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which smolt is able to adapt to seawater is limited to several weeks
(“physiological smolt window”) (Boeuf et al., 1985; Berglund et al., 1992;
Boeuf, 1993; McCormick et al., 1998). Therefore, any disturbance during the
phase of downstream movement in freshwater resulting in a late estuarine
arrival could be lethal due, fi rstly, to the loss of physiological capacities
to seawater adaptation (“physiological smolt window”) and secondly, to
unfavourable local environmental
conditions (“ecological smolt window”)
such as
high temperature, low dissolved oxygen or presence of pollutants
(McCormick et al., 1998; Hvidsten et al., 1998, Antonsson and Gudjonsson,
2002; Jutila et al., 2003). This problem is much more signifi cant in the case
of very long rivers where climatic and anthropic conditions (temperature,
fl ow, pollution and dams) can considerably infl uence and delay downstream
migration (McCormick et al., 2003).
Disruption to migratory pathways of both juveniles and adults is
contributing o the decline of salmonid populations (Raymond, 1988; Rivinoja
et al., 2001; Wilson, 2003). For instance, in the Pacifi c Northwest (USA), a
large number of man made obstacles are in place on rivers, including
hydroelectric and irrigation dams and passage through such structures
can induce a severe stress response in juvenile salmonids (Congleton et al.,
2000) and delay fi sh arrival at the ocean.
Conclusions
All these data clearly show the involvement of the thyrotropic axis
in smoltifi cation (Fig. 4). Such a major stimulatory control by thyroid
hormones has been well described for amphibian metamorphosis (Tata,
2006) and teleost fi rst/larval metamorphosis. Nevertheless, two other
axes, the somatotropic axis (with GH and IGF1) and the corticotropic
axis (with cortisol) are also necessary for the control of some aspects of
smoltifi cation (Fig. 4). GH is very important as the hormone responsible for
osmoregulation and SW adaptability. At the hypothalamic level, CRH may
represent a coordinator between thyrotropic and somatotropic axes (Fig.
4), as it is able to stimulate in fi sh both TSH (salmon: Larsen et al., 1998)
and GH (eel: Rousseau et al., 1999)
in vitro
. In addition, strong interactions
exist between these different neuroendocrine axes. Indeed, cortisol seems to
have a synergizing action with GH and TH in the control of osmoregulation
and metabolism.
In contrast, the gonadotropic axis is likely inhibitory on
smoltifi cation.
Data show that the various aspects of smolting process are likely
to be under the control of different environmental cues. For example,
physiological changes appear to precede the switch to migratory behavior,
and it has been suggested that changes in behavior require environmental
(e.g., photoperiod) and endocrine 'priming' factors that may be similar to
