Extracting water for irrigated agriculture is a common practice worldwide. It is estimated that in 2010, 45 percent of total global food production will come from irrigated lands. In developing countries at least 80 percent of water could be used for irrigation.
Irrigation in arid countries
The largest demand for irrigation is in arid and semi-arid countries. There, species diversity of native fish has most drastically declined. In general, dammed rivers experience declines in riverine fish stocks where in only the best of situations they are replaced by fish species of standing waters.
In Asia, the arid and semi-arid zones extend from the Mediterranean to the Pacific Ocean and include over a dozen of countries. In most, irrigated agriculture is an important part of the economy and a sector on which food production depends almost entirely. Irrigated agriculture in such countries is a governmental priority and dominates government decisions on the allocation of water resources. Closely connected with this is the construction of dams, reservoirs, irrigation and drainage canals, without which the sustainable production of food and other crops of economic importance is unthinkable.
In Central Asia, Kazakhstan, China and Mongolia there are rivers and lakes with no outlet to an ocean. In China there are about 430 endorheic rivers (Lu, 1994), many of which have endemic fish species. As a result of diverting water for irrigation, fish stocks in some endorheic rivers have been endangered, leading to a collapse of fisheries and, in extreme situations, to the extinction of fish species.
Irrigation reservoirs have an inherent instability: water level manipulation interferes with the basic biological functions of fish. In addition, some impoundments have been subjected to changes in water quality. Indigenous fish stocks exposed to changed conditions have proven vulnerable and their numbers have declined. To compensate for such losses, fishery managers have been translocating or introducing fish species, in some instances with fish food organisms.
In the arid belt of Asia, it is probable that now there is not a single water body left with just indigenous fish. Where there are fisheries, the proportion of introduced fish species can constitute 50 percent or more of the catch . Regular stocking of fingerlings has become routine in many irrigation reservoirs.
Negative impacts of irrigation
As damming serves to store water for diversion to irrigation canals, there is an additional negative impact on fish. Pumping and water distribution are very damaging for fish, in some cases fish mortality approaches 100 percent when irrigation uptake and diversion coincide with peak periods of fish reproduction and nursing.
It is estimated that about 90 percent of drifting young fish enter irrigation canals and perish in the irrigated fields of Uzbekistan. As well, many irrigation uptake systems, such as those on the Amu-Darya, do not have functioning fish protection devices. Large-scale irrigation in arid and semi-arid zones with high evaporation and poor water management may lead to a gradual increase in soil salinity and, consequently, in the salinity of residual drain water. When such water is returned to the river its salinity increases, threatening aquatic organisms.
The loss of large quantities of water diverted for irrigation may cause considerable damage to terminal lakes, such as in northwestern China. A well-known example is the Aral Sea which is drying and, as a result, is also undergoing a rapid increase in salinity. This has led to the decimation of Aral Sea fish stocks as well as those in numerous freshwater lakes of the Syr-Darya and Amu-Darya river deltas, both of which are the major inflows to the Aral Sea.
Management strategies for irrigation reservoirs have included the translocation and stocking of fish. In Keban and Karakaya reservoirs in Turkey the return on stocking has been 13 and 19 percent respectively (Petr, 1998). In reservoirs in Central Asia introductions often only temporarily increased fish yields. Where river basins became interconnected through canals, fish faunas of these basins intermingled through immigration of species. The new fish communities then form a mixture of indigenous and introduced species and one can assume that a balanced state is still to be achieved.
In the arid belt of Asia a governmental priority remains the use of water for irrigation and hydropower production, leaving fishery managers in the unenviable position of adjusting management to the continuously -- and often unpredictable -- changing situation. In order to increase fish production present government strategies focus on the rehabilitation of rivers and terminal lakes with the aim of arresting and reversing the salinization process and of maintaining the water level in reservoirs and lakes closer to those needed by fish. This allows fishery managers to develop plans for optimizing fish yields under more stable conditions. Culture-enhanced methodologies, such as cage and pen culture, should also lead to substantial increases in fish production from these water bodies (Petr and Mitrofanov, 1995). There is also potential for augmenting fish production in irrigation canals (Redding and Midlen, 1991).
In the humid tropics, with rice culture the major beneficiary of irrigation schemes, reservoirs may not experience large water level fluctuations thus the danger of salinization is minor compared to that in arid countries. Fish culture in irrigation reservoirs is often enhanced by cage culture and fish are produced in rice paddies.