Knowledge of the fundamental physical processes governing hydrological structures is necessary for a better understanding of complex physical - biological interactions. In this review attention is focused on spatial and temporal scales of physical processes and hydrological structures related to the life cycle of anchovy (Engraulis encrasicolus, L.) and potentially affecting the population dynamics in the Bay of Biscay. The Bay of Biscay can be considered an open oceanic bay with a large French continental shelf oriented S-N and a narrow Spanish shelf oriented E-W. The general oceanic circulation is weak and the presence of cyclonic and anticyclonic eddies is frequent. A consistent poleward flow is apparent but seasonal changes show different phases at different locations along the slope. The residual currents over the shelf are principally governed by the wind, the tides in the northern part and the water density. The hydrology is also very heterogeneous in the area. The most characteristic structures are, the cold pool extending from the southern Brittany area down to the latitude of the Girond estuary, the winter warm poleward current along the Spanish slope, the cold freshwater in the surface layers extended over the French shelf and the warm waters in the south-eastern part of the bay in late spring. The meteorological conditions show also both spatial and temporal heterogeneities. The mean wind speed is two times higher in the northern part of the bay than in the south-eastern part with an obvious seasonal pattern. The sea surface temperature shows also a clear latitudinal gradient in the summer period. Finally the freshwater runoff from the main estuaries Gironde and Loire shows a clear seasonal pattern but the two estuaries are in phase. The nature and the number of physical process involved in parallel with the fact that the main spawning period of anchovy in the Bay of Biscay takes place during the period of the most important environmental changes (decrease of freshwater runoff, increase of solar heating, daytime duration, changes in dominant winds) makes the study of physical-biological interactions and the understanding of the anchovy population dynamics difficult.