# Phenology and environmental drivers of glass eel entrance in a Mediterranean lagoon

Raphaël Lagarde, Jason Peyre, Elsa Amilhat, François Bourrin, François Prellwitz, Alexandre Perrault, Gaël Simon, Elisabeth Faliex

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Raphaël Lagarde, Jason Peyre, Elsa Amilhat, François Bourrin, François Prellwitz, et al.. Phenology and environmental drivers of glass eel entrance in a Mediterranean lagoon. ICES Journal of Marine Science, 2022, 79 (7), pp.2107-2116. 10.1093/icesjms/fsac144. hal-04569393

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# Phenology and environmental drivers of glass eel entrance in a Mediterranean lagoon

Raphaël Lagarde<sup>1,2</sup>, Jason Peyre<sup>1,2</sup>, Elsa Amilhat<sup>1,2</sup>, François Bourrin<sup>1,2</sup>, François Prellwitz<sup>1,2</sup>,
Alexandre Perrault<sup>1,2</sup>, Gaël Simon<sup>1,2</sup> and Elisabeth Faliex<sup>1,2</sup>

The European eel, *Anguilla anguilla*, is an emblematic facultative catadromous species that spawns in the Sargasso Sea and grow in continental waters of Europe and North Africa. In most of its growing habitats its population has dropped since 1980. Although Mediterranean lagoons represent particularly important habitats for eel, knowledge of eel ecology in lagoons is not as developed as it is in rivers. Particularly, data on the phenology, drivers and biometrics characteristics of glass eel entering lagoons are scarce. To address this lack of data, the abundance, pigmentation stage, size, and weight of glass eel entering the Bages-Sigean lagoon (western French Mediterranean) were monitored during 647 d from December 2018 to April 2021 using passive floating traps. Simultaneously, different environmental drivers were measured (flow velocity, temperature, rainfalls...). The highest abundances of glass eels were observed between mid-November and mid-March especially when the discharge of the main tributary of the lagoon was higher than its base flow. The glass eels captured during the peak of entrance were less pigmented, longer, and bigger than during other months. This work enabled us to identify periods when anthropogenic activities should be limited to decrease human-based impact on glass eel in similar habitats.

**Keywords:** *Anguilla anguilla*, endangered species, migration, recruitment, transitional waters.

## Introduction

The European eel, *Anguilla anguilla* L., 1758 (referred to as eel hereafter) is a facultative catadromous species that spawns in the Sargasso Sea (Schmidt, 1923; Miller *et al.*, 2019). After hatching, the leptocephalus larvae migrate to their growing habitat located in coastal and continental waters from Norway (72°N) to Morocco (30°N) and the Mediterranean Sea watersheds (ICES, 2020). Eel colonizes continental water at the glass eel (unpigmented juveniles) and elver (young pigmented juveniles) stages before settling in its growing habitat as yellow eel. The growing period of yellow eel lasts from 2 to >30 years. At the end of this growing period yellow eel metamorphoses in silver eel (the maturing stage) and migrates back to the Sargasso Sea to spawn (Béguer-Pon *et al.*, 2015; Amilhat *et al.*, 2016).

The complex life cycle of eel involving long-distance migrations implies two main consequences for its population. First, eel is impacted by numerous anthropogenic pressures (Feunteun, 2002; Drouineau *et al.*, 2018) from the global (climate change) to local scale (habitat degradation, pollution, fishing mortality...). The susceptibility of eel to several concurrent anthropogenic impacts is thought to be the main reason explaining its population collapse (Miller *et al.*, 2016). Indeed, the number of glass eels and elvers entering in their continental growing habitats in 2020 was less than 10% compared to the 1960–1979 period (ICES, 2021). This reduction of recruitment also reflected in the reduction of silver eel population (Aalto *et al.*, 2016). This drastic reduction of eel population led the European eel to be listed in 2007 on the IUCN red list as a critically endangered species (Jacoby and Gollock, 2014;

Pike *et al.*, 2020) and the European Union to mandate each of its member country to write an eel management plan to restore eel population (European Council, 2007). The second consequence of eel life cycle, is that eel population is a single panmictic population (Als *et al.*, 2011; Pujolar *et al.*, 2014; Enbody *et al.*, 2021). The panmixia in eel population plead for a global management of this population all over its transnational repartition (Als *et al.*, 2011). However, the high degree of phenotypic plasticity observed in eel (Enbody *et al.*, 2021) points out the need to describe the specificities of eel ecology in each habitat it colonizes to adapt the global conservation and restoration measures to these local specificities.

