EFFECT OF RE-OPENING OF A MIGRATORY PATHWAY FOR EEL (ANGUILLA ANGUILLA, L.) AT A WATERSHED SCALE

The construction of the Arzal dam eel ladder in 1996 allowed enhancing fluvial recruitment from a negligible level, limited to yellow eels crossing sluice and dam overflow, to a level ranging from 0.2 to 2.4 million glass eels per year. The effect of such recruitment on the eel population (Anguilla anguilla) of the Vilaine watershed was analysed at 19 electrofishing sampling stations. From 1998 to 2003, average densities of eel varied from 0.34 to 0.72 eels.m-2, with a maximum reached in 1999 of 0.82 eels.m-2. Fluvial recruitment failure, together with density dependent mortalities, explains the drop in yellow eel densities observed from 1998 in the downstream area (< 50 rkm) and from 2000 in the middle stream area (50100 rkm). The largest densities consisted of a large proportion eels classified as age 1 and the increased density in middle stream sectors was interpreted as the consequence of density-dependent migration at the periphery of a saturated area. The construction of 13 eel ladders on the Vilaine waterway in 1999 and 2000 was followed by increased densities in the upstream area (> 100 rkm) in 2001. The comparison to an electrofishing survey performed at 17 stations in 1981, ten years after dam construction, confirmed that ladder installation increased densities by a factor 6 and modified the population structure, with densities of eels age 0 and 1 multiplied 29 fold. This change corresponded to reduced escapement rates to the glass eel fishery (1% to 5%). Considering this result, a preliminary escapement target of 240 glass eel per km2 of watershed area or 1 500 glass eel per ha of water surface, is proposed and discussed for glass eel fisheries.