Beschreibung: Riparian fluvial systems are used for the irrigation of rice. This practice has led to an increasing need to conserve biodiversity of these riparian systems against intensification of rice　irrigation systems. This study examined the underlying processes that structure fish community assemblages in a traditional rice irrigation system and how these processes might inform conservation programmes. Sampling was conducted at 26 reaches of a permanent irrigation channel in September and December 2007 and May 2008. The relative roles of niche versus dispersal processes were determined by examining species richness across the local environment, such as habitat complexity (diversity of depth and velocity) and size (volume of water) or proximity from the river as potential source of species, and deterministic versus stochastic processes. Non-random or random patterns of species co-occurrence were also compared across the sites. Each site's species richness was mainly associated with habitat complexity and size, but not proximity to the river. Also, species co-occurred non-randomly in segregated patterns. Therefore, the fish community was deterministically structured by niche processes and local extinction, but not by stochastic and external immigration processes. In addition, the observed relative abundance distributions primarily fitted a log–normal distribution, indicating a ‘low stress’ condition for the fish community. To enhance co-existence of fish species, high conservation priority should be given to local factors such as hydraulic complexity and volume of water at reach scale in the rice irrigation system. Fish species preferring slow and shallow environments co-occurred less frequently among reaches, demonstrating the need to conserving hydraulic variability. The effectiveness of local factors and deterministic patterns is thought to derive from stable water flow and high internal and external connectivity, which in turn would safeguard the processes that structure fish community assemblages. Such hydrological connectivity might be the key for conservation in intensified rice-farming landscapes. Copyright © 2015 John Wiley & Sons, Ltd.