December 11, 2017

The effects of large-scale water projects on fisheries

Published as Chapter 8 of The Social and Environmental Effects of Large Dams: Volume 1. Overview. Wadebridge Ecological Centre, Worthyvale Manor Camelford, Cornwall PL32 9TT, UK, 1984. By Edward Goldsmith and Nicholas Hildyard.

Short-term successes: long-term failure

Proponents of large dams place considerable emphasis on the potential which a dam’s reservoir offers for the setting up – or, indeed, the expansion – of fishing industries. Even those who are critical of many aspects of large dams see such fisheries as providing major benefits. Professor Ackermann, for instance, regards the boost given by dams to fishing as “one of the more gratifying aspects of man-made lakes.” [1] Among other things, he sees such fisheries as slowing down the migration of young people to the cities, and considers that the new fishing opportunities may even lure back those who have already left an area.

Undoubtedly, when a large reservoir is filled, there is likely to be a dramatic rise in the population of those fish species which are favoured by the new lacustrine conditions – although, those fish which are adapted to a riverine environment will tend to disappear. All in all, however, the actual number of fish is likely to increase quite substantially as advantage is taken of the vastly expanded aquatic environment. So, too, the release of large quantities of nutrients from the rotting vegetation and soils which have been submerged by the reservoir – together with the increased populations of those microorganisms favoured by the new conditions – will encourage the expansion of fish populations.

That expansion in fish numbers, however, is likely to prove a short-lived bonanza. The submerged vegetation and soils soon rot down – thus reducing the amount of available nutrients – whilst competition and predation cuts down the inflated populations of those fish species which first dominated the reservoir’s ecosystem. As this occurs, so the lake environment will become more highly structured, more diverse and more stable.

In the case of Lake Volta, for instance, a very considerable fishing industry was indeed developed immediately after inundation. In fact, at one time, there were as many as 20,000 fishermen on the Lake, using some 20,000 canoes and catching up to 60,000 metric tonnes of fish a year. But catches rapidly fell off as the submerged vegetation below the lake rotted away and nutrients became less and less readily available.

The experience of fisheries on Lake Kariba is similar. Thus, five years after the lake was formed, some 2,000 fishermen were landing 3,628 metric tonnes of fish per annum. A few years later, however, landings had dropped off dramatically. Ten years after closure, no more than 907 metric tonnes of fish were caught and the number of fishermen fell off correspondingly. Efforts to restock the lake with new species proved a dismal failure: 26 tonnes of juveniles were introduced in the lake but very few survived. By 1978, fish catches had fallen so low that only a small part of the human population along the shores of the lake was engaged in fishing.

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Dams and the destruction of fisheries

The pattern of fish yields at Lakes Kariba and Volta would appear to be fairly typical of those at other man-made lakes. Initial success is invariably followed by long-term failure. It is important to realise, however, that a dam’s impact on fisheries does not begin and end with the fate of the fish in its reservoir. By disturbing the ecological balance of the rivers it impounds, a dam can have a serious effect on fish life within the river basin itself – and, indeed, within the seas immediately beyond its estuary.

The problem is succinctly put by Dr. David Tolmazin, former head of the Marine Economy Department at the Economics Institute of the Ukrainian Academy of Sciences.

“Rivers, sectioned off by dams, are no longer single ecosystems,” he points out. “The flow of suspended material and dissolved gases, which previously sustained the life of a river population, is interrupted. Despite the construction of artificial channels for fish and other measures for transporting them, fish migration is disrupted: spawning and fattening conditions deteriorate. The total river population decreases substantially, and some species disappear completely. The restricted storage lake ecosystem cannot ensure the survival of all life in the river .” [2]

Indeed, in terms of fish yields, the loss of fish throughout the river basin as a whole can, in most cases, equal – or even exceed -the temporary gains made in a dam’s reservoir. At that point, the claim that dams help to boost fish production wears somewhat thin.

