*Mean age of females became significantly higher only in the 2010s, R²=0.03, F(2, 712) = 10.23, p<0.0001; t-tests 1980s vs. 1990s: p=0.34 and 1980s vs. 2010s: p<0.0001.

**Mean length of females increased significantly between each decade, R²=0.09, F(2, 744) = 35.73; p<0.0001); t-tests 1980s vs. 1990s and 1980s vs 2010s: p<0.0001.

Figure 1 . Map of the study area showing the location of the river Imsa and of the trap capturing out migrating silver eels (Anguilla anguilla ).

Figure 2 : European eel otoliths after grinding, polishing, etching and staining. Annual rings are numbered on the pictures. Each otolith was assigned an Otolith Uncertainty Index (OUI) which corresponds to differences in readings between observers, level 1: less than 3 years, level 2: 3 to 5 years, level 3: more than 5 years. 2A: 15 years, OUI 1, body length: 67 cm, (In toto estimate: 9 years). 2B : 18 years, OUI 2, body length: 69 cm, (In toto estimate: 6 years). 2C: 20 years, OUI 3, body length: 67 cm, (In toto estimate: 9 years).

Figure 3 : Differences in age estimates of eel (Anguilla anguilla ) between two methods either by reading the otoliths in toto (lower end of the bar) or grinding and polishing (top end of the bar).

Figure 4 : Age distribution of European eels from the river Imsa (Norway) based on otoliths read whole (“in toto” method: IT; grey bars) or grinded and polished (GP; white bars). The IT and GP age distributions were significantly different from each other (Kolmogorov-Smirnov, D=0.925, p<0.0001).

Figure 5 : Age distribution of European eel (undifferentiated: yellow; males: blue, females: red) caught during their downstream migration in the river Imsa between 1982 and 2016. Mean (equal to median, shown as arrows) ages of females was 19 y in the 1980s and 1990s, and 21 y in the 2010s.

Figure 6 : Annual number of European eels recorded in the traps in the river Imsa, 1975-2018. (A) upstream migrating recruits (mostly young of the year elvers) in the spring; the inset zooms on the period 1992-2019 and (B) downstream migrating silver eels in the fall.

Figure 7 : Length distribution of out-migrating European eel (undifferentiated: yellow; males: blue, females: red) caught during their downstream migration in the river Imsa between 1982 and 2016. Mean length of females (closed arrow) was 60 cm in the 1980s, 63 cm in the 1990s and 68 cm in the 2010s. Median lengths (open arrows) were 61 in the 1980s and 1990s and 69 in the 2010s.