2. Inland water resources of Asia

Fish and water are inseparable, and it is necessary to consider the water resources in Asia, particularly those resources that are likely to benefit from some form of stock enhancement. Global water resources are diverse, but crucially for inland fisheries, there are only limited freshwater resources available to fisheries. The earth’s freshwater resources amount to about 35 029 000 km³ or approximately 2.5 percent of the earth’s total water resources (Shiklomanov 1993). More than 75 percent of the earth’s freshwater resources occur as glaciers and permanent ice cover, and about 23 percent is locked in ground water. This leaves les than 3 percent of the total that mightbe available for freshwater fisheries.

The Asia-Pacific region accounts for 36 percent of the global freshwater runoff, but its average per capita amount of available freshwater is the lowest in the world, amounting to only 3 690 m³/yr (UNEP 2002). This low per capita availability of water highlights the limited nature of this primary resource and consequently, competition for water will shape the future development of inland fisheries and aquaculture. The water resources available to fisheries can be categorized into: riverine resources (rivers and their floodplains), natural lakes and man-made impoundments (reservoirs).

2.1 Riverine resources

Global riverine runoff (which excludes polar glaciers and ice) is estimated to be 44 500 km³ (Shiklomanov 1993), and that of Asia is 14 410 km³, approximating 32.4 percent of the total. This is the highest among the continents, and Asia also has the largest number (49) and the highest cumulative river channel lengths (about 90 000 km). This is more than double that of North America or Africa (Kapetsky 1998).

Asia has a number of major rivers with international river basins, such as:

• the Mekong River,

• the Irrawaddy River,

• Ganges-Brahamaputra-Megna river system,

• the Indus River and

• the Red River.

Although major fisheries exist in all these river systems (Coates 2002), stock enhancement has been rarely practiced to contribute to fisheries in the river channels per se in Asia. In contrast, floodplain fishery enhancement is a common practice in Bangladesh and Myanmar. The seasonal floodplains in Myanmar are estimated to be about 8 million ha, mostly in the Irrawaddy system (with a discharge of 13 500 m³/sec (Coates 2002). Bangladesh, which is essentially a huge delta, has a seasonal floodplain of about 2.8 million ha (Rahaman 1987). In Bangladesh, the floodplains also contain deep depressions (beels), amounting to about 114 700 ha (Rahaman 1987). Also relevant to the present study are the oxbow lakes commonly found in Bangladesh (referred to locally as "baors"). Oxbow lakes are perennial waterbodies and are the remnants resulting from natural shifting of the original river channels.

2.2 Natural lakes

Asia has relatively few natural lakes, with only about 15.4 percent of the global total of 1.236 million km². Most of these Asian lakes are confined to the volcanic areas of the continent, primarily the Indonesian and Philippine archipelagos and northern India. There is one major exception, the Great Lake (Tonle Sap) in Cambodia, a lake that expands from 2 000-3 000 km² in the dry season to 10 000-12 000 km² in the flood season, as a result of the reverse flow of the Tonle Sap River. The major lakes (>5 000 ha) in Indonesia and the Philippines have a total area of 257 968 (nine lakes) and 89 820 ha (five lakes), respectively (Baluyut 1999). Indeed, the limited number of natural lakes in Asia has been linked to its relatively poor lacustrine fish fauna and consequently, the ease with which tilapias of African origin have been able to predominate in some of the lacustrine waters of Asia (Fernando 1980, Fernando and Holcik 1982).

2.3 Reservoirs

There is an ancient tradition of reservoir building for irrigation and water storage in some Asian countries such as in Sri Lanka, where this activity was an integral part of the civilization and dates back nearly 2 500 years (De Silva 1988). Elsewhere in Asia, modern reservoir building is a post-Second World War phenomenon. Large reservoirs are constructed for purposes of irrigation, hydropower generation and/or flood protection. More recently, modern large reservoir construction has come under increasing scrutiny and remains a controversial issue in most countries. A central feature in the argument relating to large reservoir construction is the relative financial and socio-economic benefits and/or impacts to the nation and impacted communities. It is not uncommon to see such issues taking centre stage in the political arena (McCully 1995, Roy 1999), and this is reflected in the establishment of the World Commission on Dams. Reservoirs are never or very rarely constructed for fisheries development, although the economic potential of fishery development is often cited as a mitigating factor to offset impacts to riverine fisheries that result from dam construction. Most typically, fisheries development in reservoirs is a secondary activity that is subsidiary to the primary purpose of the dam, which is usually hydropower or irrigation. This review will consider only the fisheries-related biological and environmental aspects of pre-existing reservoirs and not the more complex issue of the impacts of dam construction on riverine fisheries.

The reservoirs of Asia can be loosely divided into large impoundments resulting from the damming of large rivers and smaller impoundments that depend on rainfall and the runoff from local catchments (Plates 4 and 5). The aggregate reservoir resource in Asia is large (Figure 5), and currently accounts for over 40 percent of the global capacity of large reservoirs (>0.1 km³). In addition to the large reservoirs, there is a very large area of medium and small reservoirs, which tend to be located in rural areas and are primarily intended for irrigation. The distribution of different types of reservoirs in the seven main river basins of PR China is given in Table 3. It is estimated that developing countries in Asia have 66 710 052 ha of such reservoirs (FAO 1999). PR China is estimated to have 86 526 medium (66.7 to 677 ha) and small-sized (<66.7 ha) reservoirs (Table 3, Huang et al. 2001). Similarly, Sri Lanka is reputed to have over 10 000 small reservoirs, which are non-perennial and retain water up to six to nine months in the year (De Silva 1988).

Figure 5. Reservoir number and volume (in km³) in Asia at different time periods and the Asian reservoir volume expressed as a percentage of world reservoir volume. Reservoirs >0.1 km³ capacity are included [based on data from Avakvan and Lakovleva, 1998]

Table 3. Number of reservoirs of each type, and the total storage capacity in the seven main river systems in PR China (from Huang et al. 2001)