Transitional waters, and especially Mediterranean lagoons, are very productive habitats, which sustain important traditional fisheries (Pérez-Ruzafa *et al.*, 2011). The productivity of lagoons is linked to the diversity of habitats characterized by strong spatio-temporal variability of environmental gradients such as salinity, temperature, turbidity, and dissolved oxygen (Basset *et al.*, 2007). This is mainly due to their shallow water depth and to their communication with the sea, which is usually limited to one or a few channels. Their exposition to wind and to rare but heavy rainfalls can also trigger rapid changes in environmental conditions in the lagoons (Boutron *et al.*, 2021). Eel is present in >70% of the Mediterranean lagoons (Pérez-Ruzafa *et al.*, 2011) and is among the most frequent and abundant fish species in these habitats with estimated biomass as high as 30 kg ha⁻<fnref n="1" /> (Amilhat *et al.*, 2008). As a comparison, eel estimated biomass in freshwater habitats (rivers and lakes) is usually inferior to 5–6 kg ha⁻<fnref n="1" /> (Rosell *et al.*, 2005; Breteler *et al.*, 2007; Acou *et al.*, 2009) even if

<footnote n="1"><sup>1</sup>Centre de Formation et de Recherche sur les Environnements Méditerranéens, Université de Perpignan Via Domitia, UMR 5110, F-66860 Perpignan, France</footnote>

<footnote n="2"><sup>2</sup>CNRS, Centre de Formation et de Recherche sur les Environnements Méditerranéens, UMR 5110, F-66860 Perpignan, France</footnote>

<footnote>* Corresponding author: tel: +33 4 68 66 21 86; e-mail: raphael.lagarde@univ-perp.fr</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Locations of the Bages-Sigean lagoon in southern France (a), the channel that links the lagoon with the sea (b), and detail view of the channel (c). The white circles represent the locations of the three meteorological monitoring stations (the name of each station is specified) and the white triangle indicates the location of the gauging station on the Berre River (b). The white star represents the location of the Horizontal Acoustic Doppler Current Profilers (H-ADCPs) and Conductivity-Temperature-Depth (CTD) probe (c). The black squares with white font and grey and white font represent the location of the floating traps installed on 12 and 13 December 2018 and 4 October 2019, respectively (c).</figcaption></figure>

an estimated biomass >40 kg ha⁻¹ has also been documented (Acou *et al.*, 2009). Approximately 400 lagoons are present in Mediterranean countries for a combined wetted area of >500000 ha (Barcala *et al.*, 2022). These observations highlight the importance of Mediterranean lagoons for eel population and fisheries. Most studies on eel in Mediterranean lagoons are focusing on yellow and silver eels, the stages commonly targeted by fisheries (Amilhat *et al.*, 2008; Charrier *et al.*, 2012; Aalto *et al.*, 2016) as glass eel fishery is prohibited in many Mediterranean lagoons. Despite acknowledging the important role of the number of glass eel entering the lagoons for the abundance of yellow and silver eels the following years, this stage has not been extensively studied in lagoons (Lecomte-Finiger, 1983; Lefebvre *et al.*, 2003; Crivelli *et al.*, 2008) especially in recent years (Leone *et al.*, 2016).

To contribute to fill the gap of knowledge concerning glass eels entrance in lagoons, this stage was monitored daily over three years in the Bages-Sigean lagoon (southwestern French Mediterranean coast). We first hypothesized that a marked seasonal phenology of glass eel entrance in the lagoon exists and that entrance is mainly driven by local environmental drivers due to the strong spatio-temporal variability of envi-

ronmental gradients in lagoons. Second, we hypothesized that the biometric and the pigmentation characteristics of glass eels entering the Bages-Sigean lagoon were similar to those historically observed in the same site (Lecomte-Finiger, 1983) but were different compared to other eel habitats in the Mediterranean and Atlantic coasts.

