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before reaching the ocean (Duston et al., 1990) and thus impair the chance
of post-smolt survival in the ocean.
All these data show that disruption of sex hormones is subject of intensive
research in vertebrates, but thyroid hormone synthesis and signalling are
now also recognized as important targets of endocrine disruptors such as
PCBs, perchlorates and brominated fl ame-retardants (for review: Brucker-
Davis, 1998; Jugan et al., 2010; Zoeller, 2010). Thyroid-disrupting chemicals
can target the thyrotropic axis at various levels, including synthesis of TH
by thyroid gland and its regulation by hypothalamic-pituitary hormones,
the catabolism and clearance of circulating TH by the liver and kidneys, the
binding to transport proteins in the circulation, the cellular uptake of TH,
the peripheral activating/inactivating metabolism of TH by iodothyronine
deiodinases, the transcriptional activity of TH receptors and the expression
of TH-regulated genes (for review: Brucker-Davis, 1998; Jugan et al., 2010;
Zoeller, 2010). Even if data are available on thyroid-disrupting chemicals
in teleosts (for review: Brown et al., 2004; Carr and Patino, 2011), studies
in salmonids should aim at investigating their effects and mechanisms of
action during smoltifi cation. A pellet containing glucosinolate rapeseed
used as diet in the rainbow trout triggers antithyroid and anti-growth
effects (Burel et al., 2000, 2001).
Many studies showed that xenoestrogens likely exert some of their
effects on smoltifi cation indirectly by disturbing general hormonal processes
such as the thyrotropic and somatotropic axes. Sex steroids depressed
plasma thyroid hormone levels (Ikuta et al., 1987; Munakata et al., 2001),
which are involved in the stimulation of downstream migration (Iwata,
1995; Munakata et al., 2000, 2001). Plasma TH levels were clearly depressed
by 4-NP and E2 (McCormick et al., 2005). E2 has been shown to modulate
somatotroph function and/or GH dynamics in amago salmon (Miwa an
Inui, 1986) and rainbow trout (Holloway and Leatherland, 1997). During
the early temporal window within the fi nal stages of smoltifi cation, short-
term water-borne E2 and 4-NP exposures resulted in depressed growth and
plasma IGF1 concentrations (Arsenault et al., 2004). McCormick et al. (2005),
also reported decreased circulating IGF1, and as no changes in plasma GH
levels were observed, the authors concluded that the effects of E2 and 4-NP
were not through their impact at the pituitary level, but rather at the hepatic
level. Similarly, treatment of juvenile Atlantic salmon with 4-NP and E2 did
not affect pituitary GH mRNA levels (Yadetie and Male, 2002).
6.5.2 Anthropic disruptors of migration
During downstream migration, the timing between arrival in various
environments (freshwater, estuary and finally seawater) and smolt
physiological status is crucial for survival. For example, the period during
