Inter-Annual Trend in Recent Years (2001-2012) of Population Size and Reproductive Potential of Silver-Phase European Eels in the Loire River

The Loire River is one of the largest rivers in Western Europe. Its privileged situation with regard to the Gulf Stream leads to high levels of recruitment by glass eels and probably to among the highest level of silver eel biomass in Europe. However, no study attempted to estimate both number and reproductive potential of silver eels produced by the Loire R. In collaboration with professional fishermen, number of silver eels migrating from upstream part of the Loire watershed was estimated for six downstream migration seasons (2001-2004, 2008 and 2012) by a mark-recapture technique. Calculations conducted with pooled data (Petersen estimate) revealed a drop in silver eel numbers between the periods 2001-2003 (approximately 330 000 fish) and 2004-2008 (150 000 fish), whereas estimation for the last year is still in progress. The exploitation rate by the commercial fishermen observed during the period was on average 14.3 ± SD 4.3 %. To evaluate the quality of silver eels, we considered pathogens (A. crassus), fat level and the concentration of chemical contaminants (PCBS and heavy metals). Finally, the contribution to the spawning stock of the Loire silver eels that are mainly represented by females (> 98%) of 1 kg is discussed.