## Material and methods

### Study site

The Bages-Sigean lagoon, located in southwest of the French Mediterranean coast, extends over an area of 38 km² and has an average depth of 1.3 m (Figure 1a and b). It is one of the largest lagoons in France and it hosts one of the biggest eel sub-populations with an eel estimated biomass as high as 30 kg ha⁻¹ (Amilhat *et al.*, 2008). The main freshwater input comes from the Berre River, which is located in the southwest part of the lagoon watershed (Figure 1b). Glass eels were sampled in the channel that links the lagoon with the Mediterranean Sea (Figure 1c). The channel is ~2.5 km long, and 50–150 m wide with a depth ranging from 3 to 10 m. It can be considered as a micro-tidal estuary in which the current is
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Schematic representation of a floating trap used during the study. The ropes that maintained the geotextiles layers to the foam are drawn in black.</figcaption></figure>

mainly influenced by the wind and freshwater inputs in the lagoon (Fiandrino *et al.*, 2012). No sluice gate or other man-made structure prevents the migration of glass eels into the lagoon but also silver eel escapement to the sea. The channel is located in an urban environment and is used as a commercial harbour (Port la Nouvelle harbour). The substrate in the channel is almost exclusively mud, and the banks are riprap or concrete with no vegetation and little refuge for glass eels.

## Sampling method

The sampling gears used in the study were passive floating traps consisting of ten 50 × 50 cm layers of geotextile stacked (MACMAT/Geomat, MACCAFERRI), which create refuge habitats for glass eels and small elvers (Figure 2). Each set of geotextile layers was topped with a 50 × 50 cm foam square, which maintains the trap buoyancy and provides shade. The traps were attached to fixed structures to maintain them at a distance ranging from 1 to 3 m from the channel bank. Several studies used passive traps with an artificial refuge habitat to study glass eel and/or elver abundance in lentic areas (Silberschneider *et al.*, 2001; Edeline *et al.*, 2007; Leone *et al.*, 2016). These authors positioned their traps close to the bottom, but a recent study (Cellule Migrateurs Charente et Seudre, 2014) demonstrated that the number of glass eels captured in floating traps was higher than in traps positioned close to the bottom.

The entire study period spanned from December 2018 to April 2021. Ten floating traps were installed in the channel, from the closest area to the sea to the closest area to the lagoon on 12 and 13 December 2018. Another five passive traps were added in the closest area to the lagoon, on 4 October 2019 because most glass eels captured from 12 December 2018 to 3 October 2019 were collected in the floating traps close to the lagoon. Sampling was interrupted on 16 March 2020 due to restrictions related to the COVID-19 crisis. All 15 floating traps were reinstalled in the same location on 30 September 2020 until the end of the study, 2 April 2021.

The floating traps were inspected daily, using an 80 cm hand net equipped with a 1 mm mesh size net to avoid glass eel escape from the traps. To take into account the variability in the number of floating traps, the daily captures of glass eels, considered as a proxy of the number of glass eels entering the lagoon, were expressed as captures per unit effort (CPUE, number of glass eels per trap and per day) in the following analyses.

Several times a week all the glass eels captured were anaesthetized with eugenol (0.3 ml L⁻¹), measured (total length, TL in mm, precision ± 1 mm) and weighted (weight in g) with a precision scale (KERN, ALJ500-4A, precision ± 0.001 g). Their pigmentation stages (from VA unpigmented to VII fully pigmented), was determined based on the number of pigments in the cerebral area, the tail, and the body following a methodology adapted from Briand (2009). Subsequently, all the glass eels were released in the lagoon.

## Environmental parameters

Eleven environmental parameters (Table 1), known to influence glass eel entrance in continental habitats (Arribas *et al.*, 2012; Aranburu *et al.*, 2016; Podda *et al.*, 2020), were continuously collected during the study following the methods described in Lagarde *et al.* (2021). Briefly, five parameters were obtained from the three closest Météo France meteorological stations (b): rainfall (mm h⁻¹), air temperature (°C), atmospheric pressure (hPa), nebulosity (qualitative scale, from 0 clear sky to 9 fully cloudy), and wind velocity (m s⁻¹, with positive values when the wind was blowing seaward and negatives values otherwise). The discharge of the Berre River (m³ s⁻¹) was obtained through automatic measurement at a gauging station (available at www.hydro.eaufrance.fr, accessed on 22 May 2021, b). The percentage of the full moon (0% new moon, 100% full moon) was estimated with the “lunar” package (Lazaridis, 2014) in the open source R software (R Core Team, 2021). The other five parameters were measured in the channel (c). The flow velocity (m s⁻¹, with positive values when the water was flowing seaward and negatives values otherwise) was estimated based on the measurements made by two H-ADCP, (Channel Master, RDI-Teledyne) positioned 1.5 and 4 m below the water surface. The backscattered intensities of the acoustic signal of the two H-ADCP were used to estimate the suspended solids concentration (SSC, g L⁻¹). Finally, the water temperature (°C) and the salinity were measured with a CTD probe (6600 or EXO2, YSI). Temperature was monitored in four other locations in the channel using temperature loggers (PLUG & Track, iBcod). The salinity was measured manually each morning using a refractometer.

