Influence of night brightness and turbidity on the vertical migratory behaviour of glass-eels (Anguilla anguilla L.) in the Adour estuary

Glass-eels migratory movements according to environmental conditions were s tudied on Adour estuary during the 1997/1998 catch season. These observatio ns took place at night, during flood tide, 10 kilometres upstream the sea t ransversal limit, on a sector of 12 kilometres in length where we observed the propagation of tide. Glass-eels densities (in g per 100 m(3)) were estimated with fine mesh nets on 5 sampling stations. On each station, 3 longitudinal transects were sam pled simultaneously in surface and close to the bottom. Catch abundance bet ween November and March followed a dome shape tendency with a pick in Janua ry. Movements into the water column were induced by moonlight intensity. Gl ass-eels migrate mainly close to the bottom when turbidity is low and moonl ight intensity is high. Moon phases and turbidity induced modifications in glass-eels behaviour and catchability by the fishery. According to these observations, a modelisation of vertical migratory schem e of glass-eels in that estuary can be proposed : high moonlight intensity and low turbidity induced abundance of glass-eels mainly close to the botto m; low moonlight intensity and/or high turbidity induced dispersion of glas s-eels throughout the water column.