Abstract

Temporary waters are important habitats for many fish species. Nevertheless, determination of which species colonise these habitats, when this occurs and where fish distribute themselves within the temporary waters are rare. The fish assemblage on a temporarily flooded grassland and its adjacent permanent canal, together with environmental variables, were monitored throughout an entire flooding event from February to May 2006 in the Brière marsh (Northwest France). In total, 589 fish belonging to 12 species were captured, but the fish assemblage was dominated by six species in both habitats. Frequency of occurrence of fish was significantly higher in the canal than in the grassland (52.9 ± 11.9% and 13.0 ± 2.4%, respectively) and similar result was observed for relative density (1.72 ± 0.50 and 0.28 ± 0.08 fish per point sample, respectively). These parameters changed during the flooding event and maximum values were recorded in mid-April, when proliferation of zooplankton and water temperature above 12 °C were observed. The extent of fish colonisation in the grassland was influenced by water level. During the first half of the flooding event, the proportion of grassland occupied by fish represented, on average, 45.9 ± 3.7% of the available flooded habitat. Afterwards, the extent of fish colonisation in the grassland decreased when water level decreased and fish used only 17.8 ± 2.1% of the available inundated area and remained close to the canal. A lack of synchronisation in the flood event and water temperature resulted in a mismatch between fish abundance and spatial distribution within the flooded grassland.