Let us look, then, at the extent to which dams disrupt river ecosystems – and, in particular, at their effects on fish life:

  • Firstly , as Tolmazin intimated above, dams tend to reduce the catch of migratory fish by preventing them from reaching their spawning grounds. The higher the dam, the more fish will be lost trying to leap it or to swim through its turbines. Although fish are often -though not always – provided with ‘ladders’ to enable them to swim over the dam, such ladders are expensive and do not always work: where dams are more than 30 to 40 metres high, for instance, fish cannot move up the ladders without fatally delaying their migration.In some cases, the resulting decline in certain fish species has been dramatic. In California, for example, catches of salmon have fallen by 90 percent largely because dams now make it almost impossible for the salmon to travel upstream in order to spawn. So, too, in the USSR, the drastic fall in sturgeon populations has been blamed primarily on the dams which have cut the sturgeon off from their natural spawning grounds in the major rivers entering the Caspian Sea. 
  • Secondly , the creation of vast storage reservoirs tends to reduce the flow of rivers – largely because the waters which are stored are drawn off for domestic, agricultural or industrial use. Invariably the result is a fall in the water levels of those lakes and inland seas which are fed by rivers – with disastrous consequences for fish life. Thus, in the USSR, the shrinking of the Caspian and Aral Seas as a result of upstream water abstraction is partially blamed for the reduced catches of sturgeon and other commercial fish. In the United States, according to the US Water Resources Council, the rivers of fourteen states in the mid- and southwest are now so over exploited that they have barely enough water to provide for fish and wildlife. If such habitats are to be protected, then, according to a study by the US Department of Agriculture, the agricultural area under irrigation in the US must be cut by at least a fifth. [3] At present, however, every effort is being made to expand the amount of land under irrigation.
  • Thirdly , the building of storage schemes and other water development projects has led to an increase in the salinity of many rivers. In part, that increased salinity is due to the return of highly saline drainage waters from irrigated land: in part, to the reduced flow of the rivers themselves. Thus, increased water abstraction upstream not only reduces the amount of water available to dissolve the salts coming downstream but also leads to the intrusion of sea water into the estuaries and deltas of rivers. Indeed, the salt content of the lower reaches of many rivers is now so high that – on that count alone – they no longer provide suitable habitats for riverine fish.
  • Fourthly , the building of a dam traps the silt which was previously washed downstream as the river flowed unimpeded to the sea. That silt – which simply builds up behind the dam – contains nutrients which are vital to the survival of fisheries in the lower reaches of a river and in the sea beyond (see Chapter 4). Before the building of the Aswan High Dam, for example, the sardine fisheries along the eastern Mediterranean coast yielded some 13,000 tons of fish a year. Deprived of the nutrients in the Nile’s silt, however, the sardine population fell dramatically. Indeed, by 1969, catches were down to 500 tons a year – a loss which was only partly compensated by the development of a high seas fishing industry which, in 1969, caught some 5,000 tons of fish. [4]So, too, Tolmazin points out that the drastic decline in the fish yields of Russia’s Azov and Black Seas “coincided with the introduction of schemes to regulate a large number of major rivers.” The brackish coastal area was thus “substantially deprived of the minerals and detritus brought down by the rivers, so necessary for life in the estuaries and the seas.”

    The result, says Tolmazin, was a “piscatorial disaster”. Between the late 1940s and the early 1970s, the Black Sea saw the disappearance of the Black Sea mackerel, the large horse mackerel, the palamida and the bluefish. By 1979, “the only edible fish still being caught were the anchovy, a small type of horse mackerel and the occasional sprat.” In the Azov Sea, the destruction of fisheries was even more pronounced: by the 1970s, the fish catch was only one-third of what it had been in the 1930s. Worse still, it is the most valuable fish which have disappeared: species such as sturgeon, perch, pike, bream and herring which once made up 70-80 percent of the catch, now account for a mere 7-8 percent of the fish landed. [5] 

