Biology Reference
In-Depth Information
of a peptide antagonist of CRH (Clements et al., 2002). It also increased the
ability of juvenile salmonids to fi nd cover in a novel environment (Clements
et al., 2002). In a simulated stream environment, central administration of
CRH was able to increase the proportion of juvenile chinook salmon that
were distributed downstream of a mid-stream release site (Clements and
Schreck, 2004). This alteration of downstream movement was similar to
that of wild juvenile salmonids that were stressed (central administration
of CRH) during their downstream migration (Clements and Schreck, 2004).
Recently, icv injections of coho salmon held in circular-shaped channel tanks,
with CRH showed that CRH stimulated both downstream movement and
plasma T4 level (Ojima and Iwata, 2010). This last result is in agreement
with the fact that ovine CRH was able to stimulate TSH release from coho
salmon pituitary
in vitro
(Larsen et al., 1998) and that CRH-immunoreactive
fi bers terminate in pituitary regions containing TSH immunopositive cells
in Chinook salmon (Matz and Hofeldt, 1999).
Similarly, an ip injection of cortisol stimulated downstream movement
in masu salmon held in artifi cial raceways during the migratory period
(Munakata et al., 2007). Furthermore, Ojima et al. (2007) reported that
cortisol concentrations were higher in downstream migrating chum salmon
fry than in pre-migrating fry (Ojima et al., 2007).
6.3.4 Gonadotropic axis
6.3.4.1 Hormonal changes
The gonadotropic axis does not seem to be involved in the onset of
smoltifi cation. Indeed, in coho salmon, no changes in plasma levels of E2 in
males and females, or in levels of 11-KT in males, were evident during spring
when plasma T4 and cortisol were markedly elevated, indicating that the
fi sh were undergoing smoltifi cation (Patino and Schreck, 1986). Similarly,
plasma sex steroids remain relatively constant throughout smoltifi cation
(chum salmon: Parhar and Iwata, 1996).
However, two other studies reported peaks of E2 and T at the same
time as T4 (coho salmon: Sower et al., 1992; masu salmon: Yamada et al.,
1993).
6.3.4.2 Experimental induction
Early sexual maturation and sex steroid administration were able to inhibit
smolt development and downstream migration (masu salmon: Ikuta et
al., 1987; Munakata et al., 2001; Atlantic salmon: Madsen et al., 2004). Sex
steroids inhibit smoltifi cation (Aida et al., 1984; Ikuta et al., 1985, 1987;
Miwa and Inui, 1986).
