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

Abstract

1. 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). 2. In the present study, we used extensive mark-recapture data sets (13 and 17 years) on eel >150 mm to estimate accurate natural survival probabilities of two eel sub-populations, from the Oir and Fr emur 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. 3. The Fr emur and Oir rivers are two small (<100 km 2 ) river systems in close proximity (65 km apart). The Fr emur River is a small river obstructed by dams with medium water quality, high eel recruitment and density (0.37 eel m À2 ), and a male dominant population; whereas, in the Oir River, low densities of eels were observed (0.04 eel m À2 ) and the sex ratio was skewed towards females. Furthermore, previous research suggested that the lotic habitats of the Fr emur River have reached carrying capacity, whereas habitats in the Oir River are below habitat saturation. 4. In the Fr emur 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. 5. The contrasting pressures applied on these two sub-populations impact survival, which possibly lead to different life history strategies such as sex differentiation.

Sexually dimorphic gene expressions in eels: useful markers for early sex assessment in a conservation context

“…Restocking eels for conservation purpose could be a promising option for sustaining eel populations. However, this action warrants more investigation regarding the carrying capacity of the selected receiving area, since it could influence both yellow eel survival 21 59 and future sex ratio 21 23 . The combined use of the sex markers that we propose would be useful to readily assess the population sex-ratio in a specific river/lagoon, where transplantations take place.…”

“…However, there is not yet a clear understanding of what will be the long-term consequences of massive juvenile eel transplantation on future adult sex ratios, considering that sex determination is density-dependent. In eels, the sex ratio happens to be site-specific, with rivers geographically close producing a majority of males or a majority of females 21 . Usually, a greater proportion of males being observed in the downstream part of the watershed (high density), while females prefer to inhabit the rivers upstream 22 .…”

Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area

“…Polders are characterised by a network of canals, connected ponds and ditches, resulting in a high habitat diversity and thus many potential growth areas (Lasne, Acou, Vila‐Gispert, & Laffaille, ). This could lead to higher growth rates, and larger eels have a higher survival rate (Boulenger et al., ). In addition, these areas are located close to the sea and may therefore, in the absence of migration barriers, be easily colonised by glass eels (Laffaille, Baisez, Rigaud, & Feunteun, ).…”

“…and larger eels have a higher survival rate (Boulenger et al, 2016). In addition, these areas are located close to the sea and may therefore, in the absence of migration barriers, be easily colonised by glass eels (Laffaille, Baisez, Rigaud, & Feunteun, 2004).…”

Cause or consequence? Exploring the role of phenotypic plasticity and genetic polymorphism in the emergence of phenotypic spatial patterns of the European eel

“…Recently, Podgorniak et al (2015) demonstrated that human-induced obstacles to migration could act as an evolutionary pressure. Concerning this, Boulenger et al (2016) highlighted that…”