The environmental parameters selected in our study were measured at a frequency ranging from 5 min to 1 h and in one to five different locations (Table 1). As glass eels are known to be mostly active during night hours (Harrison *et al.*, 2014), the mean values of the environmental parameters from 18:00 to 6:00 the next morning were used in the following analyses. Environmental parameters measured in different locations (three climate monitoring station, two HADCPs, five temperature loggers) were correlated between locations with a Pearson's correlation coefficient (*r*) > 0.8, consequently the mean value of all the different locations was used in the following analyses. The manual estimates of salinity collected in the morning were used when the CTD measurements during the previous night were missing due to technical issues (60 d). Finally, the air and water temperatures were correlated with *r* > 0.8. To limit the redundancy of these two parameters, only the water temperature, more representative of the glass eel habitat conditions, was kept in the following analyses.

## Statistical analysis

The effect of the ten environmental parameters (rainfall, atmospheric pressure, nebulosity, wind velocity, discharge of the
<table><caption>**Table 1.** Summary of eleven environmental parameters obtained or measured during the study.</caption><thead><tr><th>Environmental parameter</th><th>Unit</th><th>Mean</th><th>Min-max</th><th>N of stations or instruments</th></tr></thead><tbody><tr><td>Rainfall</td><td>mm h<sup>-1</sup></td><td>0.08</td><td>0-7.5</td><td>3 (climate monitoring stations)</td></tr><tr><td>Air temperature</td><td>°C</td><td>12.2</td><td>1.8-22.9</td><td>3 (climate monitoring stations)</td></tr><tr><td>Atmospheric pressure</td><td>hPa</td><td>1 018.2</td><td>992.2-1 035.1</td><td>3 (climate monitoring stations)</td></tr><tr><td>Nebulosity</td><td>–</td><td>5.0</td><td>0-8.5</td><td>3 (climate monitoring stations)</td></tr><tr><td>Wind velocity</td><td>m s<sup>-1</sup></td><td>2.6</td><td>-12.3-12.5</td><td>3 (climate monitoring stations)</td></tr><tr><td>Berre River discharge</td><td>m<sup>3</sup> s<sup>-1</sup></td><td>0.48</td><td>0.01-21.8</td><td>1 (Ripaud gauging station)</td></tr><tr><td>% of the full moon</td><td>–</td><td>52.3</td><td>0-100</td><td>1 (lunar package)</td></tr><tr><td>Flow velocity</td><td>m s<sup>-1</sup></td><td>0.02</td><td>-0.34-0.38</td><td>2 (1.5 and 4.5 m depth H-ADCP)</td></tr><tr><td>SSC</td><td>g L<sup>-1</sup></td><td>0.02</td><td>0-0.06</td><td>2 (1.5 and 4.5 m depth H-ADCP)</td></tr><tr><td>Water temperature</td><td>°C</td><td>12.4</td><td>3.0-21.6</td><td>5 (CTD probe and 4 PLUG &amp; Track loggers)</td></tr><tr><td>Water salinity</td><td>–</td><td>36.0</td><td>27.5-41.9</td><td>1 (CTD probe or manual measurements)</td></tr></tbody></table>

The number (N) of stations or instruments used to measure the parameters and the type of station or instrument are specified. When more than one instrument measured the same parameter, the mean values measured by the different instruments were kept in the analyses.

