Gonadal Development in European Eel Populations of North Adriatic Lagoons at Different Silvering Stages

The European eel (Anguilla anguilla, Linnaeus 1758) is a catadromous fish with significant cultural, scientific, and commercial value. The protection of this species is particularly difficult because the biology of the eel life cycle remains unknown in many aspects. 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 of this species. This work aims to perform a histological analysis on gonad samples from European eels coming from four different lagoons of the North Adriatic at different stages of silvering, and to evaluate the maturation of the gonads. For this study, a total of 859 wild eels were captured from four different lagoons using the “lavoriero”. Subsequently, the biometric parameters were collected. Seventy-nine female eels were randomly selected, dissected, and the gonads were removed for histological analysis. Sections of 4 µm were cut and stained with hematoxylin and eosin (H&E). Histological observations of germ cells at the light microscopy level allowed for the characterization of six steps of oocyte maturation. Valle di Comacchio had the highest levels of oocyte maturation, while Valle Ca’ Pasta had the lowest. Eels with silver index III had an oocyte maturation nearly equal to that of eels at stages IV and V of silvering. Considering the results, we can affirm that eels from North Adriatic lagoons have high oocyte maturation levels and high GSI value indices at silvering stage III. The following experimental work shows that the levels of oocyte maturation are higher even at lower silver index levels. It can be hypothesized that in these eels, the transition from yellow to silver eel occurs faster in relation to the high trophic availability in North Adriatic lagoons.