Fish communities of Atlantic littoral reclaimed marshes: A heritage to be managed? The case of Bourgneuf-Machecoul marsh (Western France).

The north of the Breton marsh extends over 16,000 ha reclaimed on the bay of Bourgneuf since the 11(th) century. it comprises 3,200 km of ditches which either are connected to former salt pans or drain cultures. The corresponding water surface is of 1,700 ha. Between 1987 and 1989, 46 fish species were censused but the structure of the fish community depended upon the salinity which is controlled by sluice gates. The brackish part serves as nursery and/or permanent habitat for 26 marine species, therefore conserving the original functions of the saltmarshes. A total of 11 fish species were present during winter in extensive aquaculture pans were the average biomass was 310 kg/ha largely dominated by eels. in freshwater parts, 21 species were listed and the average biomass was 315 kg/ha. important variations were observed, the presence of fish species depending on quality and accessibility of habitats. During the study, the richness, the abundance of the community decreased significantly. The fish community was first dominated by eel (Anguilla anguilla) which decreased and was progressively replaced by brown bullhead (Ictalurus melas). The natural and human causes of such spatio temporal variations are listed and discussed.