1. Synchronous fluctuations of the abundance of several populations at a regional scale can be a sign of a climatic effect on their dynamics (Moran effect). However, the interpretation of spatial synchrony is often complicated by the interaction between climatic disturbances and migrations between the studied populations. 2. We addressed this question by studying 24-year time-series of abundance estimates for four populations of four fish species in three different catchment basins in Côte d’Ivoire (West Africa). As localities from two different basins cannot exchange individuals, dispersal cannot be considered a synchronizing factor in this system. 3. A sign test performed on the four site × site correlation matrices rejected the null hypothesis of no spatial synchrony. No relation was detected in relating the level of synchrony in species dynamics (r = 0·58 on average) to the geographical distances between the four sites (176–367 km). A more detailed analysis taking account of sampling error was carried out between two sites, and confirmed the above results. When corrected for sampling noise, high values of spatial synchrony were obtained (r = 0·88 on average). 4. Interspecific spatial synchronies (r = 0·57 on average) were almost as high as intraspecific values, suggesting that the four species are reacting in the same way to the same Moran effect. 5. A good correlation (r = 0·54) between a regional river discharge index and the total catch of the four species 1 year later was observed, suggesting that hydrological variability is one of the main synchronizing factors. Moreover, as expected on theoretical grounds, spatial synchrony in river discharge (r = 0·84 and 0·88 on a log scale) is similar to spatial synchrony in population dynamics (r = 0·88 on average and corrected for sampling error).
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