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

1. Expanding 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 reliable information on the nature and magnitude of impacts. 2. To quantify the impacts of small- to medium-scale irrigation schemes on aquatic habitat availability, fish catches, species richness and ecological composition of fish assemblages in these landscapes, we conducted a field study in Laos. The observational study was replicated at irrigation scheme level, covering 10 weir and 10 dam irrigation schemes paired with non-impacted control sites. 3. Weir schemes had no significant impact on aquatic habitat, but caused a significant decline (-36%) in fish catches that was only partly explained by a reduction in fishing effort. Weirs had no effect on species richness, but were associated with a significant increase (+17%) in the relative abundance of omnivores. 4. Dam irrigation schemes significantly reduced riverine habitat area, and increased lacustrine and dry-season rice-field areas. Dams led to a marked redistribution of catch and fishing effort from non-reservoir habitats into reservoirs, but no overall change in catch or effort occurred. The redistribution reflected a response to fishing opportunities in the reservoir, rather than a loss of productivity in non-reservoir habitats. No significant impacts were detected on fish species richness or the relative abundance of functional feeding groups. 5. Overall impacts of irrigation on fisheries were related mostly to changes in fishing effort, rather than ecological effects on the resources. The unexpectedly moderate level of ecological impacts may reflect compensatory effects at the landscape level and the fact that rice fields, which dominated the wet-season habitat, continued to be managed as rain-fed deep-water systems even where dry-season irrigation had been developed. 6. Synthesis and applications. Small- to medium-scale irrigation schemes in rain-fed rice-farming landscapes have only moderate impacts on fisheries, which remain productive and diverse. Changes in agricultural practices in the wet season are likely to have greater effects on fisheries than dry-season irrigation.