In the Camargue (southern France), movements of fish between a canal and two seasonally flooded marshes were monitored continuously by fish traps for 3 years for one marsh and for 2 years for the other. The timing of the entry and exit of the main species was determined together with species annual demographic balance, defined as the difference between the number of fish leaving and the number of fish entering. Only some of the species inhabiting the canal colonized the marshes. Except those species that reproduced in salt or brackish waters, all the species colonizing the marshes reproduced in it. The most abundant of these species were small sized (mosquitofish, sand smelt and three-spined stickleback). The unpredictability of water levels in summer was particularly unfavourable for recruitment and survival of species that breed late in the year, e.g. pumpkinseed sunfish, or which prolonged their stay in the marsh and only attempted to leave just before the connection was broken, e.g. carp. Only the three-spined stickleback always had a positive demographic balance as a result of colonizing the marshes. Adults of this species entered in winter and young-of-the-year left in April. By limiting its stay in the seasonally flooded marshes, the stickleback minimized the risks related to environmental unpredictability. These results suggest that hydrology (temporal variations of water depth) may influence the fish community structure through interspecific differences in survival and recruitment, as a result of temporal variations in the use of seasonally flooded marshes.