Between-year variation of lateral fish migrations in the Inner Delta of the River Niger (Mali, West Africa)

In the Inner Delta of the River Niger fish lateral migration from the river toward the plain was monitored at the beginning of the flood during 1991 and 1994, two hydrologically different years. Fish sampling was performed with a fyke-net (mesh-bar 8 mm) set in a small channel linking river to plain. This first step of the migration is generally less studied but yet very important for optimal colonization and natural fish farming of the food resources on the floodplain. The adults and/or the juveniles of more than thirty fish species representing thirteen families migrate yearly at the beginning of the flood from the river toward the plain. The species diversity (Shannon index) was similar for the two years (2.72 in 1991 and 2.44 in 1994). On the whole the migration into the plain was more important (+24% in number) in 1994, a high flood year. The relationships between the different migrating fish species and the water discharge, temperature and conductivity are evaluated. The between year variation of migratory behaviour was investigated. These results on the migratory behaviour could be used in the conservation and improvement of fisheries and in the development of fish farming in this area of Mali.