## 1 Introduction  
Freshwater eels, genus _Anguilla_, are distributed widely across much of the globe, from tropical to temperate regions, and they consist of 19 species and subspecies [1, 2]. Most studies in recent years have focused on eels of temperate habitats such as the Japanese eel (_Anguilla japonica_), the European eel (_A. anguilla_), and the American eel (_A. rostrata_), which are distributed in regions found between mid and high latitudes [3]. The members of the Anguilla genus are catadromous fish with an oceanic and continental phase. One of the gaps in knowledge concerning the eel life cycle relates to the oceanic phase as it is challenging to observe them during their marine life stages. The life history of this fish depends strongly on oceanic conditions; maturation, migration, spawning, larval transport, and recruitment dynamics are completed in the open ocean [4]. The European eel is distributed from North Cape in Northern Norway, southwards along the coast of Europe, all shores of the Mediterranean, and on the North African Coast [5, 6]. The available information indicates that the stock has severely declined in most of its distribution area, and the population has decreased dramatically since the 1980s [7, 8]. The European eel is now classified as \"criticallyendangered\" according to the International Union for Conservation of Nature (IUCN). Several conservation actions have been applied over the years: in 2007, the European Union adopted a management plan to recover the European eel stock through an EU regulation (EU 1100/2007). In 2010, another measure adopted was the prohibition of the trading of glass eels outside the European Union, and since 2018, 3-month fishing closures have been implemented at the EU level. In the last stage of life in freshwater, eels undergo several morphological and physiological changes to adapt to the long migration [9,10]. These changes are thought to be closely related to gonadal development [4,9,11,12]. Downstream migration is a specific life-history event for eels that marks the end of the passive growth phase (yellow stage) and the beginning of the migratory phase (silver stage). As eels grow and prepare for the long migration, it is of key importance that they accumulate a large amount of lipids, according to [PERSON] and [PERSON] [13]. Optimal trophic and environmental conditions allow for favorable lipid accumulation that is necessary for gonad development and help eels to reach sexual maturation and successfully complete migration [14]. According to this, the process of silvering is also more flexible than generally presumed [14] and can be influenced by several trophic and environmental factors [15, 16, 17, 18]. Information on the gonadal stage and morphological indices are necessary for a better selection and the release of sexually mature individuals that are ready to migrate, as requested by the EU directive. In addition, they can provide guidelines on how to regulate the various factors that influence maturation [19] and thus be of great support for artificial reproduction, which has been showing encouraging results in the last decades [20, 21, 22]. Over the years, various methods for assessing eel maturity have been developed; in European eels, one of the methods is based only on the change of patterning, and it is used to differentiate the two stages of the yellow and the silver eel, as the \"migratory\" eels often show a white-silver belly and a black dorsal back. However, most migratory eels often show intermediate characteristics, such as bronze belly and sides. Thus, the color criterion is subjective and unreliable [23]. [PERSON] [9] proposed another method for distinguishing yellow and silver female eels. The author developed an index based on eye surface area, which increases at the migratory stage, and set a threshold of 6.5 for sexually maturing European eels. Another non-invasive method is the silver index, developed by [PERSON] et al.