Environmental Factors Promoting Upstream Movement of Yellow Eel (Anguilla anguilla L.) in the Mondego River ^†

One of the main reasons for the collapse of the European eel (Anguilla anguilla L.) is habitat loss resulting from the severe obstruction of rivers throughout the species’ range. Specific fish passes (i.e., eel ladders) can be installed in weirs and dams to mitigate this impact and to promote the upstream movement of eels. In the Mondego River, one of the most important basins for diadromous species in Portugal, an eel ladder was installed in 2015 at the Coimbra weir, the first obstacle to fish migration in this basin. This ladder is equipped with a monitoring trap at the upstream exit, where the eels that have successfully overcome the eel ladder are counted and measured before being released. The timing and environmental variables that promote the upstream movement of these individuals (counts at the eel trap were considered as a proxy for upstream movement activity) were assessed between January 2017 and August 2019. A total of 12,019 eels with a length ranging from 60 mm to 287 mm (median = 138 mm) used the eel ladder to move upstream the obstacle. The upstream movement occurred throughout the year, but a clear peak in activity was observed between May and June (~74%). Eels < 150 mm appeared mainly in early summer (May–June), with 64% of the total number of individuals counted in the eel trap belonging to this size class, but no differences in total body length were found between these years. Generalized Additive Models (GAMs) were used to determine the environmental factors that explained these upstream movements. Among the predictors considered (river flow, precipitation, water temperature and photoperiod), minimum water temperature had the strongest explanatory power. The results from this study are crucial for the management of the species, particularly at obstacles where no transposition equipment exists, and human intervention may be required to assist in their upstream progression.