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.