Spatial distribution of an eel population (Anguilla anguilla L.) in a small coastal catchment of northern Brittany (France). Consequences of hydraulic works

摘要

The Frémur is a 60 km2 catchment of Brittany where many hydraulic works (dams, weirs, gauging structures, etc.) have reduced the recruitment by elvers and glass eelssince at least 50 years. In 1992, an eel lift was built on the main dam (14 m high) and in 1996, two other dams (5 m high) located downstream were equipped with eel passes toimprove upstream migration. In 1995, before the construction of the passes, eel population parameters (density, biomass and size distribution) were assessed at the scale of thewhole watershed by electrofishing (removal method) in 33 stations. The average biomass was high (19 g/m2 ) despite the numerous physical obstructions. However, there was anaccumulation of eels downstream many hydraulic works and very low densities immediately upstream. Moreover, downstream the first obstruction, at 2 km from the sea,the population is dominated by boot lace eels and elvers (< 100 mm), whereas this size class is absent in upstream reaches. These results suggest that the standing capacity ofthe watershed is not reached and that the population could be enhanced in the next years by the recently installed eel passes.