Upstream movement of juvenile eels (Anguilla anguilla L.) in a southwestern European river

The upstream movement of juvenile eels (median total length (TL) = 138 mm) in the Mondego River, Portugal, was analysed between January 2017 and August 2019. A total of 12,019 individuals (TL ranging from 60 to 287 mm) were counted and measured on an eel ladder at Coimbra weir, 44 km upstream from the river mouth, during their upstream movements. Although eels were counted throughout the year, a clear seasonal peak was detected in spring and late summer, with 93% of total movements occurring between April and September. No differences were found in the total length of individuals moving upstream between years. Generalised additive models, with an explained deviance of 83% and an adjusted R-2 of 0.88, identified minimum water temperature as the main environmental variable explaining these movements, with a threshold of 13 degrees C triggering the onset of movements, and a peak between 15 and 19 degrees C, which corresponds to the spring and summer periods. Other predictors such as photoperiod, precipitation and river flow also contributed as explanatory variables in the model but less importantly. The present results may contribute to the management of this endangered species in the southern region of its distribution range by defining adequate temporal windows to monitor and promote the movement of juvenile European eels through obstacles to migration.

Automated summary

The upstream movement of juvenile eels in the Mondego River in Portugal was studied from January 2017 to August 2019. A total of 12,019 individuals were counted and measured during their upstream movements, with a peak occurrence in spring and late summer. The main environmental variable explaining these movements was minimum water temperature, with a threshold of 13 degrees C triggering the onset of movements, and a peak between 15 and 19 degrees C, corresponding to the spring and summer periods. Other factors such as photoperiod, precipitation, and river flow also contributed to the movements but to a lesser extent.