This synthesis is based on the three case studies as presented by the individual authors, and on information obtained from several additional sources available to the Editor. The conclusions reflect the Editor's opinion and may not necessarily agree with those of the authors.
3.1 The Agno Basin (Philippines)
3.1.1 Present environmental situation:
Seven major land and water resource uses of the Agno Basin have been identified: forest exploitation, dry land agriculture, irrigated agriculture, mining, hydropower production, aquaculture and capture fisheries. They are related in simple or complex land-water relationships, and either directly affect the quality or quantity of water, or they require water as a medium.
The river basin is under stress from deforestation, resulting in heavy erosion which is further aggravated by unstable land use and improper cultivation practices. The productivity of agricultural lands in the hilly country of the upper catchment has declined by as much as 50%. The pressure on land and water resources is high, with 305 persons/km2 in the Pangasinan Province which extends through the major part of the Agno River. 54.2% of the total population are farmers, fishermen, hunters, loggers and related agricultural workers, exerting a high pressure on primary resources.
Mining is another important user of land. In spite of measures undertaken to prevent much of the mine waste and tailings reaching the river, a high number of mines in the catchment contribute suspended solids which, together with those from eroding lands, may reach catastrophic concentrations, especially under natural calamities such as floods caused by cyclones. Current estimate of the area adversely affected by sedimentation in the Agno River catchment is 27 000 ha of land.
3.1.2 Major engineering modifications in the basin:
These have involved structures for mining industry, especially tailings, dams and three major dams on the Agno River. In the 1950s two multipurpose dams were constructed on the middle Agno: the Ambuklao Dam and the lower downstream situated Binga Dam. They became fully operational in 1956 and 1960 respectively. A third dam, San Roque, is under construction. In 1957 a major irrigation system (ARIS) was constructed, which today irrigates 40,000 ha of land.
3.1.3 Pollution:
Mining, resulting in large input of mine tailings (26,380 t/day in 1978) into the river, together with the input of soil through erosional forces, has negatively impacted the irrigated crop yields, reducing them by 20 to 50%. This has been due to three major impacts: (i) silt accumulation from irrigation water, which causes cementing of the soils and stunts growth of crops by inhibiting root absorption; (ii) reduction of soil fertility, as indicated by low organic matter content and reduced concentrations of available phosphorus; (iii) the sediment-loaded irrigation water impeding nutrient availability and their uptake by crops.
The extent of organic pollution through sewage discharge is unknown. However, there has been an increase in fertilizer, pesticide and herbicide input into the river especially from the irrigated lands.
3.1.4 Fisheries:
The 270 km long Agno River has never been considered a fishery resource. The upper Agno is poor in fish and hardly ever fished except during the rainy season when water level is high. The two existing reservoirs have been stocked at irregular intervals with fish. Fisheries statistics are unavailable; a guesstimate gives a figure of some 500 part-time fishermen catching mostly tilapias and common carp. The reservoir fisheries is now providing about 50% of the protein requirement of the fishing families in the area.
The lower Agno with the exception of the reservoirs appears to be poor in fish. The production in the lower catchment comes from brackishwater fish ponds found mostly along the coast and in the delta of the Agno, and from a few freshwater fish ponds in the lowlands. In 1981, total production of 16,452 t of fish from 13,444 ha was achieved. Of these, 12,380 t came from brackishwater ponds. This represented 9% of the total brackish water pond production of the Philippines, and 1,200 kg/ha/yr as compared to the national average of 870 kg.
3.1.5 Fisheries planning and current activities:
Due to the abundance of marine fish stocks in the past, and more recently due to the intensification of capture fisheries in natural lakes and rapid expansion of aquaculture, virtually no attention was paid to the river capture fisheries potential of the Philippines. On the Agno, even the construction of the first two multipurpose dams in the 1950s has not attracted much attention as an environment with fisheries potential, although fish were stocked in them. A reservoir fisheries development plan was not available prior to the establishment of the new reservoirs, and there was no monitoring of catches through statistical data collection to assist in the evaluation of stocking, and to assess the level of establishment of native fish stocks in these reservoirs. No statistics were available on riverine fisheries either. Thus, with an increase in river pollution, its impact on fish stocks could not be assessed. Any planning of fisheries in the river and its reservoirs has to be therefore based on the current data collection, results from experimental catches and other approaches. The current collaborative effort of the National Power Corporation together with the Bureau of Fisheries and Aquaculture centres on pilot testing of tilapia cultures in floating cages placed in the reservoirs. A land-based hatchery-nursery complex is also being proposed. BFAR also considers undertaking the evaluation of the present fish stocks in the reservoirs, which should then form the basis for a rational stocking programme.
