Material and Methods
Study location
The river Imsa in southwestern Norway (Figure 1) is an unregulated
oligotrophic system. The catchment covers an area of 12800 ha, of which
1536 ha (12%) is lake surface (major lakes are Imsvatnet, 40 ha, and
Storavatnet, 819 ha). A trap catching descending silver eels as well as
ascending juveniles is located about 100 m from the river outlet in the
sea. The distance from the trap, which has been in operation since 1975,
to the upper end of eel habitat in the catchment is set at 20 km, and
the eels ascend the system up to an altitude of approximately 215 m
above sea level (Vøllestad & Jonsson 1988). Descending, predominantly
silver, eels are caught in a Wolf trap (apertures 10 mm, inclination
1:10). The juveniles entering this watershed are small yellow eels
(elvers or recruits) that are typically 70-90 mm long and weighing less
than 1.0 gram, although a few individuals may be larger. The distance
from the nearest lake along the free-flowing river to the fish trap is
970 m. There has been no stocking of eels in this watershed. Before
2006, there was a restricted seasonal yellow and silver eel fishery
upstream of the trapping station. The number of ascending recruits and
descending silver eels is given per calendar year.
Wolf traps with aperture 10 mm generally catch all eels larger than
approximately 25 cm in length, which includes all silver eels in the
river Imsa (Vøllestad & Jonsson, 1986). The elver trap leads all
ascending recruits into a capture chamber where their numbers are
recorded, and sub-sampling of size is performed, before they are
released to continue upstream. The traps are checked twice every day (at
circa 08:00 and 15:00 h).
Otolith data and age
determination
Otoliths from the historical collection (collected in the 1980s and 90s)
were initially read in toto by clearing them in 96% ethanol for
18-24 h before counting the annuli directly using a stereo microscope
and 96% ethanol as refraction medium (Vøllestad, 1985). These otoliths
were, since then, stored dry in an envelope, each labeled with length,
sex and stage (yellow or silver). For the reanalysis, a subsample was
selected from years with the highest sampling effort (1982, 1986, 1991
and 1992).
The more recent otoliths (from the 2010s) were sampled from eels caught
during their downstream run in the river Imsa. Twenty-five silver eels
were sacrificed per year (61 eels in 2016). Length, weight, fin and eye
diameters were measured for stage determination (Durif, Dufour, & Elie,
2005; Durif, Guibert, & Elie 2009). The eels were dissected for sex
determination and removal of otoliths. Otoliths from the 2010s were not
analyzed using the old in toto method.
A total of 798 fish were processed. All otoliths (historical and new)
were prepared by grinding, etching and staining and read according to
the protocol described in ICES (2009, 2011) (Figure 2). The year 0 band
was assigned as the first winter after the oceanic migration,i.e . it marked the beginning of the continental life stage. The
last year was defined as the outer edge of the otolith since eels were
caught during the fall season. Some otoliths had clear and regularly
spaced annuli (Figure 2A). Others presented numerous tight rings,
unevenly spaced, which sometimes joined in a “bundle” or fused into
one large annulus on the other side of the otolith (Figure 2B, 2C).
Whether these bundles represent one or several years is unknown. Here,
we assumed that some of the marks forming a bundle represented false
checks and thus one bundle represented one year. Otoliths were read by 2
or 3 observers, or for some samples by the same observer twice, but the
second time several months after the first reading. As expected, some
otoliths were easier to interpret than others, and the age estimates did
not always agree between observers. To characterize the uncertainty in
the readings, we assigned each age estimate with an Otolith Uncertainty
Index (OUI) corresponding to how much the observations differed between
observers/observations:
- OUI level 1: differences less than 3 years
- OUI level 2: differences between 3 to 5 years
- OUI level 3: differences of more than 5 years.
Calculations and
statistics
Indicative growth rate of each individual was calculated by dividing the
body length LT (mm) of the eel minus 70 mm, which is the mean size of
glass eels when they recruit to European coasts (Elie, 1979; Svedäng et
al., 1996), by the continental age.
Differences in mean age and mean length between decades and between OUI
levels were tested using linear regression models. Differences in
proportion of OUI levels between decades were tested using a Chi-square
test. Differences between length-weight relationships were tested with
ANCOVA after the variables were log-transformed to investigate changes
in condition over the decades. Statistics were carried out using the
statistical software R (R Core Team, 2019, v. 3.6.0).