The evolving story of catadromy in the European eel (Anguilla anguilla)

Abstract

Anguillid eels were once considered to be the classic example of catadromy. However, alternative life cycles have been reported, including skipping the freshwater phase and habitat shifting between fresh, brackish, and saltwater throughout the growth phase. There is a lack of knowledge regarding these alternate life strategies, for example, the proportion of individuals in the population that adopt them compared to classic catadromy. We provide a description of these alternate life cycle strategies in temperate anguillids, their possible drivers, and the methods available to investigate them. These methods (lethal and non-lethal), include otolith microchemistry, fatty acid and stable isotope analyses, parasite identification, blood transcriptomics, and electronic tags. We argue that since the current management framework for the European eel and other temperate eels is based mainly on the freshwater component of the population, it ignores eels growing in saline waters. Many of the factors that are thought to be responsible for the precipitous decline of the eel population are more prevalent in freshwater systems. Therefore, the contribution of saline eels may be more important than currently estimated. The habitat-shifting ability of eels may be all the more crucial for the persistence and recovery of those species that are endangered.

The Influence of Natural and Anthropogenic Environmental Pressures on European Eel Abundances in French Estuaries

“…The intensification of local anthropogenic disturbances in estuaries is also associated with a decrease in species richness and fish abundance [39,40]. Improving our knowledge of the ecological role of European eels in marine habitats is an important contribution to their management and conservation [41]. The European eel stock is currently at its lowest historical level [13], and effective eel management is difficult, partly due to a lack of understanding of the relationship between environmental and anthropogenic factors on the estuarine eel population at the regional scale.…”

To pool or not to pool data? Applying a generalized depletion model to assess American eel elver Anguilla rostrata fisheries from multiple rivers in Nova Scotia, Canada

“…Most American eel knowledge is from temperate regions in the United States and Canada, with littleknown tropical Caribbean distribution (Kwak et al, 2019). Moreover, existence of saline water cohorts (Jessop et al, 2002(Jessop et al, , 2006 is not fully recognized in the current management framework (Durif et al, 2023), which might lead to underestimating the current stock status. These uncertainties have prompted calls for assessments that pool international expertise, match the geographic scope of eel genetic structure, and provide robust advice for conservation management (Cairns et al, 2022).…”