Biology Reference
In-Depth Information
(Egginton, 1986). Silver eels have higher aerobic activities than yellow eels.
The mass-specifi c power output of slow muscle is greater at silver stage
than at yellow stage (Ellerby et al., 2001).
7.2.3.3 Migratory behaviour
Downstream/seaward migration is the last step/stage of eel silvering. It is
a process sensitive to environmental factors such as fl ood/water discharge,
water temperature, tubidity, photoperiod, light intensity and moon phase
and/or atmospheric depression (Vollestad et al., 1986; Okamura et al., 2002a
and b; Durif et al., 2008). At silvering, eels changed from sedentary fi shes
to migrant fi shes, i.e., migration from brakish waters, rivers and streams to
the sea. Silver eel migration behaviour is characterized by regular diurnal
activity, active swimming very close to the surface at night and occasional
dives of short duration (Westerberg et al., 2007). Using miniaturized pop-
up staellite archival transmitter, Aarestrup et al. (2009) were able to follow
silver European eels during their spawning migration up to 1,300 km from
release and thus to provide unique behavioral insights. They showed that
when eels moved into the mesopelagic zone they all undertook distinct
vertical migrations (DVM) between 200 and 1,000m. During the night, they
occupied shallow warm water; at dawn, they made a steep dive into the
cool dsphotic zone and at night they ascended steeply back into the upper
layer (Aarestrup et al., 2009). The authors hypothesize that DVM refl ects
thermoregulation in order to keep temperature below 11°C, delaying
gonadal development.
7.2.4 Changes linked to preparation to reproduction (Changes in
gonads)
The gonadosomatic index (GSI) increases progressively in female yellow eels
from 0.3 to ≥1.5 in silver eels with increase of follicular diameter, thickening
of follicular wall and appearance of many lipidic vesicles (Fontaine et al.,
1976; Lopez and Fontaine, 1990). This increase in gonad size was shown
to be a good criterion to estimate the state of advancement of the silvering
process in the different eels (Marchelidon et al., 1999; Durif et al., 2005). Durif
et al. (2005) described fi ve stages with physiological and morphological
validation. In this study, a growth phase (stages I and II), a pre-migrating
stage (stage III) and a migrating phase (stages IV and V) were defi ned.
Stages I and II correspond to the previous “yellow” stage with a GSI<0.4%;
the gonads show small primary, non-vitellogenic oocytes, with a dense