3.1.6 Summary of constraints to fisheries by other uses and environmental mismanagement of resources:
The constraints have been organized into a matrix relating the level of each stress to a particular type of fisheries.
The problems listed in Table 1 indicate that the upper Agno River channel is a highly stressed environment with hardly any prospects for fishery development, with the exception of aquaculture well outside the river itself. The upper course of the river has the highest sediment load and heavy metal content of the whole system. The two existing reservoirs and the third one under construction in the middle course function as a sediment trap, and although they have a multipurpose use, their fisheries potential is yet to be fully realized.
The irrigation system (ARIS) has some fisheries potential, the level of which would be controlled by the length of water retention and by sediment/mining waste and tailings inputs. Its vulnerability under the impact of sedimentation can be seen from the lowering of crop production, and fish and prawns would probably face some negative impact if introduced. Leaching of heavy metals from the deposits, and pesticide/ herbicide inputs from the irrigated fields are other potential hazards. Much work is needed to harmonize the uses of mining industries, irrigated agriculture and other water uses for establishing an environment of high water quality conducive to fisheries development in the river and the irrigation system.
The reservoirs have good fisheries potential. They trap the mining and other pollutants by retention of sediments and clarification of water. Their existence also improves the downstream water quality. When the San Roque reservoir comes into existence, further improvement is expected.
The lower Agno has very little fishery, although current surveys have shown a very promising potential for Macrobrachium fisheries (Natividad 1982). At present, the top inland fish production in the basin is brackishwater aquaculture in the lower Agno. Being outside the direct influence of the Agno River, it is not vulnerable to changes in river water quantity and quality.
| Fishery Resources use | Constraints imposed on fisheries by other users | Constrains imposed by natural catastrophies | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Large-scale deforestation of the upper catchment leading to high sediment loads | Improper agricultural practices, leading to a high sediment load | Mining waste and tailings disposal | Urban sewerage outlets | Substantial water uptake for irrigation or other uses | Regulation by damming | Heavy metals leaching from sediments | Pesticide and herbicide input from agricultural practices | Floodwater impact on mining waste/tailings storage | Floodwater wash of eroded soils | ||
| Upper Agno | Traditional fishery | H | H | H | M | H | O | H | O | H | H |
| In-stream aquaculture | H | H | H | M | H | O | H | O | H | H | |
| Pond aquaculture | O | O | L | O | O | O | H | O | O | O | |
| Agno River reservoirs | Capture fisheries | L | L | L | L | H | O | M | L | H | M |
| Stocking | L | L | L | L | H | O | M | L | H | M | |
| Cage culture | L | L | L | L | H | O | M | L | H | M | |
| Irrigation system | Stocking | M | M | M | L | O | O | M | M | H | H |
| Aquaculture | M | M | M | L | O | O | M | M | H | H | |
| Lower Agno | Capture fisheries (stream and floodplain) | M | M | M | L | M | L | L | L | M | M |
| In-stream fisheries (including Macrobrachium) | M | M | M | L | M | L | L | L | M | M | |
| Aquculture in ponds | O | O | O | O | O | O | O | L | L | O | |
| Capture fisheries (estuary and delta) | M | M | M | L | O | L | L | L | L | O | |
| Coastal | Brackish water ponds | O | O | O | O | O | O | O | L | O | O |
| Conflict level: H - high; M - medium; L - low; O - none | |||||||||||
To prevent further deterioration of the Agno River aquatic environment and to speed up ameliorative measures to rehabilitate the environment, more active dialogue is needed between fisheries and organizations responsible for mining, agriculture, forest exploitation, irrigational and other water uptakes and hydroelectricity production. The major objective of such a dialogue should be to achieve optimization of the land and water resources, to closely monitor the environment and to maintain a high quality of water. The size of the river basin with its complex interactions between the multitude of factors, magnified by the land-water resource development projects, shows the need to involve a more comprehensive approach than applied up to now to resolve the multitude of conflicts.
