Results

Age estimates

Otoliths from 41 eels (5%) were unreadable and were excluded from the analyses and the following percentages (Table 1). Most otoliths were assigned an OUI (Otolith Uncertainty Index) level 2, indicating a 4 to 5 years uncertainty (47%). OUI level 1 otoliths (1 to 3-year uncertainty) represented 14%, and OUI level 3 (over 5-year uncertainty) represented 39% of the otoliths analysed. Age varied with OUI (R2=0.04, F(2, 753)=17.8, p<0.0001): uncertainty increased with age. Length did not vary between OUI levels (R2=0.006, F(2, 750)=2.2, p=0.11). The proportion of OUI level 3 otoliths was different across decades (Χ2=45.729, df=4; p<0.0001): 51% in the 1980s, 28% in the 1990s and 34% in the 2010s.
Estimated ages from the IT method were always lower than from the GP method (Figure 3). Differences varied between 0 and 29 y with a mean and a median equal to 11 y. The correlation between age estimates was significant (R2= 0.05, F(1, 586)=30.8, p<0.0001), but still too low to infer one estimate from the other (Figure 3). The new and old age distributions were different from each other (Figure 4).

Comparison of new age estimates and length at silvering over the years

Out of the 798 eels, 751 were females, 43 males, and 4 undifferentiated. Ninety-three percent were at the silver stage, the remaining were either yellow or intermediate.
Mean age at silvering (all years) was 19 y for females and 15 y for males (Table 1). During the last decade (2010s), mean age of females significantly increased compared to the 1980s and 90s (Table 1). This is due to the disappearance of young silver eels (<15 y) during the 2010s (Figure 5). This corresponds to the lowest recruitment level which was reached in the 2000s (Figure 6), thus 12 to 15 years before the 2010s sampling.
Mean body length increased significantly from the 1980s to the 2010s (Table 1, Figure 7). Female length increased by 8 cm over the 30-year period (Table 1). When male silver eels were still caught in the trap, these migrated at a length of around 40 cm. In the 2010s, male eels disappeared, as did the contingent of smaller female silver eels (around 50 cm). We found no differences in length-weight relationship between decades (F(2, 241)=1.37, p=0.26).

Growth

Growth estimated based on the GP method were highly variable, and age of female eels was only slightly related to their body length (R2=0.08). Mean growth calculated over the entire freshwater stage of the eels by using the new age estimates (GP method) was 30 mm y-1 in females (min-max: 16 - 64 mm y-1) and 24 mm y-1 in males (min-max: 15 - 37 mm y-1). Growth calculated with the old estimates (IT method) was 77 mm y-1 for females and 72 mm y-1 for males.

Linking annual numbers of recruits and silver eels

Both the number of ascending recruits (elvers) and descending silver eels have changed substantially during the period from 1975 to 2017 (Figure 3). The number of ascending recruits demonstrated large annual variation during the period 1975-1990, with a minimum of 2400 and a maximum of 46 200 eels, and a mean number of 19 000 (± SD 15 200). From 1991 to 2017, the annual numbers of ascending recruits declined to a much lower level, varying between 5 and 8 878 eels, with a mean of 2 555 (± SD 2 512). The number of descending silver eels changed abruptly in 1988, from a mean of 5 854 (± SD 1 180) eels during 1975-1988, to 2 183 (± SD 785) during 1989-2017.
The age distribution of the female silver eels migrating to the sea each year included up to 31 age classes (from 8 years to 39 years old). Thus, each year’s silver eel run represented more than 20 age classes of recruits. We attempted to fit a Recruit-Stock analysis model by assigning descending silver eels to recruit cohorts according to mean decadal age distributions but found no significant relationship.