Berre River, moon phase, flow velocity, SSC, water temperature, and salinity) on the abundance of glass eel entering the lagoon was analysed using random forest (RF; Breiman, 2001). RF does not depend on the normality or homoscedasticity of the data, does not require transformation of the data and integrates nonlinear responses (Breiman, 2001). The latter appeared particularly important in our study as data preview highlighted non-linear relationships between the number of glass eels entering the lagoon and several environmental parameters. The sampling date, considered as a continuous variable ranging from 1 on 1st October of year N to 365 on 30th September of year N + 1 was integrated to the RF to account for temporal variability. To limit the confounding effect of seasonality on environmental parameters and eventually on the number of glass eels entering the lagoon, RF analyses were only performed during the main periods of glass eel entrance in the lagoon each studied year (October–March, see Results). Indeed, outside this period, the number of glass eels that arrive from the sea to the site is expected to be very low. Additionally, the sampling was performed only once for the months outside the main period of glass eel entrance in the lagoon (instead of two or three times for the months inside the period). Consequently, specific conditions, such as an exceptional heat wave observed in 2019, could have greatly influenced the results. The global performance of the RF model was assessed based on the percentage of variance explained by the model. The importance of each environmental parameter on the number of glass eels entering the lagoon was assessed based on the node purity index (Banerjee et al., 2019). The significance of the node purity index was estimated by comparing the predicted values for each parameter to a null distribution obtained with 1000 random permutations of the values (Archer, 2016). The effect of parameters with a significant node purity index on the number of glass eel entering the lagoon was further described based on its partial response plot.

To compare the temporal variability of pigmentation stages, TL, and weight, all glass eels for which the biometric and the pigmentation characteristics were measured during a calendar month were grouped. When the number of glass eels during a calendar month was lower than 20 individuals, pigmentation stages, TL, and weight were analysed between groups of two or three months. The proportions of the pigmentation stages between the months or groups of months were compared with a Chi² test. As the distributions of TL and weight were not normally distributed (Shapiro–Wilk normality test, *p* < 0.002), they were compared between months or groups of months using non-parametric kernel density estimates (Lan-

glois et al., 2012). The bandwidth of TL or weight was selected following the data-driven procedure of Sheather and Jones (1991), which aims at minimizing the error made for smooth densities. Finally, the significance of the difference in TL or weight distributions between months or groups of months was tested with a permutation test as described in Langlois et al. (2012).

All statistical analyses were performed with R software (R Core Team, 2021) implemented with the packages *random-Forest* (Liaw and Wiener, 2002) and *rfPermute* (Archer, 2016) for RF analyses and *Kernsmooth* (Wand, 2015) and *sm* (Bowman and Azzalini, 2014) for comparison of LT and weight distributions. The significance level was set at *p* < 0.05.

## Results

### Phenology of glass eels entering the lagoon

The number of glass eels entering the lagoon was monitored daily during 647 d split in two continuous periods from 13 December 2018 to 16 March 2020 (461 d; Figure 3a and b) and from 30 September 2020 to 2 April 2021 (186 d; Figure 3c). A total of 2657 glass eels were captured during the study. Following a general trend, captures were higher in October–March than in April–September (Figure 3a and b) with a variability in CPUE from 0 to 7.2 glass eels per floating trap per day during the most abundant period each year (Figure 3a–c). Most of the CPUEs superior to one glass eel per floating trap per day were observed from mid-November to mid-March.

### Effect of environmental drivers

The effect of the ten environmental parameters and of the sampling date on glass eels CPUE during the highest period of glass eel entrance in the lagoon (between October and March) was analysed using RF (Figure 3). A total of 351 d when glass eel CPUE as well as all environmental data were available was used for the analyses. The RF describing the CPUE of glass eels in relation to environmental parameters and sampling date did not perform well as only 25.1% of the variance of the CPUE of glass eels was explained. The only variable significantly associated with the CPUE of glass eels was the discharge of the Berre River (Table 2; *p* = 0.001). The CPUE of glass eels was higher when the discharge of the Berre River was higher to its base flow conditions (0.1–0.2 m³ s⁻¹, Figure 4; Supplementary Figure S1). Above this threshold, the CPUE of glass eels did not further increase with discharge.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** CPUE (number of glass eels per floating trap and per day) during the study (a–c). The vertical dashed lines (a and c) delimitate the higher (October–March) and the lower (April–September) periods of glass eel entrance in the lagoon.</figcaption></figure>