3.2 The Nam Pong (Thailand)
3.2.1 Present environmental situation:
The Nam Pong river basin, situated in a monsoonal climate, has a relatively small catchment of only 12,560 km2. Prior to the construction of Ubolratana Dam in 1965, much of the river watershed was forested, and there was no extensive clearance for planting upland crops. The only large town in the river basin was, and still is, Khom Kae. Now, soils in the catchment above the Ubolratana Dam are highly eroded and poor in nutrients. The major tributaries to the reservoir are faecally polluted. Industrial pollution is probably negligible, as industries are mostly located downstream of the dam. The major engineering modifications in the catchment include three dams and an irrigation system. The major benefits come from the dam and power station which have been in operation since 1966, and from irrigation water uptake started in 1966. An unanticipated benefit from the project has been fish production from the reservoir.
The creation of the Nam Pong reservoir increased upland agriculture and the establishment of resettlement areas has tremendously affected the character of land use in the Nam Pong Basin. The major changes have been (i) rapid change of forest land into land for farming, (ii) change of originally flood-inundated areas into land under controlled irrigation for rice and miscellaneous crop cultivation, (iii) seasonal use of the reservoir drawdown areas for agriculture.
3.2.2 Major engineering modifications in the basin:
These include three dams: a small reservoir on the Nam Phom River, upstream of the Nam Pong reservoir, and the Nong Wai diversion dam further downstream of it. The Nam Pong (Ubolratana) reservoir, the major engineering impact on the river basin, is a multipurpose project which serves hydropower generation, irrigation, fisheries, flood control and recreation. The reservoir which covers 410 km2 at fully supply level, proved soon after impoundment to have eminent fisheries importance. The irrigation system extends on both banks of the Nam Pong River, downstream of the Nong Wai diversion dam. An industrial centre is established between the Ubolratana and Nong Wai dams, and a major town, Khon Kaen, is downstream of the Nong Wai diversion dam.
3.2.3 Fisheries:
Prior to damming the Nam Pong River, only subsistence fishery by part-time fishermen, mostly farmers, was practised in streams and rivers, lakes, canals and paddy fields. The fishing season coincided with the rainy season, and during the dry season fishing was negligible. The damming of the Nam Pong resulted in a decrease in number of fish species which were purely of riverine character. The commercial catch over the last 15 years has ranged between 30 and 63 kg/ha/yr, with the cyprinid fish dominating the total and with six major fish species forming 79.65%. With 14 fishermen/km2 in 1978, the fishing pressure was very high, resulting in a modest average annual catch of only 340 kg/fisherman. There has been no limit on entry, no closed seasons or restricted areas to fishing. Among the 19 species of fish stocked, several were indigenous species. Of these Puntioplites proctozysron and Puntius spp. are among the six major species dominating the catch. However, insufficient data on recapture do not allow assessment of the portion of the catch derived from stocking. During the four-year period of 1969–1972 another introduced fish, sepat siam (Trichogaster pectoralis), dominated the yield.
Outside the Ubolratana reservoir aquaculture practices extended over 665 ha of ponds, 437 ha of paddy fields and 46 690 ha of other water-bodies. The Nong Wai irrigation system has a strong fisheries component with 183 fish ponds, 96 paddy fields and 16 impoundments stocked with fish. The average production of small impoundments is 300 kg/ha. The marketing to home consumption ratio differs in the Nong Wai irrigation area from that of the lakeside fisheries, with 80% of fish captured from the former being for home consumption and 20% sold and bartered, while from the reservoir 80% is sold and 20% home-consumed.
3.2.4 Cost and benefits of the river basin development:
An economic evaluation of the project published in 1981 concluded that, based on a project life of 50 years, the project provided negligible financial benefits; in other words it was a poor investment. The economic evaluation considered as costs were power, irrigation, resettlement, loss of forests and paddy fields, while benefits were power, irrigation, flood control, fishery and recreation. Social benefits including those accrued from better nutrition have not been considered.
3.2.5 Modelling for environmental management, including fisheries:
An integrated management oriented simulation model of the Nam Pong Basin, taking into account the biological, physical and socio-economic aspects of the system, was prepared in 1980 and finalized in 1981 (Mekong Secretariat, 1982), 15 years after the Nam Pong reservoir was formed. The model consists of four interacting subsystems encompassing the dynamics of water management, land-use patterns, socio-economics and reservoir fisheries. The fisheries submodel has been based on the understanding that during the last few years the reservoir fisheries have been declining. Several reasons for this have been identified, among which figures prominently lack of control of the number of fishermen, which has resulted in overexploitation and a steep decline in catch per unit effort. The fish species composition has been shifting to small herbivores of low economic value.
