influx of sediments from the Agno river, due to extensive erosion in the watershed and the discharge of tailings from the two mines in the upper Agno region. As a result, the irrigation canals have silted up and become contaminated with heavy metals toxic to both crops and aquatic organisms (Baluyut, 1985).
The discharge rate of the canals in the ARIS system has fallen from 28 m/s to 7.5 m/s, resulting in a loss of 27 000 hectares of land from the scheme, and a 20–50% loss in crop production (Baluyut, 1985). It is estimated that 26 380 million tonnes of mine tailings are released into the Agno river each year, but it is not known how much enters the ARIS system. Considering the high level of toxic wastes and suspended solids in the irrigation canals, there is little potential for fish production in this situation.
Decreased silt loading resulting from dam construction has been seen in many systems, particularly the Aswan Dam in Egypt and the Danjiangkou Reservoir in China (Dakang, 1987). The resultant reduction in this important component of the nutrient supply to the canal system, may indirectly affect the diversity and number of aquatic organisms. In addition the reduction in nutrients will ultimately affect crop production and intensify the use of fertilisers, which will have water quality implications for fish production.
A major problem which could result from reduced silt loading in the canals, is an increase in the production of aquatic weeds, which would take advantage of the greater light penetration in the clearer water. In this situation there may be a reduction in suitable breeding areas for substrate spawners such as salmon, steelhead, and Oreochromis sp. On the other hand, surface or vegetation spawners, such as carp, may gain considerable benefit from this.
Conversely, reduced turbidity would enhance the production of phytoplankton, and will therefore be of benefit to fish production in the system. It is not clear what levels of phytoplankton production are typical of irrigation canals, so the degree to which reduced turbidity would be of benefit is difficult to estimate. Natural phytoplankton production even in large canals with slowly flowing water will be low. However, where levels receive surface water from reservoirs, one may expect, at least seasonally, high phytoplankton densities.
High silt loading may have a number of effects. Certain species of fish are susceptible to high levels of silt in the water, which can abrade and clog their gills causing severe haemorrhaging, osmotic imbalance and respiratory difficulties. In salmon smolt for example, it is not uncommon for high mortalities to occur after heavy rain. This can be directly attributed to the increased silt loading of the supply water.
In the context of irrigation canals, the consequences of high sedimentation rates can be deleterious to fish culture. For example, in the case of pen culture in canals, the walls of the pen, although allowing passage of water, may slow its flow rate sufficiently to cause sedimentation. The fish pen, in this scenario, would act as a sediment trap - a situation which would create management difficulties and so reduce the viability of the culture operation. Furthermore, it has been recorded that high turbidity can reduce fishing success in capture fisheries (Ojiako, 1988).
The problem of heavy metal contamination of the water and sediments should always be of concern. Although the effects on the fish in such an environment may not be apparent in terms of loss of production, the consequences for human health may be severe. It is well established that certain pollutants, pesticides as well as heavy metals, are very persistent in the environment and become concentrated in the body tissues of animals higher in the food chain. Fish are particularly susceptible, and there are numerous records of heavy metal poisoning as a result of fish consumption - perhaps the most notorious case being that of ‘Minamata disease’. In fact a severe case of mercury poisoning, from which hundreds of people in a Japanese fishing village died.
Crops follow a seasonal pattern of growth, and consequently their water requirements fluctuate on a seasonal basis. In terms of fish production, this presents a problem because of the consequent changes in the aquatic environment of the irrigation canals, i.e. many irrigation systems remain static for long periods at a time, whilst at other times of the year they have constantly flowing water (although the follow rates may vary considerably). When static, the water in such canals may develop high phytoplankton and zooplankton concentrations.
Despite the considerable potential for fish production in irrigation canals, the problem of varying flow rates is one of the prime constraints to integrating fish production with crop irrigation.
As explained in section 3.2.1, it is difficult to estimate the exact water requirements for crops, and consequently to make accurate allowances for abstractions (by altering the rate of water supply to the canal system). Thus water levels and flow rates in canals may fluctuate. This problem is particularly apparent in poorly managed schemes and where there is uncontrolled abstraction by farmers.