  • Fifthly , the invasion of artificial reservoirs and their associated waterways by aquatic weeds has seriously reduced fish yields both upstream and downstream of dams. The weeds – whose proliferation is the direct result of interfering with river ecosystems – affect fish populations in a number of ways. Firstly, they increase water losses to evapotranspiration, thus reducing the water level of the reservoir. Secondly, by virtue of their sheer mass, they inevitably reduce the effective capacity of the reservoir, hence restricting the habitat available for fish life. In addition, when they rot and die, they use up valuable oxygen, which can therefore result in high fish mortality. Moreover, by diminishing the sunlight both at the surface of a reservoir and in the waters below, weeds reduce the biological productivity of a reservoir – which, in turn, reduces the number of micro-organisms on which fish can feed. Certain weeds also produce toxins, whilst others give rise to rotting scums: both types have been implicated in causing fish kills.In addition to their adverse effect on fish life, weeds also interfere with fishing activities. When a reservoir is created in a forested area, for instance, and the forest is not cleared before flooding, there tends to be a proliferation of the weed salvina . Where this occurs, the salvina frequently forms broad ‘mats’ (which find anchorage amongst the partly submerged trees and shrubs) not only severely restricting the movement of boats – to the detriment of the local fishing industry – but also blocking hydro-electric turbines and even harbours. Although those problems can largely be avoided by clearing forested areas before flooding, such clearance is rarely undertaken. Where forests have been cleared, there tends to be a greater diversity of aquatic plants among which fish, such as tilapia, can thrive. 
  • Finally, aquatic weeds lead indirectly to the loss of fish life as a result of the herbicides which are used to eliminate them – albeit temporarily. Such chemicals kill certain species of aquatic life and leave others to proliferate, thus disrupting the aquatic ecosystem still further. Because the weeds soon reappear after spraying, the herbicides must be applied on an almost continuous basis if the lake and its waterways are to be kept weed-free. Over the years, therefore, the active ingredients of the chemicals tend to build up in the water and the sediment, eventually leading to the contamination of the lake and the poisoning of fish.
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Pesticide pollution and the destruction of fisheries

Unfortunately, the herbicides used to control aquatic weeds are not the only chemicals which are likely to pollute a lake and its waterways. As we have seen in Chapter 7, large-scale water development projects greatly increase the habitat for the vectors of waterborne disease. To control those vectors, vast quantities of various pesticides are applied every year to both reservoirs and lakes. Herbicides are sprayed to kill the vegetation they feed off; moluscicides to kill the snails that carry schistosomiasis; and insecticides to kill the blackfly which transmit onchocerciasis (river blindness) and the mosquitoes which transmit malaria.

Many of the pesticides currently used in the Third World are known to persist for long periods in the environment and to pose considerable health risks. In 1976, it was estimated that half the pesticides used were organo-chlorines, a group whose use has been severely restricted in the industrialised world on both health and environmental grounds. [6] Moreover, the use of such pesticides (many of which are suspected carcinogens and mutagens) has often been profligate. In 1966, for instance, large quantities of DDT were poured into the River Niger in order to combat onchocerciasis. In Uganda, the White Nile was repeatedly sprayed with DDT to protect workers from the disease whilst they were working on the Owen Dam. Large quantities of DDT were used for the same purpose during the building of the Volta Dam. [7]

Although, recently, less poisonous insecticides (such as Abate R) have been employed to combat onchocerciasis, it is questionable whether one can continue to use them year after year with impunity. Eventually, unacceptable levels of their residues are likely to build up in the sediment of the lakes and rivers where they have been sprayed. Indeed, Professor John Hunter expresses the fear that, as a result of spraying programmes, Lake Volta may become “an insecticide sink with biological repercussions yet to be determined.” [8]

He goes on to point out that very little research has been undertaken on the effects of insecticides on water quality: in the case of the Volta Scheme, for instance, only $162,000 were allocated for such research – a sum that amounted to no more than 0.9 percent of the total budget for onchocerciasis control in the area. That paltry research budget, says Hunter, clearly reflects “a low priority for ecosystem stability.” It also explains why the ‘biological repercussions’ of present and past spraying programmes have ‘yet to be determined’ – and suggests, very strongly, that they are unlikely to be understood until it is, quite probably, too late.

