Large-scale synchrony and inter-annual variability in roach recruitment in the Rhone River: the relative role of climatic factors and density-dependent processes

We report patterns of temporal variation in the recruitment of roach (Rutilus rutilus). The data consist of the annual abundance of the first two year-classes, 0+ and 1+ fish, at four sites in the Rhone River (France) between 1987-97. Over this 11-year period, both 0+ and 1+ fish fluctuated strongly. Cross-correlation indicated high spatial synchrony in 0+ dynamics, although correlations among sites in 1+ dynamics were weaker. No clear pattern was apparent in the relationship between the level of synchrony and distance between pairs of sites. The spatial synchrony in 0+ fish could be attributable to large-scale variations in weather, influencing water temperature. Total length in 0+ roach was correlated with water temperature (expressed in degree-days over 12°C), and water temperature was the main factor explaining inter-annual variation in 0+ cohort size. Monthly variation in abiotic factors (measured by standard deviation in water temperature and discharge) did not influence 0+ fluctuations. Correlations with June water temperature suggest that year-class strength was mainly determined by abiotic factors during the first few months of life. The absence of spatial synchrony in 1+ fluctuations suggests either correlation between survival and abiotic conditions during the first year of life, other factors influencing survival. Survival in the first year was density-dependent. Intraspecific competition within the 0+ cohort could thus influence the fluctuations in recruitment to older age-classes. The implications of age- or stage-dependent level of synchrony for species with complex life histories are discussed. Studying spatial synchrony for the different life history stages could enhance our understanding of the population dynamics of spatially structured species.