Irrigation waters are either partially or fully controlled, thus flow rates within the canals may be variable throughout the year and may also become static at times.
As many irrigation schemes serve a dual function of flood control, they are therefore disproportionately affected by heavy rainfall, and spate conditions may be common during the rainy season. In extreme cases flooding may occur, as in Bangladesh in 1987 and 1988. As these events are unpredictable, this factor is particularly important. Sudden inundation will not only wipe out the crops but also any low lying aquaculture facilities. Similarly, droughts are not consistent in their timing as has been seen in Brazil (Hall 1978) and many other countries, e.g. Sudan, Ethiopia etc. Thus the water required from year to year may also vary considerably.
In many countries water can or is abstracted without consent of the water authorities. Large abstractions will have an effect on the water depth and flow rates in canals. In the lower Indus, for example, fishing has suffered in some years as a result of ALL water being abstracted for use in the irrigation schemes during the winter. In Kenya it is thought that the regulatory effects of the irrigation dams on the Tana river will adversely affect the floodplain fisheries of the lower reaches by affecting the extent and duration of the flood cycle (Litterick, 1981).
Welcomme (1979 a,b) suggested that fish production from floodplain fisheries is correlated with the flood cycle - greater production being seen after peak floods. The widespread construction of flood control systems in Africa in the last two decades has led to the disruption in floodplain ecology and production (Awachie, 1981). With such a potential decline in the river fishery then pressure to produce a protein source through other methods such as aquaculture in one form or another increases.
Some studies have shown that species may be at risk of extinction as habitats decline in both quality and extent due to the annual spring or dry season fall in water levels. In the USA, the reduction in the population of the pupfish, Cyprinodon elegans, found in irrigation ditches in Texas (Phantom Cave Springs) has been attributed to the shrinkage of its habitat. This is brought about by periodic draining of the canals in which the pupfish lives for routine maintenance (Davis 1979). In many other systems, e.g. in China (Tapiador and Coche, 1977), routine drainage is also used to allow canal maintainance.
Large or frequent abstractions, where they resulted in a significant drop in the water level, would obviously pose problems for any type of intensive aquaculture or fisheries in such canals since stocking density, oxygen supply, and waste removal depends on the flow rate and the depth of water.
Smaller fluctuations in water level and flow rate also have implications for fish production. In respect of fisheries, frequent water level fluctuations will have a negative impact on the growth of aquatic macrophytes along the canal margins. Whilst this may be desirable from the irrigation engineers point of view, it would be detrimental to the fish community through habitat reduction (see section 6.2).
The effect of the more minor fluctuations in water level and flow rate are more apparent in the case of aquaculture. Here fish are grown in confined conditions, and depend upon an adequate supply of water to replenish oxygen and to remove excretory wastes from their immediate environment. Whilst not so critical at low stock densities, intensive, and perhaps semi-intensive, systems are particularly vulnerable to reductions in the water supply as the stock density is carefully calculated so as to make full use of the available water.
Increases in the flow are also of concern. High flow rates may lead to high levels of suspended solids. The implications of this are examined in section 3.3.3. Additionally, flooding is potentially a serious problem. Irrigation schemes often have a dual function for flood control, and are therefore disproportionately affected by heavy rainfall. Most fish culture cages and pens would be destroyed under spate conditions, and were the banks of the canal breached, most production would be lost.
Water management practices in irrigation systems are often of a regular, seasonal nature, and their worst effects can be avoided by asynchronous seasonal fish production. In tropical climates, most cultured fish can be grown to market size in six months or less, so year-round use of the canals is not necessary.
Natural waters carry a certain amount of dissolved mineral salts and naturally occurring organic compounds leached from the soil and rocks. Pollution is the result of additions to this natural loading of chemicals. Uses may be for industrial, agricultural (irrigation), or municipal purposes. Potential additions, either direct or indirect, include: insecticides, herbicides, fungicides, bactericides, nematocides, plant hormones, detergents, heavy metals, salts and numerous organic compounds (Law and Skogerboe, 1972). Pollutants present in sufficient quantities will render the water unfit for irrigation, consumption and fish production.
