Biology Reference
In-Depth Information
rate as growth rate varied with both latitude and longitude. Silvering seems
to be triggered by a period of high growth. Indeed, an important increase
in GH was observed in pre-migrant European eels (stage III = beginning of
silvering) (Durif et al., 2005). Using primary cultures of eel pituitary cells,
it was shown that insulin-like growth factor IGF-1 (produced by the liver
under GH stimulatory control) was able to inhibit GH (Rousseau et al., 1998)
and stimulate LH release (Huang et al., 1998), leading to the hypothesis that
this growth factor could be the link between body growth and induction
of puberty in the eel. The high GH levels observed during spring in the eel
could stimulate liver secretion of IGF-1, which could trigger the initial LH
synthesis and decrease in GH. In addition, various studies showed that
favourable growth conditions cause eels to silver rapidly (Vollestad, 1988,
1992; De Leo and Gatto, 1995), such as is the case in aquaculture, under
experimental conditions (Tesch, 1991; Fernandez-Delgado et al., 1989) or in
brackish water and at low latitudes (Lee, 1979; Fernandez-Delgado et al.,
1989), involving environmental factors as triggerers of silvering.
7.3.2.2 Swimming
Lipid mobilization and initiation of maturation that occur during silvering
are linked to migration, as showed by the positive correlation between
intermediate stages and migration (Durif et al., 2005) and the negative
correlation between migration distance to the spawning rounds and GSI
at the start of oceanic migration (Todd, 1981b).
The swimming physiology of European silver eels and the effects on
sexual maturation and reproduction has been recently reviewed (Palstra
and van den Thillart, 2010).
Swimming induced an increase of eye diameter in different swim trials
(Palstra et al., 2006, 2007, 2008a). However, changes in the length and shape
of the pectoral fi ns, which are indicative of the degree of silvering, were not
reported in any of the swim-trials (Palstra et al., 2007).
Swimming induced changes in gonad histology of both females and
males (Palstra et al., 2007, 2008b). In contrast to resting eels, swimming
female eels from Lake Balaton had higher GSI and oocyte diameter and
some of their oocytes were in the lipid droplet stage 3 (Palstra et al., 2007).
Swimming male eels had also a higher GSI, refl ecting thick layers of
connective tissue and more stage 2 and late type b spermatogonia compared
to resting males (Palstra et al., 2008b).
Swimming induced changes in pituitary and blood plasma/expression
of maturation parameters. After swimming 5,500-km (173 days) in
freshwater, young farmed female eels showed increased plasma levels of
estradiol and 11-ketotestosterone, pituitary levels of LH (Van Ginneken et
al., 2007a). In this study, no signifi cant differences were measured in eye