Despite the lack of research on specific chemicals, however, we know enough about the long-term ecological effects of biocides in general to state categorically that the systematic spraying of reservoirs, rivers and irrigation canals with herbicides, molluscicides and insecticides is quite incompatible with the maintenance of a healthy fishing industry. Commenting on the use of pesticides in South-east Asia, for instance, Professor Daget of the Museum of Natural History in paris points out: “Since the effect must be to kill off insects and plant life, they must necessarily reduce the total quantity of natural food available to fish, especially in the paddy fields. [9]

A recent report, undertaken as part of UNESCO’s Man and the Biosphere programme, comes to the same conclusion. In particular, it points to the vulnerability of Daphnia, the water-flea which is an essential component of many freshwater ecosystems and which is known to succumb to a wide range of pesticides. So too, phytoplankton and algae (which, because they are at the bottom of the aquatic food chain, are essential to the maintenance of the various forms of life at other levels of the chain) are rapidly destroyed by pesticides. [10]

The problem is compounded both by industrial pollution (whose effects on fisheries we shall consider later in Chapter 17) and by the increasing use of agricultural chemicals. With regard to the latter, it is important to note that the land brought under perennial irrigation by water development schemes is invariably turned over to intensive plantation agriculture (see Chapter 13). The resulting increase in the use of pesticides and artificial fertiliser – without which such farming could not be practised – has led to algae blooms and to the widespread pollution of waterways through chemical run-off.

As a result, many areas have now been rendered unfit for fish life. In India, for example, pesticide use has led to the complete loss of fish life in some rivers, reservoirs and estuaries. [11] Elsewhere in South and South-east Asia, the story is the same – particularly in those areas where new ‘high-response’ varieties of rice and other crops have been introduced as part of the Green Revolution. Such crops are extremely vulnerable to insect depredations and, therefore, require the application of large quantities of pesticides – often with devastating consequences.

In 1983, for example, more than a million fish were killed by biocides in Thailand’s Suphanburi province in what has since been described as “the country’s worst man-made ecological disaster.” [12 ] The biocides – notably paraquat and Dieldrin – had been sprayed to protect rice crops in the region.

Just as it is the deltas of large rivers which suffer most from silt deprivation and increased salinity, so such areas are worst hit by pollution of a river’s higher reaches. Moreover, the destruction of delta ecosystems poses a particularly serious threat since deltas tend to be extremely rich in fish life. The Mekong Delta, for example, provides its 20 million inhabitants with an estimated 200 million tons of fish a year. Yet, such areas are being systematically destroyed by the activities of man.

In that respect, the experience of the Delta Lakes of the Nile is eloquent. Thus, Dr. Carl George of Union College, New York, notes how artificial fertiliser run-off “has created areas of anaerobic waters which are becoming an increasing problem in the shallow brackish waters of the Delta Lakes.” [13] So too, “periodic massive fish-kills have been reported . . . as a result of run-off from insecticides, herbicides and moluscicides.”

In addition to that chemical assault on their ecological integrity, the Delta Lakes have also suffered from silt deprivation, reduced flow and increased salinity. Indeed, the whole ecosystem is now so seriously disrupted that it is rapidly ceasing to provide a suitable habitat for fish life.

In areas where people have traditionally depended on fish to provide them with animal protein, the pollution caused by agricultural chemicals has particularly serious implications. In many parts of South and South-east Asia, for instance, aquaculture is widely practiced and yields large quantities of fish. ‘Cage’ culture, in particular, is highly efficient: according to V. R. Pantalu, up to 25,000 kilograms of fish a year can be produced by suspending a single cage, measuring 5 metres by 3 metres by 45 metres, in a river or large stream. [14]

Yet, as Peter Freeman, a freelance consultant and the author of one of the few overviews on the environmental effects of large dams, points out, such methods of aquaculture are “incompatible with the cultivation of high-yielding rice varieties that require pesticides.” [15] Unfortunately, the UN Food and Agricultural organisation is totally committed to the expansion of such pesticide-dependent agriculture. Indeed, it foresees global pesticide use increasing fivefold between now and the end of the century – with average pesticide use per hectare in the Third World doubling. [16]

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Dams, fishing and the net loss of protein

Even without considering the reduction in fish catches attributable to the various types of ecological disruption we have described above, it would seem doubtful whether the fisheries provided by a man-made lake can compensate for the food resources lost to flooding. In that respect, the work of Eugene Balon is particularly relevant. Thus, he points to the protein value of the fish caught in the river before a dam is built: of the crops in the farmland which is flooded; and of the wild game which inhabits the often extensive croplands, rangelands and forests that are drowned by a reservoir. When those food resources are taken into account, argues Balon, a dam may well be found to cause a net loss in available protein.

