## 3 Results
### Histological Analysis  
Histological observations of germ cells allowed for the characterization of six steps of oocyte maturation--five steps in primary growth and one in secondary growth (Figure 2):  
* One-nucleoli (PGon): characterized by only one nucleolus inside the germinal vesicle, the absence of lipid droplets, and a reduced oocyte diameter of about 32.5 \(\upmu\)m;
* Multiple nucleoli (PGmn): characterized by multiple nucleoli inside the germinal vesicle arranged randomly. There is also the presence of globular masses called Balbiani's bodies within the cytoplasm; \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Lagoon (Tot. Eels** & \multirow{2}{*}{**Parameter**} & **Resident** & **Pre-Migrant** & \multicolumn{2}{c}{**Migrant**} \\ \cline{3-6}
**Sampled)** & & **FII** & **FIII** & **FIV** & **FV** \\ \hline \multirow{6}{*}{**Valle DI OMACCHIO (320)**} & \% & 3.2 & 29.7 & 65.9 & 1.2 \\ \cline{2-6}  & K & 1.79 \(\pm\) 0.17 & 2.06 \(\pm\) 0.22 & 2.23 \(\pm\) 0.23 & 1.95 \(\pm\) 0.17 \\ \cline{2-6}  & GSI & \(n\). 32 & 0.71 \(\pm\) 0.17 & 1.65 \(\pm\) 0.23 & 1.70 \(\pm\) 0.16 & 1.74 \(\pm\) 0.22 \\ \cline{2-6}  & Body Weight (BW) & 404.3 \(\pm\) 145.7 & 996.45 \(\pm\) 119.36 & 1366.94 \(\pm\) 221.99 & 755.00 \(\pm\) 175.04 \\ \cline{2-6}  & Total Body Length (BL) & 601.1 \(\pm\) 52.7 & 786.12 \(\pm\) 43.12 & 848.67 \(\pm\) 50.56 & 726.25 \(\pm\) 57.17 \\ \hline \multirow{6}{*}{**Valle NUOVA (254)**} & \% & 5.5 & 73.2 & - & 21.3 \\ \cline{2-6}  & K & 1.75 \(\pm\) 0.22 & 1.81 \(\pm\) 0.15 & - & 1.83 \(\pm\) 0.23 \\ \cline{2-6}  & GSI & \(n\). 25 & 0.62 \(\pm\) 0.26 & 1.63 \(\pm\) 0.19 & - & 1.96 \(\pm\) 0.12 \\ \cline{2-6}  & Body Weight (BW) & 433.92 \(\pm\) 130.46 & 557.09 \(\pm\) 89.00 & - & 554.29 \(\pm\) 113.00 \\ \cline{2-6}  & Total Body Length (BL) & 620.00 \(\pm\) 62.52 & 673.61 \(\pm\) 36.20 & - & 670.87 \(\pm\) 46.26 \\ \hline \multirow{6}{*}{**Valle CAPAST (112)**} & \% & 1.8 & 76.8 & 7.1 & 14.3 \\ \cline{2-6}  & K & 1.52 \(\pm\) 0.07 & 1.94 \(\pm\) 0.17 & 2.18 \(\pm\) 0.14 & 1.92 \(\pm\) 0.18 \\ \cline{2-6}  & GSI & \(n\). 11 & 0.46 \(\pm\) 0.04 & 1.53 \(\pm\) 0.26 & 1.77 \(\pm\) 0.25 & 1.53 \(\pm\) 0.18 \\ \cline{2-6}  & Body Weight (BW) & 413.50 \(\pm\) 8.77 & 747.43 \(\pm\) 128.71 & 1020.48 \(\pm\) 146.79 & 737.50 \(\pm\) 90.23 \\ \cline{2-6}  & Total Body Length (BL) & 648.50 \(\pm\) 4.95 & 726.43 \(\pm\) 43.26 & 775.13 \(\pm\) 33.63 & 726.69 \(\pm\) 35.60 \\ \hline \multirow{6}{*}{**Valle SAN (168)**} & \% & 1.8 & 55.3 & 3.0 & 39.9 \\ \cline{2-6}  & K & 1.66 \(\pm\) 0.25 & 1.70 \(\pm\) 0.17 & 1.99 \(\pm\) 0.24 & 1.65 \(\pm\) 0.20 \\ \cline{2-6}  & GSI & \(n\). 17 & 0.70 \(\pm\) 0.10 & 1.46 \(\pm\) 0.33 & 2.08 \(\pm\) 0.39 & 1.57 \(\pm\) 0.13 \\ \cline{2-6}  & Body Weight (BW) & 492.37 \(\pm\) 57.53 & 616.80 \(\pm\) 118.34 & 1143.90 \(\pm\) 194.42 & 577.56 \(\pm\) 112.70 \\ \cline{2-6}  & Total Body Length (BL) & 668.67 \(\pm\) 34.00 & 711.19 \(\pm\) 41.61 & 831.80 \(\pm\) 52.77 & 702.85 \(\pm\) 39.52 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Morphometric parameters and silvering indices of the four eel populations. Data are reported as mean \(\pm\) s.d. Perinucleolar (PGpn): characterized by the presence of the first lipid droplets and the increase of the oocyte diameter to around 60 \(\upmu\)m;
* Circumnuclear oil droplet (PGod): characterized both by the position of nucleoli that is arranged peripherally and by the increase of lipid droplets around the germinal vesicle. The oocyte diameter is around 75 \(\upmu\)m;
* The cortical stage (PGca) is characterized by an increase in lipid droplets arranged around the germinal vesicle and the presence of cortical alveoli in the periphery. The oocyte diameter is around 80 \(\upmu\)m;
* Early secondary growth (SGe): this stage is characterized by the presence of yolk at the periphery of the oocyte. The oocyte diameter is around 95 \(\upmu\)m.  
Figure 2: **(A)** One-nucleoli stage. (**B**) Multiple nucleoli stage. (**C**) Perinucleolar stage. (**D**) Circumnuclear oil droplets (stage IV of pre-vitellogenesis).