No abstract

One of the actions that has been implemented to support the recovery of the panmictic population of European eel is stocking of waterbodies where natural recruitment is low or null. However, growth conditions of the stocked eels can vary greatly. This circumstance emphasises the importance to determine ideal habitat conditions to contribute to the success of stocking actions and, consequently, to increase the production of silver eels. This study aims to evaluate the early settlement and growth of stocked glass eels in the upper reaches of a fragmented river. Stocking was carried out, in 2014, at three sites of an inland tributary of the Mondego river basin (Central Portugal), and its monitoring was conducted during the following two years, until 2016, along with the collection of environmental and hydromorphological parameters. The results showed a successful dispersion throughout the study area. Growth varied spatially, although environmental parameters have not clearly explained this variation, but overall with high growth rates and a positive allometric growth in this early stage of stocking, suggesting a good condition of the stocked individuals. This study showed that these upper reaches of fragmented watercourses, a shared feature amongst most European rivers that are currently inaccessible for natural recruitment, may be suitable habitats for eel stocking.

The catadromous life cycle of the European eel encompasses a spawning migration to the Sargasso Sea. Prior to their migration, eels develop a silver coloration being referred to as “silver eels.” Due to the dramatic European eel recruitment decline, it is crucial to quantify silver eel escapement to evaluate the success of measures taken under Eel Management Plans (Regulation EC (1100/2007)). Thus, the percentage of silver eels escaping from the River Mondego (Portugal) was estimated during two consecutive spawning seasons (2014–2016) and their migratory behaviour was studied. Thirty‐six male silver eels were tagged with acoustic transmitters and their downstream migration was tracked using automatic receivers deployed at four locations along the river course, from the first impassable obstacle up to the river mouth (67 km). Among the tagged eels, 15 (42%) escaped to the sea. In both years, individuals displayed faster migration speed in the downstream reaches of the river. BRT (“Boosted Regression Trees”) models were used to explain downstream migration and escapement of silver eels. The analyses revealed that the downstream migration is triggered by several environmental variables with water conductivity as the most influential predictor. Escapement of eels from the estuary occurred a few hours after their arrival and was primarily explained by the offshore wind intensity, which promotes a current in the coast that may favour their transport to the open sea. These findings contribute to a better understanding of silver eel migration in southern latitudes and highlight and reinforce the need to monitor spawner escapement.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).