The reservoir fisheries submodel simulates the dynamic of the Nam Pong reservoir fisheries as a function of hydrology and fishing effort. Monthly biomass is predicted for four groups of fish, each consisting of several species: small herbivores, large herbivores, small carnivores and large carnivores. Biomass predictions are based on reservoir size, inflow, nutrients, fishing effort, predator biomass and prey availability. Fishing harvest predictions are calculated from the previous year's harvest and the current reservoir elevation.
Only those model scenarios which implemented explicit control of the number of fishermen (or limited access) predicted significant increases in fish stock and harvest and also maintained a viable population of the carnivore fish group. The most effective strategy for protection of fish stock with the aim of maintaining a high recruitment for the following year was achieved in the simulated model by reducing fishing intensity during the minimal water level period.
3.2.6 Coordination of resource management:
There has been no coordinating authority for the basin, which is the major obstacle to the realization of the potential benefits (Mekong Secretariat, 1982). At present more than 10 departments are actively engaged in developmental efforts in the Nam Pong Basin, with little or no coordination of operations. The major problems have been identified as lack of unity of command, lack of coordination in resource management, lack of qualified personnel, in particular a shortage of competent planners and project analysts responsible for development planning at the provincial level, and lack of goal consensus. Optimization of the reservoir fisheries under the multiple-use system is being coordinated by the Ubolratana Reservoir Fishery Research Centre (URFRC) which reports directly to the Department of Fisheries in Bangkok. The four major goals of the URFRC are to maintain or, if possible, to increase the fish yield from the reservoir, to protect the fish spawning grounds, to educate local people through extension programmes in conjunction with conservation projects and to use the Ubolratana reservoir to test fishery management activities for possible application to other reservoirs in Thailand (Mekong Secretariat, 1982).
3.3 The Mahaweli (Sri Lanka)
3.3.1 Present environmental situation:
Although the Mahaweli is the largest river in Sri Lanka, the total area of the basin covers only 10,420 km2. Prior to the Accelerated Mahaweli Development Programme, initiated in 1979, the basin had a 44.5% forest cover and cash crops on 26.4%. It also had 4 000 ha of seasonal tanks, 12,800 ha of flood lakes and 9,951 ha of existing reservoirs. The current population density in the upper basin of the river is 372 persons/km2 and that in the lower basin 88 persons/km2; the projection for the year 2000 for the whole of Sri Lanka is 358 km2. To provide enough food for the growing population and to increase its living standard, the Government of Sri Lanka, since independence, has given top priority to food production. This has resulted in a gradual increase of areas under cultivation, rehabilitation of the old and introduction of new irrigational facilities. The development of new lands for cultivation could only proceed at the expense of the natural habitat and its associated fauna and flora. The major changes in the environment of the basin, imposed under the present Accelerated Mahaweli Development Programme which is nearing completion, have included resettlement of people from areas to be flooded by reservoirs and transfer of elephants into new wildlife habits, including four new national parks. Large areas are to become irrigated lands.
3.3.2 Major engineering modifications:
The Mahaweli River flows through the intermediate and dry zones which, although receiving a substantial volume of rainfall during the major monsoon season, are dry for the greater part of the year. This makes the creation of irrigation facilities a prerequisite for the expansion of farming activities in these areas. Under the Accelerated Programme five major storage reservoirs are being constructed in conjunction with agricultural and related development in a 420,000 ha portion of the dry zone. When fully developed, the programme will provide settlement for nearly one million people, boost hydroelectric power output and enable achievement of self-efficiency in agricultural products. The new irrigable areas will cover 119,000 ha of land. The total surface area of the new four major reservoirs (Victoria, Randenigala, Ulhitiya and Moragahakanda) in the Mahaweli Basin will be 9,720 ha and that of Maduru Oya in the neighbouring catchment, 3,480 ha. Four dams (Victoria, Kotmale, Randenigala and Maduru Oya) will have a 477.5 MW power generation capacity.
