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McCormick et al. (2007) demonstrated that smolt GH/IGF1 and cortisol axes
had greater capacity to respond to increased daylength than parr ones.
6.4.2 Temperature
Temperature is less important than photoperiod in regulating physiological
smolt development, but still has substantial impact (Boeuf, 1993; McCormick
et al., 2000, 2002). According to Björnsson et al. (2011), temperature is
likely to control ecological conditions that determine smolt survival
during downstream migration and early ocean entry (an “ecological smolt
window”). In at least some species of anadromous salmonids, temperature
regulates the length of time during which the smolts are capable of good
survival after ocean entry, with elevated temperatures narrowing this
“physiological smolt window” (Björnsson et al., 2011).
ATPase activity was decreased and migration reduced, when steelhead
trouts were subjected to temperatures of about 13°C or greater (Zaugg and
Wagner, 1973). In Atlantic salmon smolts, an early increase in temperature
from 5 to 12°C has been shown to advance the development of seawater
tolerance compared with controls raised at ambient water temperature
(5-6°C) (Solbakken et al., 1994). Similarly, the development of hypo-
osmoregulatory ability and increase in gill Na+, K+-ATPase activity during
smolting were signifi cantly infl uenced by freshwater temperature, with
earlier development of seawater tolerance in salmon smolts at higher
temperatures: in smolts raised at 12.0°C, maximum gill Na+, K+-ATPase
activity was reached in late April, compared to late May and mid-June in the
8.9°C and ambient groups (Handeland et al., 2004). Staurnes et al. (1994) even
observed the development of typical smolt characters (marked silvering,
high seawater tolerance and high Na+, K+-ATPase activity) in groups of
Atlantic salmon reared at constant long day and seasonally changing water
temperature that increased during spring, suggesting temperature can act as
a cue for smolt development in the absence of an appropriate photoperiod
stimulus. McCormick et al. (2000) were the fi rst to demonstrate that low
temperature limits the ability of photoperiod to advance the physiological
aspects of smolting (condition factor, gill Na+, K+-ATPase). They clearly
showed that smolting of Atlantic salmon was advanced by increased
day length when fi sh were held at elevated temperatures (10°C) through
the winter, but not when fi sh were held at cooler, ambient temperatures
of 2°C.
Similar effects of temperature were reported for coho and chinook
salmon by Zaugg and McLain (1976) and Clarke et al. (1981). In contrast,
no effect of a rise in temperature from ambient to 10-11°C during spring
on smolt development was observed by Dickhoff et al. (1989) and Duston
and Saunders (1995) in these species.
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