<table><caption>**Table 2.** Relative importance of the ten environmental parameters and the sampling date, explored to explain the daily number of glass eels captured over the three highest periods of glass eel entrance in the lagoon (December–March 2018–2019; October–March 2019–2020 and 2020–2021) from the RF analysis.</caption><thead><tr><td>Variable</td><td>Index of node purity</td><td>p-values</td></tr></thead><tbody><tr><td>Berre River discharge</td><td>34.1</td><td>0.01</td></tr><tr><td>Water salinity</td><td>19.2</td><td>0.12</td></tr><tr><td>Sampling date</td><td>18.4</td><td>0.16</td></tr><tr><td>Flow velocity</td><td>12.4</td><td>0.79</td></tr><tr><td>Water temperature</td><td>12.2</td><td>0.74</td></tr><tr><td>Rainfall</td><td>12.2</td><td>0.10</td></tr><tr><td>SSC</td><td>10.8</td><td>0.91</td></tr><tr><td>Wind velocity</td><td>10.0</td><td>0.93</td></tr><tr><td>Atmospheric pressure</td><td>10.0</td><td>0.95</td></tr><tr><td>% of the full moon</td><td>9.9</td><td>0.95</td></tr><tr><td>Nebulosity</td><td>4.1</td><td>1.00</td></tr></tbody></table>

The variables are ranked by decreasing index of node purity and the *p*-values associated with the index of node purity are specified.

## Pigmentation stages of glass eels

The pigmentation stages were determined on a total of 1065 glass eels. Due to the low number of glass eels captured in

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Partial response plot of the RF analysis describing glass eel abundance (CPUE, in glass eels per floating trap and per day) as a function of the discharge (m³ s⁻¹) of the Berre River.</figcaption></figure>

2019, all glass eels sampled from January to February, March to April, May to July, and August to September were grouped (Figure 5). Additionally, due to the restrictions related to the COVID-19 crisis, only four glass eels were sampled for the pigmentation determination in March 2020. These individuals were removed from the analyses. The relative propor-
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Proportions of the pigmentation stages identified in glass eels sampled during the study. The number of glass eels (N) sampled each month or group of months is specified.</figcaption></figure>

Proportions (%) of the identified pigmentation stages were significantly different between months or group of months ($Chi^2_{test, Chi^2_{104}} = 399, p < 0.001$). Glass eels were less pigmented from November to February (higher proportion of stages VA, VB, and VIA0) than from March to October (Figure 5).

## TL and weight distributions of glass eels

TL and weight were measured on the same 1065 glass eels used for the identification of pigmentation stages. The TL of glass eels varied from 51 to 76 mm (mean ± standard deviation [SD] = 66 ± 4 mm) and their weight varied from 0.10 to 0.44 g (0.23 ± 0.05 g). TL and weight distributions were significantly different among months or groups of months (kernel density estimate, band widths = 1 mm for TL and 0.02 g for weight, *p* < 0.001, Figure 6a and b). The greatest TL and weight were observed in November 2019 and 2020, and the smallest TL and weight in March–July 2019 (Figure 6a and b).

## Discussion

### Phenology of glass eels entering the lagoon and effect off environmental drivers

The entrance of glass eels in the Bages-Sigean lagoon took place throughout the year with a main period from October to March and the highest peaks observed from mid-November to mid-March each year. This result corroborates historical observations made during the 70s and 80s at the same site (Finiger, 1976; Yahyaoui, 1983). Therefore, the main period of glass eel entrance in the lagoon appears to be stable over several decades. More recently similar periods of higher number of glass eels entering the Canet lagoon (France—40 km south of the Bages-Sigean lagoon) in 2008 (Elsa Amilhat, Perpignan

University, unpublished data) and Ter River estuary (Spain—100 km south of the Bages-Sigean lagoon) in 2019 (Zamora and Costarrosa, 2019) were also observed. Interestingly, the main period of glass eel entrance observed in other Mediterranean areas, in the Vaccarès lagoon (Crivelli et al., 2008), the Tiber (Ciccotti et al., 1995), and the Pramaera rivers (Podda et al., 2020), was slightly different with higher entrance between late December and late April. These habitats are located 140 km northeast, 750 km east, and 650 km southeast from the Bages-Sigean lagoon, respectively. The difference of glass eel entrance periods between these areas highlights that the environmental specificities of geographically close habitats may influence the phenology of this life history stage. The specificities of glass eel entrance phenology in these habitats point out the importance to harmonize the number of glass eel monitoring sites over the eel repartition area.

Overall, the environmental drivers studied had little effect on the number of glass eels entering the Bages-Sigean lagoon (25.1% of the total variance explained by the RF model). This result may be partially explained by the influence of drivers, other than those studied, on the number of glass eels entering the lagoon. For example, the number of glass eels that arrive from the ocean to the Bages-Sigean channel could be variable in relation to more global environmental drivers, such as oceanic currents, climate changes, North Atlantic oscillation index or food availability from the Sargasso Sea to the Mediterranean (Arribas et al., 2012). Integrating these global parameters in further studies could improve our understanding of glass eel ecology in the Mediterranean.

