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Towards improved management of reservoirs in the semi-arid Northeast region of Brazil

May 17, 2000

In the Northeast region of Brazil, reservoirs called a'§udes are the main water resource during periods of drought, which are frequent and consequently often catastrophic in this part of Brazil. IRD has since 1995 been conducting a research programme on these reservoirs, in conjunction with the Federal Rural University of Pernambuco's Department of Fisheries and the Pernambuco Secretariat of Rural Production and Agrarian Reform (SPRRA). This body of research aims to gather new information needed for better management of these vital water reserves. Focused on the biology and fish ecology of the a'§udes, one of the principal parts of the programme should contribute to improved use of small-scale fishing and aquaculture. These activities, which have been declining for several years, are now considered to be a sector of production which is reemerging.

With 70 000 shallow reservoirs, or a'§udes, of surface area greater than 1000 m_ distributed over 11% of national territory, the semi-arid Northeast region of Brazil has the world's second largest density of such reservoirs after India. They were constructed according to a XIXth century policy adopted to alleviate water shortages caused by frequent severe drought. The Nordeste region is again faced with a worrying deficit of water, notably after the past two years when the El Ni'±o effect was particularly strong. Between 1997 and 1999 one of the main States of this region, Pernambuco, lost 90% of its surface water reserves. Owing to the scarcity of rainfall, the a'§udes, which are shallow (less than 10 m on average) and rich in nutrients (nitrogen and/or phosphorus), generally remain at about 10% of their capacity and only very rarely reach their maximum level. Fed with water by ephemeral rivers for only a few weeks per year, they are subject to intensive evaporation (3 m/year). This can induce salinization of the water, rendering it unusable for any purpose, or can even dry them up completely.
In the Northeast region, people use the a'§udes for domestic water supply, livestock rearing and irrigation. For several decades now, fishing and aquaculture have also been practised. In the public-sector reservoirs, the introduction by the National Department for Work against Drought of more than 100 million alevins (more often omnivorous species coming from diverse areas of Brazil or from other continents) has contributed to a growth in productivity and encouraged small-scale fishing. At present, however, the potential of these reservoirs for fishing activities is insufficiently exploited.
Until the research programme was launched, no assessment of the impact of stocking the a'§udes with young fish on fishing productivity had been undertaken. The researchers therefore first set about to draw up an inventory of the fish communities and of the phyto- and zooplankton and evaluate their capturability. They did this by running experimental fishing expeditions in 13 reservoirs and six catchment basins situated in Pernambuco State. The species richness of fish turned out to be low, with 3 to 13 species per reservoir. These could be divided up into omnivores, macrocarnivores (predator fish), microcarnivores (smaller species, constituting the prey of the former) and benthophage fish (feeding off bottom-dwelling organisms). Fishing yields there are comparable with those of reservoirs and rivers in other areas of Brazil.
The investigation has shown that in some cases fishing could be developed. It appeared that, in the reservoirs where environmental and biological conditions encourage the presence of species that reproduce in small sizes, a reduction in the permitted mesh dimensions of fish nets could be envisaged without jeopardizing fish stocks. With the aid of legislation specially adapted to these a'§udes, fishing, which currently involves mainly tilapias, could thus be extended to prolific smaller species (like piabas, piaus and sardines) which up to now have been little sought after. The biological equilibrium of these ecosystems would not suffer any disturbance.
The study has moreover emphasized that, depending on the fish species introduced into these reservoirs, it was possible to decide whether to designate them mainly for fishing or for water supply. Thus, if the water is for domestic use, in order to improve its quality it is necessary to increase the number of carnivorous species and reduce the stock of omnivores. The latter tend to dominate fish biomass and eject large quantities of nutrients. Growth in phytoplankton is therefore stimulated and together these factors lead to eutrophication which harms water quality. Conversely, if fishing is to be the end-use, it is better to increase the number of omnivores. Their rapid growth which comes with a highly diversified diet (phytoplankton, zooplankton, benthic organisms and various sorts of detritus) contributes to a rapid increase in biomass in the reservoir which can then be used for fishing.
The researchers also tested in fish breeding ponds a model for farming several species at once intended for small private or publicly owned, a'§udes, combining fish with different dietary requirements. After three months of rearing, the biomass multiplied nearly tenfold to reach 680 kg/ha. This extensive 'polyculture' requires just one stock introduction with a primary fertilization and thereafter only the food input necessary to feed the fish. It does not appears to spoil the water quality. The model proposed is inexpensive and therefore would meet the socio-economic constraints of the Northeast region. It proves to be well adapted to the reservoirs' naturally high productivity.
The results so obtained from the A'§udes programme stress the need for a regular monitoring of the reservoirs' water level and the biological quality, fish populations and fishing activities. It will therefore be possible to gear reservoir management closely to the priorities defined for their use. Technical measures based on these findings, already being gradually implemented by the Pernambuco Secretariat for Water Resources (SRH-PE) and SPPRA/DERH, are being accompanied by a campaign to increase awareness of local people to the environment, another important part of the overall programme.

Institut de Recherche pour le Développement, Paris (IRD)