Biology Reference
In-Depth Information
6.4.1 Photoperiod/melatonin
Photoperiod is the most important environmental factor modulating the
timing of smoltifi cation in Atlantic and coho salmon (McCormick, 1994).
6.4.1.1 Manipulation of photoperiod
ATPase activity was decreased and migration reduced, when the length
of increasing photoperiods approximated that of the summer solstice in
steelhead trout (Zaugg and Wagner, 1973). An artifi cial shift in photoperiod
from short to long days advances the onset of smoltifi cation, whereas short-
day exposure delays it (Saunders and Henderson, 1970). Smoltifi cation of
masu salmon (
O. masou
) can also be experimentally induced by artifi cial
manipulation of photoperiod from short to long days (Ikuta et al., 1987). In
addition, the larger the difference in day length after increasing photoperiod,
the earlier smolts appear (Okumoto et al., 1989). Atlantic salmon exposed
to continuous light into fall and winter showed a delayed parr-smolt
transformation (delayed increases of branchial Na+,K+-ATPase activity and
seawater tolerance: Saunders et al., 1985; McCormick et al., 1987; Saunders
and Harmon, 1990).
6.4.1.2 Melatonin treatment
The onset of smoltifi cation in pinealectomised Atlantic salmon was delayed
by 3 weeks and melatonin implants, which elevate circulating melatonin to
constant high levels, advanced the timing smoltifi cation in pinealectomised
Atlantic salmon (Porter et al., 1998). In contrast, in masu salmon, melatonin
feeding, which reproduces melatonin profi les under short photoperiod,
delays (but did not completely suppress) smoltifi cation induced by long-
day photoperiodic treatment (Iigo et al., 2005).
6.4.1.3 Possible mechanisms of action
Atlantic salmon juveniles exposed to abrupt increases in daylength (LD
15:9) in February or March exhibited earlier increase in plasma GH, with no
changes in plasma TH (McCormick et al., 1987, 1995, 2007). When exposed
to short daylength (LD 9:15) from January to May, fi sh had delayed increase
in plasma GH (McCormick et al., 1995). The spring rise in plasma GH levels
observed in Atlantic salmon in natural conditions or in simulated natural
photoperiod was absent in salmon exposed to continuous light (Björnsson
et al., 1995). Increasing daylength also increased plasma GH levels in masu
salmon, while not affecting plasma T4 and T3 levels (Okumoto et al., 1989).
