The hydrological regime of the inner delta of the River Niger, situated in Mali, is subject to strong annual and indeed intra-annual variability. This delta ecosystem has a characteristic feature, a three-phase cycle. The first, a period of flood, starts in July marking the beginning of the cycle; then, after several months of rising water-levels, the flood recedes, between November and January; finally, a period of low water prevails between March and June.
The river’s various fish species are adapted to this cycle of alternating conditions. Feeding, growth and mortality depend on that rhythm. The flood scatters the fish away from the river bed and brings abundant food. It provides refuge areas, environments where reproduction can take place undisturbed. Growth then proceeds until the waters are in recession, a period of high natural mortality. Fishing effort has to follow the rhythm set by the succession of flood and recession. Most campaigns are concentrated in the period of flood retreat which heralds the return of fish into the fluvial zone and their unavoidable movement through the channels fishermen know well – and when their capturability is highest. Activity diminishes and the season ends with the onset of the next flood, when again the fish are dispersed into flooded areas.
Fishing activity is therefore dependent on the hydrological seasons. Two measurable hydrological parameters can express these: rainfall and river discharge. IRD scientists have sought to determine the extent to which these two variables can provide the basis for a model for predicting annual capturable fish stocks. The team focused first on defining which of the indicators was most pertinent, secondly on finding the number of years’ worth of data necessary for obtaining a reliable forecast.
Marie-Lise Sabrie | alphagalileo
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