Environmental influences on the recruitment dynamics of juvenile European eels, Anguilla anguilla, in a small estuary of the Tyrrhenian Sea, Sardinia, Italy

Abstract

The European eel, Anguilla anguilla, is a catadromous and migratory species of commercial importance. Its complex life cycle results in its exposure to many risk factors, which have resulted in stock declines across all life stages since the 1970s. The temporal recruitment dynamics of juvenile eels (glass eels and elvers) were investigated in a small Mediterranean estuary (Sardinia, Italy). The composition of the population and the monthly and seasonal variations in the abundances of juvenile eels was assessed over 78 sampling events (from February 2017 to February 2018). Furthermore, the effects of abiotic variables on the abundances of glass eels and elvers were investigated using generalized additive models (GAMs). Glass eels had the greatest abundance during the winter months, whereas elvers had the greatest abundance during spring. Modelling revealed that the abundance of glass eels was mostly explained by the combined effects of water temperature (12.3–14.5 °C), tidal coefficient (40–110 cm), moon phase, season, and river mouth condition, whereas the abundance of elvers was associated with water temperature (14–21 °C), dissolved oxygen content (>7 mg/L), and season. These results suggest that the annual recruitment of juvenile eels occurs throughout the year, with clear seasonal migration dynamics. The use of multiple statistical approaches allowed us to identify the importance of several environmental variables in regulating the recruitment dynamics, providing useful information for conserving eel stocks through the restoration of the natural flow regime and the connectivity between freshwater habitats and the sea.

When the Eel Meets Dams: Larger Dams’ Long-Term Impacts on Anguilla anguilla (L., 1758)

“…The Sardinian hydrographic network is characterized by a reduced number of perennial rivers and the prevalence of intermittent streams (Palmas et al, 2022). Mediterranean climate is typically bi-seasonal with severe drought summer and rainy autumn/winter that determine irregular flow and strong seasonal hydrological fluctuations (Sabatini et al, 2011(Sabatini et al, , 2018Palmas et al, 2020;Podda et al, 2020). Average annual precipitations range from 500 to 900 mm (De Waele et al, 2010;Moccia et al, 2020).…”

“…High connectivity between freshwater and marine habitats facilitates the exchange of matter, energy, and nutrients, with species contributing to the longitudinal transfer and supporting important ecosystem services (Holmlund and Hammer, 1999). For instance, anthropogenic intervention in river discharge result in reduced flushing up to unnaturally extended periods of estuary mouth closure and reduce the connectivity between freshwater habitats and the sea (Potter et al, 2010;Lloyd et al, 2012;Podda et al, 2020). In many estuarine systems, especially those with Mediterranean climate, sand bars are deposited during low flow periods at the river mouth, resulting in truncation of the connection between freshwater and marine habitats (Potter et al, 2010;Suari et al, 2019;Podda et al, 2020).…”

“…For instance, anthropogenic intervention in river discharge result in reduced flushing up to unnaturally extended periods of estuary mouth closure and reduce the connectivity between freshwater habitats and the sea (Potter et al, 2010;Lloyd et al, 2012;Podda et al, 2020). In many estuarine systems, especially those with Mediterranean climate, sand bars are deposited during low flow periods at the river mouth, resulting in truncation of the connection between freshwater and marine habitats (Potter et al, 2010;Suari et al, 2019;Podda et al, 2020). These interruptions and the river flow alteration can have severe implications especially for diadromous species (Gillanders et al, 2003), like the European eel Anguilla anguilla (L., 1758).…”

Hard times for catadromous fish: the case of the European eel (Anguilla anguilla, L. 1758)

“…Success and extent of eels' recruitment depends both on global (Knights, 2003;Kettle and Haines, 2006;Bonhommeau et al, 2008;Pacariz et al, 2014;Gutierrez-Estrada and Pulido-Calvo, 2015;Bornarel et al, 2018) and local factors, whose interaction modulate spatial and temporal dynamics of recruits entering brackish environments and freshwaters (Gascuel et al,1995;Arribas et al, 2012;Harrison et al, 2014;Trancart et al, 2014;Aranburu et al, 2015). Recruitment dynamics at the local scale can vary daily, seasonally, and annually (Bru et al, 2009;Laffaille et al, 2007;Zompola et al, 2008;Arribas et al, 2012, Podda et al, 2020, are well known for Atlantic estuaries and rivers of Europe (Beaulaton and Castelnaud, 2005;Harrison et al, 2014), and relatively less known for the estuaries located in the southernmost distribution area (Arribas et al, 2012).…”

“…Many factors have been identified as recruitment short-or medium-term drivers but, so far, it has been difficult to reach clear conclusions about what are the primary drivers of its decline. Multiple environmental factors (e.g., river flow, changes in the North Atlantic Oscillation, warming of sea surface temperature, currents) probably affected the documented decline (e.g., Gandolfi et al, 1984;Domingos, 1992;Elie and Rochard, 1994;de Casamajor et al, 1999;Prouzet, 2002;Jellyman and Lambert, 2003;Knights, 2003;Polyakov et al, 2005;Bouvet et al, 2006;Bureau Du Colombier et al, 2007;Friedland et al, 2007;Laffaille et al, 2007;Adam et al, 2008;Bonhommeau et al, 2008;Crivelli et al, 2008;Kettle et al, 2008;Zompola et al, 2008;Miller et al, 2009;Durif et al, 2011;Kettle et al, 2011;Arribas et al, 2012;Baltazar-Soares et al, 2014;Hanel et al, 2014;Milardi et al, 2018;Podda et al, 2020).…”