Water quality generally is of more concern to aquaculture than to capture fisheries, a result of the high stock densities compared to the natural environment. This is the case for all water bodies in which fish are cultured, so will not be considered here. However, irrigation canals differ in that they are the recipients of the residual loads of the wide range of chemicals used in crop production, and often exhibit high levels of pesticides and fertilizer.
A study of the effects of two irrigation schemes on the Anambra River, Nigeria, showed high levels of pollutants discharging from the canals (Ojiako, 1988). Chemicals used to reduce the effects of salinisation (gypsum, pyrite, and dry lime sulphide), and pesticides and fertilizers are regularly applied to the land. Residues from these applications enter the canals via direct runoff and subsurface drainage, particularly during the first rains. River water below the discharge sites had lower dissolved oxygen, higher BOD and turbidity, and higher pH. These factors had reduced the capture fishery in the river, and lead one to seriously doubt the viability of a capture fishery in any irrigation canals under these conditions.
The widespread use of pesticides and their presence in waters used for fish production are of particular concern. Many pesticides are complex organic molecules which are very persistent in the environment, and are passed on through the foodchain. Thus, carnivorous fish can accumulate high levels of pesticides from their prey. In Malaysia and Thailand reduced fish yields have been attributed to the levels of various persistent pesticides found in rice fields (Koesoemadinata, 1980; Khoo and Tan, 1980).
Pesticide usage for agriculture in developing countries is constantly increasing, and was estimated to be 36–40% of the world total in 1975 (approximately 890 000 tonnes) (Alabaster, 1981). In industrialized countries there is also an increase in the amounts of chemicals used. For example, in Arkansas the increase in productivity of the agricultural sector has been brought about by increased irrigation and pesticide use. This is a familiar story throughout the USA, and as a result several states have recorded pesticides in ground waters in the last fifteen years (Lavy et al., 1985).
Many of the insecticides, herbicides and fungicides used in developing countries have been totally or partially banned from industrialized countries. Their increased use has been partly due to the growth in irrigated agriculture (Michael, 1987), and partly due to the ready availability of these effective, but often environmentally damaging, chemicals on the world market. Also, the widespread use of herbicides to control the prolific water hyacinth has exacerbated the situation.
The quantity of chemicals being used on the land is difficult to assess, but should be investigated before any form of fish production is considered in particular drainage canals. In Sudan, for example, the main crop is cotton which requires approximately 4 kg/ha/yr of various pesticides to maintain an economic level of production. The amounts of various pesticides used in the Gezira irrigation scheme in 1977 on cotton alone, were (in lbs active ingredient); DDT 3 x 106 lbs, Dimethoate 1.7 x 106 lbs, Toxaphene 1 x 106lbs. Five other chemicals were used in similar amounts (Zorgani et al., 1978; Coates and Redding-Coates,1981). Fish mortalities in the irrigation canals of the Gezira scheme have been attributed to pesticide contamination, and pesticide residues have been found in fish body tissues (George, 1975).
In other African countries pesticide use is increasing. In Kenya, for example, 5 000 tonnes per year of pesticides are used on a small area which includes an irrigation scheme (Alabaster, 1981). As a result breakdown products of pesticides were detected in the water and in aquatic organisms. Therefore, the unrestricted use of chemicals such as DDT, dieldrin, and aldrin is of some concern. In Burundi, Tanzania and Zambia there is no legislation which relates specifically to the use of pesticides, and DDT is widely used in irrigated areas. In Malawi, where fisheries are an important part of the economy, low levels of DDT have been found in fish tissue. Lack of funding for research in most of the African countries has contributed to the unrestricted use of such chemicals (Alabaster, 1981). There is now an international code on pesticides, formulated under the guidance of FAO, which will hopefully improve the prospects for fish production in irrigation systems.
The results of contamination are evidenced by fish kills, reduced fish productivity and greater levels of potentially harmful chemicals in fish tissues (Alabaster, 1981). In Canada, the use of the pesticide Thiodan for control of pests on potato crops is known to kill fish 200 m downwind and many others have the same harmful effect if carried downwind from the target areas (Fish Farming, Canada 1989). Considering the potential for fish production in irrigation canals, Pantulu (1980) cites the level of pesticide usage in irrigated areas as a, perhaps the, major constraint.