An Accelerated Mahaweli Development Programme has been planned to be completed within 6 years. 45,000 people have already been settled from the area of Victoria Reservoir, which formed in 1984.
3.3.3 Fisheries:
Prior to the Accelerated Mahaweli Development Programme, most of the fish came from the existing reservoirs. Some fishery, mostly for shrimps, took place in the estuary, while the river itself supported largely subsistence fishery. In 1980 the reservoirs yielded 3,000 t of fish, averaging 375 kg/ha/yr. The major fish captured have been the introduced tilapia, (O. mossambicus), but the indigenous, Puntius sarana and Labeo dussumieri, have also contributed significantly to the total fish catch. The fish species are not yet in an equilibrium, as shown by an increasing importance of Hyporamphus gaimardi in fish landings from reservoirs.
Data on the fish yield for the 12,800 ha of floodplain lakes (villus) are fragmentary. The largest villus of the Mahaweli Basin yields 141 kg/ha/yr, with 65% of the catch represented by O. mossambicus. There is also seasonal fishing for Macrobrachium. It is believed that floodplain fishery could be intensified without endangering the fish stocks. Seasonal tanks have also a considerable fisheries potential.
3.3.4 Fisheries development:
During the planning for fisheries in the new reservoirs constructed under the Accelerated Programme it was predicted that for the capture fisheries a fish yield of 2,950 t/yr could be achieved. However, a detailed programme of stocking the new reservoirs will be necessary to realize the fishery potential as soon as possible. Experience from the existing reservoirs indicates that the new reservoir fish stocks would become dominated by O. mossambicus, and that stocking of O. niloticus would enhance the utilization of the high level algal production of the early post-impoundment period. The minor fish species such as cyprinids which are currently not exploited in the existing reservoirs could also be utilized. Under an inland fish farm programme, a fish breeding and research station has been established near the Ulhitiya reservoir. Fingerlings of Indian carps and two species of tilapia have been stocked, and 80 small boats have been distributed within the area. The scheme operates under a 90% subsidy from the Ministry of Fisheries.
In spite of the predicted losses of fish from floodplain fishery, there has been a net increase of a total of some 3,200 t of fish from the capture fishery in new waterbodies established under the Accelerated Programme. The total non-aquaculture fish yield from the Mahaweli and Oya basins is expected to be about 9,000 t. The seasonal tanks should provide a further 2,250 t, and aquaculture 3,750 t. The total of 15,000 t/yr should satisfy the requirements of about one million inhabitants expected to inhabit these basins in 1990.
The increasing importance of inland fish in nutrition of the people of Sri Lanka is reflected in the inclusion of a fisheries component in overall planning considerations for the Mahaweli (and Oya) river basin development. The Government has also encouraged development of aquaculture in the area.
3.3.5 Summary of constraints to fisheries:
The impact of land mismanagement in the Mahaweli Basin on the river fish has not been evaluated, perhaps because the river fish never formed an important component in nutrition of the people. Natural backwaters and existing reservoirs have been little affected by the changes in water quality and quantity as a result of the increased erosion and faster and more irregular runoff. There are no major pollutant inputs into the river system although some waterbodies are undergoing eutrophication, as evidenced, for example, by massive growth of exotic weeds such as Salvinia molesta in some reservoirs and tanks.
Environmental measures to rehabilitate the upper river catchment have involved reforestation of the highlands where large areas were destroyed by chena (slash and burn) cultivation, which in the meantime has been outlawed by the Government. Siltation is a major problem for irrigation schemes, and to reduce erosion such reforestation has been going on since 1981. As the inundation caused by damming of the Mahaweli and the Maduru Oya eliminates one-fifth of the remaining tree cover, planting of trees on a large scale will be a necessity to provide fuelwood for cooking, still used in over 90% of Sri Lankan households.
The yield of exotic tilapias which forms the basis of current fishery in existing reservoirs, as well as that of indigenous and some other introduced fish, has been found to be influenced especially by water level manipulation, predatory and fishing pressures. At present, in the two large existing reservoirs (Parakrama Samudra and Minneriya) in the Mahaweli river basin for which data are available over a 5-year period, the very high increase in fishing pressure has resulted in a significant decline in the catch per unit effort (De Silva 1985), while some other reservoirs remain underfished. In the new reservoirs, water level manipulation and fishing pressure may be expected to become major factors determining the fish yield.
