Seasonal dynamics of estuarine migration in glass eels

An analysis of statistical data available in eight glass eel fisheries distnbuted over the French Atlantic coastline is used to define the upstream migration dynamics of Anguilla anguilla in estuaries. The CPUE (Catch Per Unit of Effort) data reveal three different types of seasonal trends that reflect different migration kinetics. The recruitment abundance is studied compared to the salinity of the estuary. The first two types of trends correspond to years of high recruitment andor of reduced fishing effort. In brackish estuarine zones, the abundances then follow a symmetrical curve, with a peak in JanuaryFebruary. This time course is due to the passage of a wave of migrants. Further upstream, in tidal freshwater zones, the abundances follow an asymmetrical curve which may describe the progressive accumulation of eels between November and March/April. When recruitment is low andlor the fishing effort is high, CPUEs vary during the season following a plateau curve, with values staying more or less constant between November and March. This latter trend represents a balance status between immigration and capture. In this case, abundance indices can only be established with total catches. The curves are explained by the migration behaviour of glass eel which exhibit a resting phase in brackish zones and an accumulation in tidal oligohaline zones. These results have direct consequences particularly for the determination of recruitment abundance indices.