## 4 Discussion  
The objective of this study was to evaluate the gonadal maturation stage of European eels at various stages of silvering in four lagoons of the Po River Delta. Generally speaking, the results showed a strong presence of silver eels (SI III-V) in the populations surveyed (>94.5%): the percentage of migrating eels with silver index IV-V was highest in Valle di Comacchio, followed by Valle San Carlo, Valle Nuova, and finally, Valle Ca' Pasta, with 20% of eels ready to migrate. This difference between the lagoons can be explained by the different environmental and trophic conditions [25; 32; 37; 38] found in each valley despite their proximity and belonging to the same geographical area. [PERSON] and [PERSON] [39], for example, have shown that Valle di Comacchio, characterized by strongly brackish waters, has an incredibly large and abundant spectrum of prey (mainly clupeids and engraulids), therefore eels can grow well because there are favorable environmental conditions, not only for growth but probably for gonadal development as well. Likewise, [PERSON] et al. [40] demonstrated that brackish waters such as the Vaccares lagoon (France) were more favorable growth sites than freshwater environments. This could explain why the eels from Valle Ca' Pasta, the only freshwater eels, had the lowest GSI values. The results also confirm the hypothesis of [PERSON] et al. [14], i.e., environmental properties could lead to large differences in lipid storage metabolism, which is crucial for coping with migration and promoting gonadal maturation [22; 41]. Concerning the results obtained on the levels of oocyte maturation, it was seen that the number of mature eels in the four considered lagoons tended to be high. Valle di Comacchio had the highest levels of oocyte maturation, while Valle Ca' Pasta had the lowest. It should be underlined that eels with silver index FIII had an oocyte maturation nearly equal to  
Figure 6: Relation between silvering stages and GSI values of sampled eels. that of eels at stages FIV and FV of silvering. Therefore, eels that the [PERSON] et al. [15] classification considered pre-migratory presented a similar oocyte maturation level to that of eels considered to be migratory. Furthermore, a strong, positive correlation was observed between GSI values and maturation levels, thus strengthening histological observation. Many studies have been conducted on eels from different environments, and all of them show how the status of morphological and histological characteristics at sampling change from area to area [22; 25]. Having histological data that confirm high oocyte maturation for eels considered to be pre-migratory is of paramount importance because it allows us to have a deeper understanding of the characteristics of the North Adriatic Italian eel populations. These logoons are confirmed as favorable environments for the growth of eels; the same study of [PERSON] et al. [15] argued that the process of silvering is conditioned by various environmental factors that play a key role in this process; in fact, favorable growth conditions make the eels' second metamorphosis quicker [39; 42; 43] The hypothesis that North Adriatic logoons are suitable habitats for gonadal growth and maturation is further strengthened by [PERSON] et al. (unpublished), who showed that eels at silvering stage FIII are very young eels and superimposed on the age of eels classified as migrants (48% FIII age 7-7+; 50% FIV age 7-7+). Considering the results, we can affirm that eels from North Adriatic logoons are eels with high levels of oocyte maturation accompanied by high indices of GSI values already at silver stage FIII (pre-migrant). In this regard, considering and comparing the Atlantic eels--on which the silver index protocol has been developed--with those of the North Adriatic, it can be observed that North Adriatic populations appear not to mirror the gonadal maturation pattern of Atlantic eels.