Impacts of irrigation on fisheries in rain-fed rice-farming landscapes: Impacts of irrigation on fisheries

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1Expanding irrigation development threatens the productive and diverse fisheries of rain-fed rice-farming landscapes. Environmental management of irrigation can minimize negative impacts on fisheries, but its effectiveness is constrained by a lack of
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    Impacts of irrigation development on capture fisheries in the rice-based farming systems of southern Laos Sophie Nguyen Khoa, Kai Lorenzen and Caroline Garaway  Department of Environmental Science & Technology, Imperial College, London, UK Bounthanom Chamsingh  Regional Development Committee for Livestock and Fisheries (RDC), Savannakhet, Lao PDR Correspondence: Dr Kai Lorenzen, Department of Environmental Science and Technology, Imperial College, London SW7 2BP, UK. E-mail: k.lorenzen@imperial.ac.uk   2 Summary Fish populations inhabiting the rivers, streams, natural wetlands and paddy fields of rice-based farming systems support capture fisheries of major importance to local livelihoods. Water resources development for irrigation has the potential to impact significantly on these fisheries by modifying habitats and their connectivity, as well as patterns of exploitation. We conducted a field survey to establish the impacts of small to medium sized weir and dam irrigation schemes on local fisheries in the rice-based farming systems of southern Laos. The survey was replicated at the irrigation scheme level. Impacted sites were paired with non-impacted controls within the same watershed. Weir schemes were associated with a significant decline in households and per-area catches which as not fully explained by a concomitant change in fishing effort. Dam schemes caused no significant overall decline in catches, but a very significant re-distribution of catches and effort into the newly created reservoirs. In both weir and dam schemes, changes catch were largely explained by changes in fishing effort. No significant impacts on fish species richness were detected. Small-to-medium sized irrigation schemes have only moderate impacts on local fisheries in rice-based farming systems. Net impacts of weirs may be more pronounced than impacts of dams. Rather than being fundamentally degraded as often assumed, fish populations and the fisheries they support can remain productive and diverse within irrigated rice systems. Protecting and enhancing wild fish stocks in such systems is likely to generate social and ecological benefits. Key words: irrigation development, tropical inland fisheries, impact assessment   3 Introduction Inland fisheries play an important role in rural livelihoods in many developing countries, in particular within the most resource-poor sections of the population (ADB, 2002). The productivity and diversity of the aquatic resources upon which these fisheries depend is closely linked to the functioning of aquatic ecosystems, hence inland fisheries link environmental and food security issues in a unique way. The productivity and diversity of inland aquatic resources are strongly affected by the development of water resources for irrigation, hydropower generation, and urban and industrial uses. In rural areas of the developing world, irrigation is widely seen as the single most important intervention impacting on fisheries. Irrigation systems may abstract and deplete (through evapotranspiration) a large proportion of the annual flow from tropical river basins, and reduce the overall availability and ecological connectivity of aquatic habitats. In addition to direct physical and ecological impacts, irrigation development may affect fishing practices, use rules and the opportunity costs of fishing. Hence the overall impacts of irrigation development on fisheries are multiple and varied. It is now recognized that fisheries impacts should be considered in the planning and management of irrigation development, but there is a lack of reliable quantitative impact assessments to inform this process (WCD, 2000). Although substantial qualitative knowledge has been generated in recent years (mainly through detailed descriptions of impacts on fish ecology), to date and to our knowledge, no quantitative assessment of the   4 impacts on fishing activities and their consequences for rural livelihoods has substantiated the qualitative information. As a result impacts on fisheries are often effectively ignored in performance analyses of irrigation schemes. Despite growing awareness, aquatic ecosystems continue to be degraded by unsound forms of utilization (ADB, 2002). The aim of our study is to assess quantitatively the fisheries impacts of irrigation development within rice-based farming systems. Rice-based farming systems support a large share of the rural population in South, Southeast and East Asia, and in parts of West Africa. Rainfed rice paddies are designed to store water for extended periods, creating aquatic ecosystems with many similarities to natural floodplains (Heckmann 1979). Like floodplain habitats paddies are colonized by fish during the wet season and contribute substantially to the overall fisheries production of the river-floodplain system. Fishing, often carried out on an occasional or part-time basis, makes a significant contribution to livelihoods in many rice-based farming systems. Irrigation development in rice-based systems aims to allow cultivation of a second crop of rice or another field crop, or at the very least to secure a single crop in dry years. The present study is concerned with small to medium scale irrigation schemes, irrigating command areas of up to about 500 ha. The schemes considered in this study use gravity to supply water to fields via canals, but differ in the diversion structures used abstract water from rivers. Weirs are relatively low structures that divert water without creating a significant storage reservoir. Dams involve significant storage of water in reservoirs.   5 Weir irrigation schemes have the effect of exacerbating the natural seasonality of river flows, abstracting water mostly during the dry season when irrigation demand is highest. Dam schemes on the other hand tend to attenuate natural flow patterns, retaining a large share of peak flows in the reservoir but increasing dry season flows due to drainage and seepage. We carried out a quantitative assessment of irrigation impacts on artisanal fisheries in Southern Laos. The study was designed as a paired comparison of household fishing effort and yield as well as local fish stock diversity between irrigated and non-irrigated sites. Material and methods STUDY AREA In Laos, the climate is tropical with an average daily temperature of 31°C and an average annual precipitation of 1500 mm, about 75% of which occurs in the monsoon season (May to October). Rice is the single most important crop in Lao agriculture, accounting for about 80% of the cultivated area. More than 85% of rice produced is of traditional varieties of the glutinous type, and annual yields are in the range of 1.5 to 2.8 t/ha. Only 3% of the paddy area is irrigated, and the dependence on rainfed systems is seen as the major constraint to the expansion of rice production (Suan 1989; IRRI 1999). Agricultural production in the rainfed areas tends to be subsistence-oriented.
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