Fish production from irrigation canals can be acquired through aquaculture or capture fisheries. The potential for fish production by both of these methods is examined in this section, as are the most appropriate practices of aquaculture and capture fisheries to irrigation canals.
Owing to the extensive collection of data on primary production and fish populations in reservoirs and rivers, it has been possible to formulate models which, given certain parameters, can be used to predict fishery yields or fish production in these environments (see Beveridge, 1984). Unfortunately none of these models can be applied with any confidence to irrigation canals because of their unique environmental conditions. These characteristics are examined in the following paragraphs.
Water in irrigation systems may originate from reservoirs, rivers, wells, springs, lakes or artesian waters. The natural flora and fauna in the canals will reflect the water source, the physical characteristics of the irrigation scheme, and will also be influenced by the use of chemicals on the irrigated land. Thus each irrigation system may differ substantially from the surrounding natural waterways.
The energy in an aquatic ecosystem is derived either from external (allochthonous) sources, or internal (autochthonous) sources. Allochthonous sources include organic matter and nutrients from the source waters, or that washed off the land or originating from overhanging plant growth. In an irrigation canal the latter sources are unlikely to contribute much to the system, and it is probable that the major source of allochthonous energy in this habitat is from the source waters.
Irrigation canals may draw their water from rivers or reservoirs. In the case of reservoirs, this may provide a rich source of allochthonous energy to the canal, in the form of plankton. However, plankton can be damaged by the high temperatures (Daget, 1976) and turbulence in shallow canals, and thus production may be lost. In rivers, primary production is usually lower than in lentic systems. The short residence time of rivers does not provide the stable environment, characteristic of lentic systems, conducive to planktonic production.
Canals usually have flowing water, although there may be periods when the canal is static. This tends to preclude abundant planktonic growth, such as that often seen in enclosed water bodies, as nutrients and planktonic organisms are continuously flushed out of the system. In certain instances canals may be considered as ponds, for example, the road and railside borrow pits in Bangladesh (Marr, 1986) (i.e. when the water is static), but this is not for continuous periods and is not always predictable.
In many canals with flowing water the main form of autochthonous production is in the form of aquatic macrophytes and not the phytoplankton community. Whilst production can be very high in aquatic macrophyte communities, this production is not immediately available to fish, and may even be removed from the system during canal maintenance. This will restrict the species of fish found naturally in the canals as there are few species (grass carp being one notable exception) which are able to directly utilise this form of plant material.
Thus, the hydraulic characteristics and the trophic status of canal habitats may be significantly different from those in natural waters.
Habitats within a canal may not be suited to the endemic species of the area. For example there may be a lack of breeding sites, stimulants to breeding, or the correct food.
Most relevant data relates to the effects of hydro-electric and irrigation construction work, and the effects of channelization on rivers (Adkins and Bowman, 1976; Bailey, 1978; Marsh and Waters, 1980; Bailey and Cobb, 1984; Bernacsek, 1984). These studies arrive at varying conclusions, some suggest that faunal diversity and abundance in irrigation canals would be less than in the source waters. Other authors are of the opinion that channelization has little effect on the natural fish populations. Differences in the complexity of the ecosystem in the source waters may account for some of the differences between these studies.
Various studies have shown that channelised sections of rivers, and irrigation canals, exhibit lower species diversity than nearby rivers or static water bodies (Daget, 1976; Tarplee et al., 1971). Tarplee et al. (1971) found that the fish biomass in a channelised stream was 31% lower than in an unchannelised stream and that a 78% reduction in macro-invertebrate biomass had occurred.
In the Gezira irrigation system in Sudan it was found that only 19 out of 34 species of fish present in the source waters (the Blue Nile) were represented in the minor and field canals (Coates 1984), a species deficit of 45%.
These differences between natural waters and associated canals can be attributed to lower habitat diversity in canal systems. The shallows and deeps which characterise natural rivers and contribute significantly to their diversity of habitats and species, are not present in irrigation canals, bankside and littoral vegetation is often discouraged, and the benthic fauna is reduced (Aliyev, 1976).
In many irrigation and river development plans the effects of construction on fisheries is often overlooked (Grover, 1980). Destruction or alteration of the riverine environment, restriction of migration to spawning areas by dams, pollution of these habitats by agro-industrial chemicals, and flood control measures which have removed the spawning stimuli for many fish species are all commonly associated with irrigation systems. A summary of the effects of hydraulic works on riverine fish communities is given by Welcomme (1985). In Washington State, USA, fish propagation and migration has been affected by the use of water resources for hydro-electric schemes and irrigation withdrawal (Funk, 1984). This, in turn, may affect the species composition and levels of production within the canal system.
Welcomme (1979a, 1979b, 1979c, 1985) proposed that production in floodplain rivers is proportional to the extent and duration of the flooding in any particular year. In irrigation canals the irregular flow rates and the lack of seasonality in the water level would not encourage the breeding of species with marked seasonal cycles of production, especially those related to the flood cycle. In addition, the construction of flood control devices is known to seriously affect the natural fisheries production in rivers (Anon, 1977; Graham, 1980). In some areas the threatened extinction of several species has been directly related to reduced flows in the rivers resulting from water abstraction for irrigation (Davis, 1979; Gowan and Kevern, 1985). In Michigan, USA, it is thought that irrigation withdrawals had the effect of reducing the habitat of the native brown trout by 19.6% over a one-year period (Gowan and Kevern, 1985).
Studies on the effects of river regulation on canal ecosystems have shown, in most cases, that irrigation canals differ from their source waters (rivers and lakes/reservoirs) in a number of ways. The overall effect of these differences appears to be a reduction in species diversity and abundance in the canal habitat, and possibly a reduction in primary production. There are two consequences of this for the fisheries biologist:
It would be difficult to establish a self-sustaining exploitable fish population without strict management of the stock, and a comprehensive restocking policy.
It is not possible to use existing models for estimating primary production in fresh water (which are based on riverine and lacustrine ecosystems) to make estimates of primary production, and hence potential fisheries production, in irrigation canals.
Thus managed fisheries in irrigation systems are something of an unknown quantity. Primary production, on which the productivity of a fishery is based, cannot be reliably estimated in canals, and there would almost certainly be problems establishing a self-sustaining fishery in an irrigation system. This is, of course, a generalization, but it emphasises the fact that each irrigation system must be examined individually with regard to its fishery potential.