A multi-approach study to reveal eel life-history traits in an obstructed catchment before dam removal

River fragmentation is expected to impact not only movement patterns and distribution of eels within catchment, but also their life-history traits. Here, we used otolith multi-elemental signatures to reconstruct life sequences of European silver eels within an obstructed catchment, just before the removal of hydropower dams. Beyond providing an initial state, we hypothesized that otolith signatures can provide crucial information on the way eels use the watershed. Indeed, their spatial distribution is expected to shape life-history traits, including condition coefficient, trophic level, growth rate, or infection by metazoan parasites. While Sr:Ca and Ba:Ca ratios were complementary in tracing fish movements between freshwater and estuary, the Ba:Ca variations allowed to discriminate three freshwater sectors. The eels assigned to the midstream sector were more mobile and exhibited lower growth rates, probably in response to higher competition at the vicinity of dams. While most eels are currently produced by downstream and midstream sectors, eels assigned to upper reaches of connected tributaries generally display higher richness in native parasite and higher body condition and lipid reserve, known to promote the success of migration and reproduction. In the near future, the dam removals will represent an outstanding experimental framework for evaluating impacts of catchment reconnection.

Automated summary

River fragmentation affects the movement patterns, distribution, and life-history traits of eels. This study used otolith multi-elemental signatures to reconstruct the life sequences of European silver eels in an obstructed catchment. The results show that eels' spatial distribution influences their life-history traits, including condition coefficient, trophic level, growth rate, and infection by metazoan parasites. The dam removals will provide an important opportunity to evaluate the impacts of catchment reconnection.