Swoon over the moon: The influence of environmental factors on glass eels entering Mediterranean coastal lagoons

The European eel (Anguilla anguilla, Linnaeus 1758) is a critically endangered species that completes its life cycle between the freshwaters of Europe (and North Africa) and spawning grounds in the Sargasso Sea. Its marine larvae are planktonic and metamorphose into post-larval glass eels near the coast. The relationship between environmental variables and the migration of glass eels into brackish water and estuaries is not well understood, but has important implications for European eel conservation and management. Using linear mixed effects (LME) models, we investigated the combined role of meteorological (wind direction and strength and rainfall), oceanographical (tide amplitude), astronomical (moon phase and illumination), temporal and water quality variables on the timing and magnitude of glass eel migration from marine to brackish waters in the Comacchio Lagoon, Northern Adriatic Sea. We used this site as a test case because it was historically prominent in glass eel wild harvest for extensive aquaculture in the Mediterranean region. We found that meteorological factors and tide amplitude had a weak effect on the migration of glass eels into the lagoon. On the contrary, astronomical variables strongly influenced glass eel movement, with waxing phases and intermediate illumination enhancing movement towards the lagoon. We also identified at least two distinct migration pulses during our study period, which were not affected by temporal and water quality variables. Temporal and water quality variables affected body size of glass eels, with bigger glass eels caught later in the season. Identifying the importance of environmental variables affecting glass eels recruitment to brackish waters can help promoting eel stocks restoration through local management.

Automated summary

The European eel is critically endangered and completes its life cycle between European freshwaters and the Sargasso Sea. The migration of glass eels into brackish waters is influenced by astronomical factors more than meteorological and oceanographical factors. Identifying the environmental variables affecting glass eels migration can help in promoting eel stocks restoration.