Discussing the example of Lake Kariba, for instance, he writes:

“Wild animals alone, if harvested, could have yielded the same amount of protein as the lake. Their densities, however, were never studied before the filling of the lake and, when revealed during a rescue operation, took everyone by surprise. In addition, there was space along the river for intensive agricultural use and a much higher potential harvest is possible at a lower energy cost in the river alluvium than on the escarpment or plateau.” [17]

Elsewhere, Balon has calculated the protein loss which is likely to result from building the proposed Treng Dam in Cambodia. If it goes ahead, the dam will flood 8,000 square kilometres of the North Cambodian Plains. Those plains, which adjoin the Mekong River, are particularly rich in wildlife – supporting populations of ungulates which are as dense as those in East Africa. Indeed, Balon insists that game farming on the plains could produce as much animal protein as fisheries in the dam’s reservoir: in fact, he says, “the potential for protein production seems about the same whether or not the dam is built.” [18]

The key difference, however, is that the dam will silt up in anywhere from 50 to 200 years, whilst the plains and their river valley would continue to produce game and crops indefinitely. Moreover, the food derived from the river valley is likely to be richer, more diverse and more dependable than that obtained for a few decades from fishing the reservoir.

When the protein gains of a dam’s fisheries are set against the protein losses caused by flooding, the fishing opportunities provided by man-made lakes hardly seem worth the candle. At the very best, they can only offer short-term compensation for the sustainable food-producing capacities of the river valley which a dam floods. When the loss of fish life in the waters downstream of a dam are also taken into account, the much-vaunted benefits of fishing man-made lakes quickly turn to costs. Indeed, in the long-term, the net result is a diminution of fish yields and other sources of protein throughout the river basin. Is it really a cost we should be prepared to go on paying?

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1. William C. Ackermann et. al. (Eds), Man-Made Lakes: Their Problems and Environmental Effects, American Geophysical Union, Washington, D.C. 1973, p.33.

2. D. Tolmazin, ‘Black Sea, Dead Sea?’ New Scientist, December 6, 1979, p.766.

3. Bruce Stokes, Bread and Water: Growing Tomorrow’s Food, Unpublished manuscript, (circa 1980), Section 3, p.7.

4. For a detailed discussion of this subject, see Carl J. George, ‘The Role of the Aswan High Dam in Changing the Fisheries of the Southeastern Mediterranean’ in Taghi Farvar and John Milton (Eds), The Careless Technology, Tom Stacey, 1973, pp. 159-179.

5. D. Tolmazin, op.cit. 1979, p.769.

6. Peter Freeman, Environmental Considerations in the Management of International Rivers, Threshold Foundation, Washington D.C. 1976, p. 16. For a recent discussion of the trade in those chemicals which have been banned in the industrialised world, see David Weir and Mark Shapiro, The Circle of Poison: Pesticides and Poisons in a Hungry World, Institute for Food and Development Policy, San Francisco, 1981.

7. Mohammed Kassas, ‘Environmental Aspects of Water Resources Development’ in Asit K. Biswas, et. al. (Eds), Water Management for Arid Lands in Developing Countries, Pergamon, Oxford, 1980, p.74.

8. John M. Hunter, ‘Progress and Concerns in the World Health Organization Onchocerciasis Control Program in West Africa,’ Social Science and Medicine Vol. 150, Pergamon, Oxford, p.271.

9. Jacques Daget, ‘La Production des Poissons de consommation dans les ecosystemes irrigues’ in E. Barton Worthington, Arid Land Irrigation in Developing Countries: Environmental Problems and Effects, Pergamon, Oxford, 1977, p.300.

10. MAB Technical Notes 8, UNESCO, paris, 1978, p.39.

11. See The State of India’s Environment 1982, A Citizen’s Report, Centre for Science and Environment, New Delhi, 1982, pp. 17-25.

12. ‘Troth-Tiranti Killer Cocktails’, New Internationalist, July 1983, p.5.

13. Carl George, op.cit. 1973, pp. 159-160.

14. V. R. Pantalu, quoted by Peter Freeman, op. cit. 1976, p. 17.

15. Peter Freeman, op.cit. 1976, p.17.

16. FAO, Agriculture: Toward 2000, FAO, Rome, 1983.

17. Eugene Balon, ‘Kariba; the Dubious Benefits’, Ambio, Vol. 7, No. 2, p.47.

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