BIO-ECOLOGICAL AND ANATOMICAL ASPECTS OF THE EUROPEAN EEL ANGUILLA ANGUILLA (L. 1758)

Catadromous fish are considered important species for their ecological, economical, and cultural value. Because their complex life cycle, they are subjected to cumulative effects of anthropogenic threats that resulted in worldwide decline since the beginning of the 20th century. Among the most iconic catadromous species, the European eel Anguilla anguilla (L.) has aroused considerable interest since ancient times, though, to date, many aspects of its life cycle remain relatively unknown. A. anguilla showed a progressively decline since the 1970s. Starting from 2007, has been protected by the European Council Regulation 1100/2007, and successively classified as Critically Endangered (CR). Although conspicuous efforts have been conducted, to implement knowledge and identify possible solutions to preserve the future survivorship of A. anguilla, there is an urgent need to gain further insights into its life history. A review of the bibliography allowed me to obtain an updated state of the art of knowledge about the European eel life cycle, biology, ecology, allowing me to identify main topics that structured my thesis project. Topics have been developed with the general aim of providing a contribution to the implementation of the knowledge of the bio-ecology and anatomy of this species in relation to the various environmental and anthropic factors through a multidisciplinary approach that involved numerous sampling activities accompanied by complex data collection, laboratory experiments, and statistical analyses. First, I demonstrated the presence of synaptic development in the olfactory bulb, in terms of morphology and density of dendritic spines, that is shaped according to a pattern linked to the migratory life stages of the European eel (glass eels and silver eels). Second, I described the glass eels’ migration temporal peak for the first time in the central-western Mediterranean in relation to the effects of main environmental factors. I also identified a spatio-temporal variability between four sites in the western Mediterranean in terms of the peak of recruitment and the biometric and pigmentation patterns of glass eels. Moreover, I tested new floating traps as an alternative sampling method in three Sardinian estuaries to further deepen glass eels’ recruitment dynamics in Sardinia. Third, I analysed eels’ survival and growth performances during a rearing experiment with wild glass eels. I estimated recapture and growth rates on marked and unmarked farmed eels after restocking after a four-year study. Results suggest a good readjustment to the wild environment and demonstrated that this technique could represents a valid alternative restocking approach. Furthermore, I studied eels’ growth also in terms of otoliths’ shape differences. I found a distinct clustering among rivers and lagoons, with more rounded otoliths shapes in eels from rivers than lagoons, but with annual body growth quicker in lagoons than in rivers, showing that eel’s growth can differ according to habitat types. I fitted Von Bertalanffy’s growth curves for female and male eels from a Sardinian stream, that exhibited the already-known sexual dimorphism in growth parameters with higher values in females. Finally, I pinpointed an overall decrease in eels’ occurrence in the Sardinian river network because of dams’ building features and the time from their construction. Achieved results might have several implications beyond the regional interest and add new points from which to start further investigations that, over specific actions, may guide the implementation of new studies and appropriate monitoring programs. Findings highlighted also an urgent need for collaboration between the various stakeholders, researchers, decision-makers, authorities, professionals, and common people as an essential step to creating a sense of awareness on the criticalities that impact this species and the need to mitigate them.