Gonadal development of European eels, Anguilla anguilla, at different stages of silvering

The European eel (Anguilla anguilla, Linnaeus 1758) is a catadromous fish which has a great cultural, scientific, and commercial value. The protection of this species is particularly difficult because the biological eel’s life cycle remains in many aspects still unknown. The European eel (A. anguilla) population has declined alarmingly over the past 30 years, this condition has led to questions about the long-term welfare status of this species. The objective of this thesis is to perform a histological analysis on gonad samples of European eels from 4 different lagoons of the North Adriatic at different stages of silvering and to evaluate the maturation of the gonads. For this study were captured a total of 859 wild eels from 4 different lagoons using the “lavoriero”. Subsequently the biometric parameters were collected. A total of 79 female eels were randomly selected, dissected and the gonads were removed for histological analyzes. Sections of 4 μm were cut and labeled with hematoxylin and eosin (H&E). Histological observations of germ cells, at the level of light microscopy, allowed the characterization of six steps of oocyte maturation. Valle di Comacchio is the one with the highest levels of oocyte maturation while Valle Ca Pasta has the lowest. What needs to be stressed is that eels with silver index III have an oocyte maturation nearly equal to eels at stage IV and V of silvering. Considering the results, we can affirm that eels from North Adriatic lagoons are eels with high levels of oocyte maturation accompanied by high indices of GSI values at silver stages III. The following experimental work showed that the levels of oocyte maturation are higher even at lower silver index levels. It can be hypothesized that in the case of North Adriatic eels the transition from yellow to silver eel occurs faster in relation to the high trophic availability in North Adriatic lagoons.