Biological characteristic of glass eels (Anguilla anguilla) at the time of transition in estuary.

During the season of migration 1999/2000, from November to March, a glass eels sampling was carried out in the south-west part of France. The objective of this sampling was to determine from the biological characteristics (size, mass, pigmentation, DNA/dry Weight, water content and otolithometry) the modifications of the biological parameters of glass eels between coastal zone and estuary. A slimming of the individuals was observed during the crossing of the estuary in December but not in February. However, some individuals captured at sea present a mark of transition in estuary particularly in February. The development of pigmentation follows the same tendencies as the development of otoliths except for the glass eels at the stage V-B. The definition of the type of otolith is an indicator more precise than the pigmentary development to determine the physiological state of the individuals. Measurements of the distances from the various structures registered on the otolith reflect a similar larval past. Only the total ray of the otolith differs between glass eels captured at sea and in estuary because of the growth of the otolith during the phase of transition. It appears starting from the data biochemical, otolithometric and pigmentary a significant mixture of flows of glass eels in February.