Despite the limited performance of the model explaining the number of glass eels entering the lagoon, the CPUEs of glass eels were significantly higher when the discharge of the main tributary of the lagoon was more elevated than its base
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Frequency distributions of TL (a) and weight (b) of glass eels. The number of glass eels in the samples for each month or group of months is the same as in Figure 4. The vertical dashed lines represent the median values of TL (66 mm) and weight (0.24 g).</figcaption></figure>

flow. Similar observation was made by Crivelli *et al.* (2008) in Vaccarès lagoon where peaks of glass eel captures were positively correlated to freshwater discharge. The positive effect of freshwater (river) discharge on the number of glass eels entering the lagoon is probably explained by the attractiveness of “inland water odour” (Tosi and Sola, 1993) and/or low salinity water (Tosi *et al.*, 1990) for glass eel. Interestingly, previous studies in the Bages-Sigean lagoon demonstrated a positive effect of strong wind blowing from the northwest (seaward) on the number of glass eels entering the lagoon (Finiger, 1976; Lecomte-Finiger and Razouls, 1981), which was not observed in our study. This observation is probably explained by the removal of sluice gates that separated the lagoon from the channel before 2004. These sluice gates limited the mixing of water between the sea and the lagoon increasing environmental gradients. Additionally, they were generally only opened when

the wind was blowing from the northwest, in order to limit the risk of flooding. In these conditions, glass eels were probably strongly attracted by the flow of water from the lagoon when the sluice gates were opened.

Finally, several environmental drivers, such as water temperature, turbidity, moon phase, tide, or meteorological parameters (wind, rainfall, cloud cover, atmospheric pressure), known to influence the number of glass eels entering continental waters (Elie and Rochard, 1994; Harrison *et al.*, 2014; Lanzoni *et al.*, 2022) were not significantly associated with glass eel CPUE in our study. The local specificities of the lagoon watershed and the channel in which we captured glass eels may explain this observation. Glass eels are known to preferentially enter continental waters when the water temperature is between 5 and 17°C (Podda *et al.*, 2020). These temperatures are frequently observed during our study with of only 7 d
(2% of the data) with temperatures lower than 5°C and 31 d (9%) with temperatures higher than 17°C in October–March. Thus, limiting temperatures for glass eel migration are rare in Bages-Sigean lagoon, which explains the absence of effect of this parameter on glass eel entrance in the lagoon. The effect of moon phase and turbidity on the number of glass eels entering continental waters is generally explained by a diminution of the luminosity during which glass eels tend to move more actively (Bardonnet et al., 2005; Podda et al., 2020). The channel that links the Bages-Sigean lagoon to the sea is constantly enlightened with public lighting, which may limit the effect of turbidity and moon phase on luminosity. Tide is also frequently associated with the number of glass eels entering continental waters habitats (Podda et al., 2020) as individuals can use tidal current as a transport vector (Harrison et al., 2014). The studied channel is a micro-tidal estuary in which the current is mainly influenced by the wind and freshwater inputs in the lagoon (Fiandrino et al., 2012). In this context, tide has only a limited influence on the currents in the channel and consequently on the number of glass eels entering the lagoon. Atmospheric phenomena such as wind, rainfall, cloud cover, or atmospheric pressure usually have an indirect effect on glass eel entrance in continental waters through their influence on aquatic environment (discharge of the rivers, water flow, salinity, turbidity etc.). However, the effect of atmospheric phenomena on aquatic environment is not straightforward. For example, the effect of rainfall on river discharge depends on the intensity of rainfall, its spatial extent, and the level of water in the aquifers before the rain. In this context, it is unsurprising to observe a more important effect of the aquatic environment drivers than the atmospheric phenomena on the abundance of glass eels entering the lagoon.

