Factors determining survival of European eels in two unexploited sub-populations

Estimating accurate age-specific survival probabilities and understanding the processes (density dependent or independent) that regulate this demographic parameter are fundamental to propose sustainable management options for the endangered European eel (Anguilla anguilla). In the present study, we used extensive mark-recapture data sets (13 and 17years) on eel >150mm to estimate accurate natural survival probabilities of two eel sub-populations, from the Oir and Fremur rivers, western France, and then we analysed survival probabilities in relation to density-independent (temperature) and density-dependent factors to identify those causing survival variability. The Fremur and Oir rivers are two small (<100km(2)) river systems in close proximity (65km apart). The Fremur River is a small river obstructed by dams with medium water quality, high eel recruitment and density (0.37eelm(-2)), and a male dominant population; whereas, in the Oir River, low densities of eels were observed (0.04eelm(-2)) and the sex ratio was skewed towards females. Furthermore, previous research suggested that the lotic habitats of the Fremur River have reached carrying capacity, whereas habitats in the Oir River are below habitat saturation. In the Fremur River, there were significant spatial and temporal variations in the survival probabilities. However, survival probabilities observed in the Oir River were stable over time and space. The results highlight that the differences in the characteristics of the two systems and the two sub-populations prompt different responses to regulatory processes. The contrasting pressures applied on these two sub-populations impact survival, which possibly lead to different life history strategies such as sex differentiation.