Biology Reference
In-Depth Information
In masu salmon, methyltestosterone treatment was able to induce
desmoltification (Yamazaki et al., 1973). Moreover, administration of
methyltestosterone in early spring to yearling masu salmon parr inhibited
natural smoltifi cation; fi sh resembled parr or dark parr and seawater
tolerance did not develop (Ikuta et al., 1985).
Aida et al. (1984) demonstrated that precocious gonadal maturation
of masu salmon inhibited smoltifi cation in the following spring since
smoltifi cation was induced in the completely castrated precocious male
fi sh, but not in the fi sh, which had a small piece of remaining testis.
In masu salmon, sex steroids were shown to inhibit springtime
smoltifi cation (Ikuta et al., 1987). Indeed, administration of E2, T or 11-KT
to masu salmon from February to May inhibited smoltifi cation, but had
no effect from September to December. Moreover, T and 11-KT were able
to inhibit smoltifi cation of masu salmon reared under artifi cially increased
daylength.
Using artifi cial raceways and masu salmon with implants containing
sex steroids, Munakata and collaborators (2001) showed that dowstream
migration of masu salmon smolts was inhibited signifi cantly by T, E2 and
11-KT. Using very few fi sh in artifi cial stream channel, Munakata et al. (2001)
found evidence that E2 (and androgens) inhibited downstream movement
in smolts after 2 weeks of treatment. An inhibitory effect of downstream
smolt migration by androgens was also reported in Atlantic salmon by
Berglund et al. (1994).
Sex steroids may inhibit the normal body silvering of the smolting
salmon (Madsen and Korsgaard, 1989), inhibit the smolt-associated increase
in gill Na+,K+-ATPase activity (Madsen et al., 1997, 2004), negatively affect
smolt growth rate (Arsenault et al., 2004) and impair the development of
SW-tolerance during smolting (McCormick et al., 2005).
Ikuta et al. (1985, 1987) in masu salmon showed that treatment with
sex steroids usually lowered plasma TH levels. These data suggest that sex
steroids may inhibit smoltifi cation by modulating thyrotropic axis.
6.4 Environmental/external Factors Involved in the Control of
Smoltifi cation
Among the external factors implicated, temperature and photoperiod are
reported as the major actors in the initiation and the control of downstream
migration (Boeuf, 1993; McCormick et al., 1998; Björnsson et al., 2011).
Photoperiod is very active on the control of growth in fi sh (Boeuf and Le
Bail, 1999) and parr-smolt transformation and largely used by farmers to
stimulate it in using treatments consisting in increasing day length.
