PHYSICAL PROCESSES AND HYDROLOGICAL STRUCTURES RELATED TO THE BAY OF BISCAY ANCHOVY

Knowledge of the fundamental physical processes governing hydrological structures is necessary for a better understanding of complex physica l - biological interactions. In this review attention is focused on sp atial and temporal scales of physical processes and hydrological struc tures related to the life cycle of anchovy (Engraulis encrasicolus, L. ) and potentially affecting the population dynamics in the Bay of Bisc ay. The Bay of Biscay can be considered an open oceanic bay with a lar ge French continental shelf oriented S-N and a narrow Spanish shelf or iented E-W. The general oceanic circulation is weak and the presence o f cyclonic and anticyclonic eddies is frequent. A consistent poleward flow is apparent but seasonal changes show different phases at differe nt 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 are a. The most characteristic structures are, the cold pool extending fro m the southern Brittany area down to the latitude of the Girond estuar y, the winter warm poleward current along the Spanish slope, the cold freshwater in the surface layers extended over the French shelf and th e warm waters in the south-eastern parr of the bay in late spring. The meteorological conditions show also both spatial and temporal heterog eneities. The mean wind speed is two rimes higher in the northern part of the bay than in the south-eastern part with an obvious seasonal pa ttern. The sea surface temperature shows also a clear latitudinal grad ient in the summer period. Finally the freshwater runoff from the main estuaries Gironde and Loire shows a clear seasonal pattern but the tw o estuaries are in phase. The nature and the number of physical proces s 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: incre ase of solar heating, daytime duration, changes in dominant winds) mak es the study of physical-biological interactions and the understanding of the anchovy population dynamics difficult.