Anadromous estuarine migration of adult allis shad Alosa alosa, shape of the phenomenon and influence of the tidal cycle.

To reach their spawning grounds, adults of Allis shad Alosa alosa L. 1758 have to 1 go through an estuary. In this area, according to the tidal rhythm, hydraulics conditions are, sometimes favorable and some times unfavorable to their upstream migration. In this work we focus our attention on the Allis shad anadromous migration during their crossing of the Gironde estuary (France). We based our analysis on daily catch by unit of effort (CPUE) data from commercial fishermen. They have been collected in two sectors of the estuary during migration seasons 1985-1988. These CPUE time series which represents the evolution of the migration have been analyzed with signal analysis methods : trends extraction, residuals variability analysis. We established that the shape of the phenomenon is uni or bimodal, the trend depends of the temperature evolution during the migration season. The time lag between peaks in the two sectors enable us to assess the migration speed of the shad in estuary : 17-23 km.d(-1). Residuals time series exhibit cyclic components, in the downstream sector they are significant and periods are : 14-16 d and 28 d in the upstream sector their shape is less regular and they are not significant. On the lower sector the lag cross correlation between residuals and tide magnitude is significant, the time lag varies from year between 0 and 6 d. This could come from the crossing by the migrants of estuarine tidal current fields, In the upstream sector correlation with tides is 3 times lower in intensity and less regular. This could result of the progressive smoothing of the tidal cycle and of the appraisal in the migration phenomenon of a 7 d periodicity which could be linked to the fisheries working.