Biology Reference
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with TH receptors during smolting, suggesting that olfactory tissues may
be particularly sensitive to effects of TH at this time (Kudo et al., 1994).
The timing of olfactory imprinting is associated with elevations in
plasma thyroid hormone levels (Hasler and Scholz, 1983; Morin et al., 1989a,
1994) and over the course of the parr-smolt transformation, changes in the
density of cells in proliferation showed a positive relationship with natural
fl uctuations in plasma T4 (Lema and Nevitt, 2004). The olfactory epithelium
may thus be better able to mirror subtle changes in plasma T4 triggered by
changes in the fi sh's immediate environment. Whether this process extends
to other parts of the brain potentially involved in imprinting (e.g., olfactory
bulb and telencephalon) will be an interesting area for further research. T4
treatment depressed olfactory bulb responses to L-alanine (Morin et al.,
1995) and lowered dopaminergic and serotonergic activity in the olfactory
system (Morin et al., 1997).
6.3.1.2.5 Osmoregulation
Thyroid hormones do not seem to have a major role in osmoregulation
and seawater adaptation in salmonids. Even if
such action is feasible (for
review: Fontaine, 1975; Donaldson et al., 1979; Higgs et al., 1982; Dickhoff
et al., 1982; Refstie, 1982; Sullivan et al., 1987), many studies did not
demonstrate improved SW adaptability following treatment with thyroid
hormones (Miwa and Inui, 1983, 1985; Ikuta et al., 1985; Saunders et al.,
1985; Omeljaniuk and Eales, 1986; Iwata et al., 1987; Sullivan et al., 1987;
Madsen, 1990a; Boeuf et al., 1994).
6.3.1.2.6 Migratory behaviour/downstream migration
TH treatment decreased the aggressive behaviour of pre-smolts (Pacifi c and
Atlantic salmon: Hoar, 1951; Godin et al., 1974; masu salmon: Iwata, 1995).
Before the migratory period, chum salmon fry and coho salmon yearlings
preferred shaded areas more than open areas. When immersed in T3 for 4
days, 80-90% preferred open water to shaded areas (the effect remaining
for a week after treatment), while the controls and more than 78% treated
with thiourea preferred to be in shade or to be in shelter (Iwata et al., 1989).
This suggests that TH caused the fi sh to move to open water in daytime
where it may be advantageous for the fi sh to form schools and migrate
seaward (Iwata, 1995). During the pre-migratory season, chum salmon fry
swam against the fl ow of current. T3 treatment changed their swimming
direction to downstream (with the fl ow of current).
The involvement of TH in stimulation of downstream migratory
behaviour has been well-documented (Baggerman, 1963; Godin et al.,
1974; Fontaine, 1975; Youngson et al., 1985; Iwata et al., 1989; Boeuf and Le
