SEASONAL DYNAMICS OF ESTUARINE MIGRATION IN GLASS EELS (ANGUILLA-ANGUILLA)

An analysis of statistical data available in eight glass eel fisheries distributed over the French Atlantic coastline is used to define the upstream migration dynamics of Anguilla anguilla in estuaries. The CPU E (Catch Per Unit of Effort) data reveal three different types of seas onal trends that reflect different migration kinetics. The recruitment abundance is studied compared to the salinity of the estuary. The fir st two types of trends correspond to years of high recruitment and/or of reduced fishing effort. In brackish estuarine zones, the abundances then follow a symmetrical curve, with a peak in January/February. Thi s time course is due to the passage of a wave of migrants. Further ups tream, in tidal freshwater zones, the abundances follow an asymmetrica l curve which may describe the progressive accumulation of eels betwee n November and March/April. When recruitment is low and/or the fishing effort is high, CPUEs vary during the season following a plateau curv e, with values staying more or less constant between November and Marc h. This latter trend represents a balance status between immigration a nd capture. In this case, abundance indices can only be established wi th total catches. The curves are explained by the migration behaviour of glass eel which exhibit a resting phase in brackish zones and an ac cumulation in tidal oligohaline zones. These results have direct conse quences particularly for the determination of recruitment abundance in dices.