Biology Reference
In-Depth Information
7.3.2 Internal factors linked to fat
7.3.2.1
Initiation of silvering/Age at silvering
7.3.2.1.1 Fat stores
In general, a minimum, critical amount of fat is needed, as a trigger for
metamorphosis or sexual maturation, in order to withstand the metabolic
needs due to metamorphosis and after migration and reproduction. In the
eel, there is a wide variability in the age at silvering (Rossi and Colombo,
1976; Vollestad and Jonsson, 1986; Poole and Reynolds, 1996; Svedäng et
al., 1996). In 2006, Durif et al showed, in a study collecting European eels
from 5 different locations in France, that age ranged from 5 to 24 years.
The idea of a 'critical fat mass' is discussed in Larsson et al. (1990), in
which the authors examined the percentage of lipid in “muscular tissue”
(15-20 g tail, including skin) in yellow and silver eels (all females). They
found that a fat % of 28 (in the tail) may be a prerequisite for the change
from yellow to silver stage and under a critical fat mass, silvering may
not even be initiated (Larsson et al., 1990). However, a number of large
yellow eels had fat above 28 and a number of silver eels had fat below
28, indicating that a certain fat % is not the 'triggering factor', but may be
necessary and need to be associated with other triggering factors (internal
and external). Svedäng and Wickström (1997) suggested that silver eels
with low fat concentrations may temporarily halt migration, revert to a
feeding stage, and “bulk up” until fat reserves are suffi cient to carry out
successful migration to the spawning area. Combining both Boëtius and
Boëtius (1985) and Larsson et al. (1990) studies, Larsen and Dufour (1993)
found that silver eels (regardless of sex) rarely have a fat % lower than 20.
Careful analysis of the energy budget of migratory eels have been made
(Boëtius and Boëtius, 1980, 1985; van Ginneken and van den Thillart, 2000;
van den Thillart et al., 2004) and showed that silver eels contain energy
reserves and organic material suffi cient for both the long migration and
the extensive gonadal growth. By artifi cially inducing sexual maturation
with CPE injections and monitoring individual characteristics, Durif et al.
(2006) demonstrated that the initial state of the eel in terms of energy stores
partly explained the variability of subsequent sexual maturation. Indeed,
the best response to CPE treatment came from eels with the highest initial
condition factor and largest initial body diameter.
7.3.2.1.2 Growth
Comparing maturing European eel from 38 different locations throughout
Europe and South Africa, Vollestad (1992) reported geographic variation
in age at metamorphosis, which could be explained by variation in growth
