Contrasted Demographic Parameters (growth and survival) of Eel Populations in Two Small Coastal Rivers of Western France: The Role of Habitat and Management Implications

Despite intensive research on eels, their population dynamics in continental waters remain poorly documented which creates serious gaps in planning the restoration of the European eel population. We selected two European sub-populations of western France, at same latitude and separated by 67 km that vary substantially in degree of human development. The Oir is a river characterized by a high water quality and connected habitats, whereas the river continuum of the Fremur is impeded by many dams and characterized by poor water quality. Over a 6-year period (2008-2013), we completed a comprehensive population assessment, including abundance estimates and mark-recapture studies (1753 PIT-tagged fish) of growth and survival. Different population dynamics patterns were observed with a total abundance twice higher in the Fremur R. than in the Oir R. (respectively 0.22 and 0.09 eel. m^-2) despite a significant drop through the years in both rivers. While estimated annual growth rates were equivalent (approximately 20 mm/year), survival rates appear to have different trends with an inter-annual variation in the Fremur contrary to the Oir where survival rates were stable. Determining whether these differences are natural inter-annual fluctuations or whether they are caused by anthropic changes must be a priority for conservation.