This becomes a serious problem in countries where fish constitutes a substantial proportion of dietary protein. In Africa the dietary percentages of fish protein are fairly high, 20% in Kenya and Sudan, 20–24% in Burundi, Tanzania and Zambia (Coche, 1980), up to 65% in Malawi (Alabaster 1981), and over 70% in Bangladesh (Marr, 1986).
There is an increasing volume of literature on the environmental impact of aquaculture (Nature Conservancy Council, 1987). This mainly centres around nutrient enrichment of the environment (eutrophication) by faecal wastes and waste feed. The main results of this are anoxic sediments and algal blooms.
Of particular concern in irrigation canals used for semi-intensive or intensive cage or pen culture, the settling out of faecal wastes and waste feed results in a richly organic, anoxic sediment. This can have implications for the culture operation itself, and for the environment in general.
The effect of eutrophication in irrigation canals has not, to the author's knowledge, ever been examined. The critical factor as to whether nutrient enrichment is likely to cause a serious problem, is the residence time of the water in the system. In the case of most irrigation systems this would be comparatively short, so the problem should be negligible or, at worst, localized. However, this is not necessarily the case in downstream lakes or reservoirs, which generally have long residence times and are therefore susceptible to eutrophication, and in which the influx of enriched irrigation water may be significant. Large-scale intensive aquaculture could contribute to eutrophication downstream of the irrigation scheme.
Fish wastes can also have a direct effect, in the case of pen and cage farming, on the culture operation from which they originate. The solids falling to the sediments are enriched with nitrogen, phosphorus and carbon (Merican and Phillips, 1985). If these sediments become anoxic through microbial decomposition there may be a generation of gases such as hydrogen sulphide, ammonium, and methane which are potentially harmful to fish (Enell and Lof, 1983). In Japanese yellowtail farms the incidence of disease has been correlated to the concentration of hydrogen sulphide in the sediments (Arizono, 1979).
The water quality of canals fed by reservoirs will, to a large extent, depend on the depth from which the water is taken and the degree of stratification of the water body. Stratification occurs in deeper lakes in which the surface waters (epilimnion) are warmed to a significantly higher temperature than the deep waters (hypolimnion). The thermocline thus produced prevents wind-induced mixing. Because of the differing conditions in each layer, the water chemistry of each may be significantly different. Stratification is often a permanent feature of deep, tropical lakes, but only a seasonal occurrence in temperate waters.
Shallow reservoirs (epilimnetic water)
Shallow reservoirs from low diversion dams (up to 8 m in height) will show little change in water quality from dam to canal. Generally these waters are eutrophic, with elevated water temperatures and little yearly stratification. With age there tends to be an increase in the level of algae and other organisms, with the danger of occasional toxic algal blooms (Hotes and Pearson, 1977). Water from such a source would be beneficial to fish production, since the higher temperatures would increase fish growth (providing the temperature did not reach extremes), and the level of primary production would be sufficient to support a fishery or aquaculture system using planktivorous fish species. The likelihood of toxic blooms should be investigated, in order to make some assessment of the risk from this source.
Deep reservoirs (hypolimnetic water)
Deeper reservoirs are often stratified and hypolimnetic water can be low in oxygen, temperature and plankton biomass, and high in nutrients such as phosphorus and nitrogen. High levels of hydrogen sulphide, iron and other heavy metals may also be present. The surface waters will have a richer plankton community, but this, combined with the higher water temperatures, may lead to oxygen depletion. Aeration of the waters being discharged into the canals will improve the oxygen content, but the quality of hypolimnetic water would probably remain unsuitable for fish production in the canal section close to the dam.
Water from the epilimnion would be more appropriate for fish production in irrigation canals with the temperature and water quality being more favourable to aquaculture and fish production in general. In Iran, trout hatchery situated close to a dam on a canal receiving hypolimnetic cool water of standard temperature, is producing both fry and fingerlings and marketable-size fish.
In summary, the supply of water for irrigation, and fish culture in irrigation canals could be, but usually are not, complementary activities. Problems encountered in irrigation systems detrimental to efficient use of the water for agriculture are also detrimental to fish production. However, with some effort, at least some irrigation canals can be adapted to fish production.