Evolutionary Ecology of the European Eel
The ongoing paradigm on the preservation of living resources argues that both the ecological and evolutionary background of exploited species should be taken into account as to devise effective and sustainable management practices. This perspective stems from the apparent ineffectiveness of traditional methods in preserving and predicting the dynamics of marine fish stocks: worldwide collapsing fish populations show no signs of recovery. These drastic scenarios severely compromise not only the ecosystems biodiversity and viability of the species, but also affect the social and economical welfare of communities that are dependent on those resources. The European eel constitutes a critical example. For centuries, it served as prominent fishing item to a large number of communities all across Europe. However, the steep recruitment decline that occurred in the 1980s drove the eel population to the low numbers still observable nowadays. Management attempts to raise eel abundance were apparently inconsequent and culminated with a “critically endangered” conservation status attributed to the species and with the enforcement of a European-wide eel management plan in 2007. Still, the 1980s decline triggered an extensive scientific research on the European eel, which translated in significant improvements concerning the knowledge on its ecology and evolution. Amongst those one can name the identification of potential ecological drivers for the decline and the disclosure of the complex evolution of the species, where the apparent absence of a structured population – panmixia – became paradigmatic. Surprisingly, and despite those achievements having wide application in the conservation of the European eel population, the link between ecology and evolution only has seldom been investigated. The main objective of the present doctoral thesis was therefore to shed light on the evolutionary ecology of the endangered European eel. First, we employed a multidisciplinary approach, which incorporated ocean modelling techniques and population genetics theory, to investigate the role of ocean currents in shaping the evolution of the European eel. Here we provided evidence for an oceanographic origin of the decline. A positive correlation between observed and modeled recruitment allowed the identification of a wind-driven oceanic pathway connecting the Sargasso Sea (the eel spawning ground) to the Gulf Stream. We were also able to put forward an alternative hypothesis to the paradigm of panmixia, namely, female philopatry within certain areas of the Sargasso Sea. The formulation of this hypothesis found support in empirical and modeled genetic differentiation amongst locations within the European eel’s continental range. Second, we investigated the consequences of both the ocean-driven recruitment decline and of the introduction of the swim bladder parasite, Aguillocola crassus, in the post-decline dynamics and adaptive potential of the species. For that, we compared the genetic diversity of neutral markers and that of an adaptive immune gene between two distinct generations of European eels. Results indicated that the European eel population might be experiencing a recovery, as suggested by an increase of the genetic diversity between generations, particularly in the case of the immune gene. The detection of a recent bottleneck in the genetic diversity of the adaptive immune gene further suggested that despite the ongoing recovery, the adaptive potential of the species might still be severely affected. We suggest that the critical endangered status of the species should not be lifted. Lastly, we explored whether signs of the possible female philopatric strategy suggested in Chapter I could be detected. Based on indirect measurements of gene flow, we tested if matrilineages identified in the eel population could impose evolutionary constrains to complete panmixia. Results suggested that indeed that could be the case: not only was a structured population model statistically favored over complete panmixia, but also asymmetries in the migration rates amongst the matrilineages were detected. By observing that those asymmetries were mainly due to the predominant matrilineage supplying migrants to the others, we suggested that the existence of segregated reproductive units at the Sargasso Sea might be the mechanism maintaining not only the viability but also the evolutionary potential of the species.