## Glass eel biometric characteristics

The average size (TL, 66 mm, range 51–76 mm) and weight (0.23 g, 0.10–0.44 g) of glass eels captured in Bages-Sigean were slightly above the size (58–64 mm) and weight (0.16–0.30 g) observed in the Vaccarès lagoon (Lefebvre et al., 2003). Oppositely, Desaunay and Guerault (1997) and Lecomte-Finiger (1992) described a slightly larger size of glass eels on the Atlantic coast: 68–73 and 64–69 mm in the Vilaine and Loire estuaries, respectively. Desaunay and Guerault (1997) also described slightly higher weight (0.21–0.31 g) of glass eels captured in the Vilaine estuary. These observations highlight the spatial and temporal variability of glass eels' biometric characteristics. One hypothesis, that could explain the variability in biometric characteristics of glass eels is that glass eels could use different migration routes from the Sargasso Sea. Glass eels arriving on the Atlantic Coast may follow the North Atlantic current (Gulf Stream), while those arriving in the Mediterranean may follow the Azores Current (Miller and Tsukamoto, 2016). These different migration routes can result in different migration time and/or variable growing conditions of the leptocephalus larvae.

At the seasonal scale, size and weight of glass eels were maximum in October–December and steadily decreased from December to March. In parallel, the proportion of glass eels in early pigmentation stages (stages VA–VIA0) was higher from November to February compared to March–October. The diminutions of TL, weight, and proportion of early pigmentation stages in glass eels from November to May were previously observed in the Bages-Sigean (Finiger, 1976) and

Vaccarès (Lefebvre et al., 2003) lagoons. The different pigmentation stages observed between months may reflect the arrival of various cohorts of glass eels in the lagoon. In contrast, this difference of pigmentation stages may be related to faster or slower pigmentation speed depending on environmental factors of the month of arrival. Briand et al. (2004) demonstrated that glass eels pigmented faster when water temperature increased and salinity decreased. In the Bages-Sigean lagoon and adjacent sea waters, the lowest water temperatures were observed from December to February. During these months, glass eels might pigment slowly, which could explain the higher proportion of glass eels in early pigmentation stages during this period. Over the study period, the salinity in the channel fluctuated from 28 to 42, which is well above the salinities (from 0 to 12) observed to influence pigmentation speed in Briand et al. (2004). The seasonal differences in TL and weight could also result from different migration routes from the Sargasso Sea and/or different growing habitats of the leptocephalus larvae (Desaunay and Guerault, 1997). Alternatively, the diminution of TL and weight from November to March could be a confounding effect of the diminution of the proportion of glass eels in early pigmentation stages as size and weight usually decrease with latter pigmentation stages (Lambert et al., 2003; Bureau du Colombier et al., 2007).

## Conclusion

The main period of glass eel entrance (mid-November–mid-March) in the Bages-Sigean lagoon and the pigmentation stages, size, and weight of glass eels were similar to historical studies in the same site. Our results also highlighted that glass eel entrance in the lagoon is more important when the discharge of the main tributary of the lagoon is higher than its base flow. When these conditions are observed, anthropogenic activities likely to impact glass eels, for example, dredging (Lagarde et al., 2021), should be avoided.

The phenology and biometric characteristics of glass eels entering in the Bages-Sigean lagoon differed compared to other habitats in the Mediterranean and Atlantic coasts. These observations plead for an integration of more monitoring sites of glass eel in the Mediterranean. The presence of historical observations and the characteristics of the channel linking the Bages-Sigean lagoon to the sea, such as the absence of migration barriers and the important population of eel in the lagoon (Amilhat et al., 2008), make our study site a prime candidate for long-term monitoring of glass eels.

## Supplementary Data

Supplementary material is available at the ICESJMS online version of the manuscript.

## Funding

This study was conducted with the financial support of Agence de l'eau Rhône Méditerranée Corse (2–4 allée Lodz, 69363 Lyon cedex 07, France), Région Occitanie (201 avenue de la Pompignane, 34064 Montpellier cedex 02, France), Ministère de l'Agriculture et de l'Alimentation (Hôtel de Villeroy, 75007 Paris, France), and préfecture de la Région Occitanie (Place Saint-Etienne, 31038 Toulouse cedex 9, France).
**Data availability statement**

Survey data cannot be made public for legal reasons but they are available upon request from the authors and data producers.

**Authors contributions**

Conceptualization: RL, EA, FB, and EF; methodology: RL, EA, FB, EF, and GS; collection of data: RL, JP, FP, and AP; formal analysis: RL and FB; first draft: RL; review and editing: RL, JP, EA, FB, FP, AP, GS, and EF.

**Acknowledgements**

We are very thankful to all the people who assisted us during the study and to R. Lecomte-Finiger for her advices on a previous version of this manuscript. Finally, we are grateful to the three anonymous reviewers for their useful comments and suggestions on a previous version of this manuscript.

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