## 4 Discussion  
To date, two approaches are used for the recovery of endangered species such as eel. The main one involves the search for candidates in the local stock, usually individuals at the advanced stage of silvering (SI-IV/V), to release the best quality spawners for migration to the Sargasso Sea [48]; this is one of the actions also present within the LIFEEL project. The second, a tool used for restocking several endangered fish species such as sturgeon, is artificial spawning with the aim of producing juveniles for release into the wild; this action is present within European projects for sturgeon (LIFE11 NAT/IT/188, Be Natur), but also for eel as the LIFEEL(LIFE19 NAT/IT/000851). Based on the results obtained in the weaning of Japanese eel larvae and the first positive results on European eel, this method is gaining ground [49; 50; 51; 52]. There are several protocols that often start with breeding animals whose specific silvering status has not been taken into account. It is clear that the wrong choice of female can lead to unsatisfactory reproductive results. However, the method is not fully standardized. In artificial reproduction techniques, the initial stage of maturation before hormonal induction is important, as several studies have examined the impact of the eel's stage of maturation on reproductive performance [53]. These studies on Japanese eels [33] and European eels [31] report that the quantity and quality of eggs obtained are highly dependent on the initial maturation stage. [PERSON] et al. [33] tested eels at different stages of maturation and showed that under-mature eels did not respond to hormone treatment. In this study, the reproductive performance of eels at the beginning of the metamorphosis (SI III) was not statistically different from the eels at the migrating stage (SI V). The positive response of the stage III eels to the hormonal-induced stimulus can be explained  
Figure 2: This graph shows all qualitative characteristics expressed in percentages of the two groups (migrant, pre-migrant). Data are given as the mean \(\pm\) SD. by the overlap of IGS of the pre-migrant eels with that observed by [PERSON] et al. [32] in Atlantic migrating eels (IV). Apparently, the pre-migrant eels of the North Adriatic Sea, if subjected to an artificial reproduction programme, reach sufficient gonadal maturation to give a hormonal response. This is confirmed by the study of [PERSON] et al. [54], which investigated gonadal maturation at the histological level of different silvering eels from the North Adriatic lagoon area, showing that pre-migrant eels, SI III eels, had overlapping oocyte maturation confronted with eels in later stages of maturation. In addition, this early gametogenic activity was found to be associated with accelerated weight growth in the pre-migrating eels studied, a phenomenon normally seen in Atlantic migrant stage IV eels. The general picture of the females from Valle Bertuzzi is certainly very peculiar, since the total absence of migratory stage IV eels is compensated for by a high concentration of pre-migratory eels, which represent almost 75% of the total population; a plausible explanation could be that the pre-migratory females from this _valle_, despite having external morphometric values typical of eels at the beginning of metamorphosis, actually seem to have all the characteristics to be considered migratory eels. Identifying the levels of silvering in these environments using the method of [PERSON] et al. [32] would give an ambiguous picture of the state of the local stock and is therefore not sufficient. Possibly, the longer distance to travel during migration, compared with Atlantic eels, on which the silvering index is based, combined with the shallow waters throughout the North Adriatic, may have delayed certain morphological changes such as the dilation of the eyes and the elongation of the pectoral fin. Thus, North Adriatic eels, although morphologically not classified as migrants, are physiologically ready for migration.