Concepts for characterizing spawning biomass of the European eel (Anguilla anguilla) in catchments

Silver eels biomass production is a primary management target to be urgently achieved for starting the restoration of the European eel (Anguilla anguilla) population. An assessment of the proportion of individuals actually escaping from catchments-and able to reproduce-compared to a theoretical pristine production under no human intervention, is of critical importance for preserving this resource, and EU urges Member States to implement such tools now. This preliminary approach, developed during the EU program INDICANG, proposes to clarity some of the basic concepts needed to implement assessment tools for a characterization of the production of spawning biomass in a catchment. These concepts rely mostly oil the influence of the catchment context (conditions for the eel growth) on the biomass of future spawners produced, and oil the notion of breeding potential (production potential of future spawners, because year-to-year effective downstream migration is unpredictable from the structure of the local eel stock). In order to allow its implementation in data-poor catchments, this assessment does not aim to be fully quantitative, but to give a semi-quantitative estimation of the silver eel potential production. After being reduced by coefficients of anthropogenic mortality (fisheries, hydroelectric turbines, dams and reservoirs), this breeding potential should give a reference value for defining the future management targets and their monitoring.