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 R² of 0.88, identified minimum water temperature as the main environmental variable explaining these movements, with a threshold of 13 °C triggering the onset of movements, and a peak between 15 and 19 °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.

The green crab (Carcinus maenas) population was investigated in a coastal lagoon (Santo André) located on the southwest coast of Portugal. This study assessed the spatial-temporal variability of green crab abundance in the lagoon and its relationship with environmental variables. Experimental fishing was used to collect data on abundance, size structure and other demographic characteristics. Sampling was conducted monthly in 2019 at five sites, using fyke nets. From a total of 15063 individuals collected, 3898 were analysed. The highest catches were obtained in winter and at sites closer to the sea. The population was male-biased (58.70% of males and 41.30% of females), with an average carapace width of 48.81 mm for males and 40.79 mm for females. Ovigerous females were most abundant in December and January, and 50% were sexually mature at 45.11 mm. Based on data from the experimental fishing and fishermen’s logbooks, the estimated annual catch of green crab ranged from 1873 to 3354 kg/ha, confirming the potential value of its fishery in the lagoon. This resource can be exploited for multiple purposes, including its meat or shell waste, thus contributing to the circular economy. Considering that green crab is a bycatch of the eel fishery, fishing regulations need to be modified to safeguard the stocks of both species.

The European eel population has declined over the last 50 years and is outside safe biological limits. In 2007, the European Commission enforced a regulation to ensure that all Member States implement Management Plans to achieve an escapement rate of 40% for silver eels. However, escapement is assessed using diverse methods, and therefore, estimates of different countries are not comparable. Thus, European scientists and managers face the challenge of finding a method that could be applied all along the European eel distribution range using widely available data. During the SUDOANG project, the Eel Density Analysis model (EDA) was used to estimate the silver eel escapement in Spain, France, and Portugal. EDA can estimate silver eel escapement at different scales (basin, EMU, country, international) using data from routine electrofishing surveys, such as those collected for the Water Framework Directive (WFD). EDA is based on open-source software and is widely applicable to European rivers. A chained river network in the three countries was built, including the hydrological attributes, the location, and the characteristics of 10,574 obstacles. A total of 46,118 electrofishing operations from 1985 to 2019 were also compiled and projected to the hydrographic network. Finally, EDA was implemented through the sub-models on eel presence–absence, density, size structure, and silvering. EDA estimated a total fluvial population of 12.3, 11.7, and 5.8 million silver eels for 2015 in France, Spain, and Portugal, respectively. Eel presence and abundance decreased as the altitude and the distance to the sea increased. Finally, by setting the cumulated height of dams to zero, EDA provided an estimate of the effect of habitat loss due to dams on the eel population. These results correspond to the first assessment of silver eels carried out simultaneously and with the same methodologies in three European countries.

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.