Biology Reference
In-Depth Information
Protein catabolism is accelerated (Malikova, 1959).
Depletion of protein
reservoirs was observed, both as depleted whole-body protein (Fessler
and Wagner, 1969) and depletion of protein in muscles and liver (see in
Nordgarden et al., 2002). These results constrast with Woo et al. (1978) who
reported equal muscle and liver protein levels in smolt and parr, and with
Sweeting and McKeown (1989) who reported stable muscle protein levels
throughout the smoltifi cation process.
6.2.6 Migratory behaviour
Timing of downstream migration varies among species. Smolts usually start
their downstream migration at the beginning of the spring for the individuals
from South latitudes (for example, France and Spain: Boeuf, 1993; Utrilla
and Lobon-Cervia, 1999), and in August for the individuals from the highest
latitudes (for example, Iceland: Antonsson and Gudjonsson, 2002).
The salmon, which at the parr stage is sedentary, territorial and presents
an aggressive behaviour, becomes a gregarious and migratory fi sh as a
smolt (Jonsson and Jonsson, 1993; Thorpe, 1994). Behavioural changes
include a decrease in territoriality and aggression and the subsequent
formation of aggregations for the downstream migration (Iwata, 1995).
There is a gradual decrease in aggressive behaviour as smolting progresses
and reach minimum levels at the peak of smolting. Fish become aggressive
again after the peak of smolting. The reduction of aggressive behaviour is
a key factor in downstream migration, as it allows fi sh encounters to lead
to schooling formations.
Downstream movement takes place preferentially in groups from the
same parents and is stimulated by smolts coming from upstream areas
(Hansen and Jonsson, 1985; Hvidsten et al., 1995; Olsen et al., 2004). A recent
study indicated that solitary movements may initiate the downstream
migration, followed by the schooling behaviour (Riley, 2007).
Downstream migration in freshwater is believed, by some authors, to
be a passive displacement with the current (Huntsman, 1939; McCleave,
1978; Tytler et al., 1978; Thorpe et al., 1981). However, observations of smolt
swimming actively have been reported (Kallenberg, 1958; Solomon, 1978;
Hansen and Jonsson, 1985; Fängstam, 1994), namely with the smolts actively
seeking areas of high velocity (Jonsson et al., 1991).
Generally, the downstream migration of salmonids occurs during the
night (Neave, 1955; Aarestrup et al., 2002; Riley et al., 2002; Carlsen et al.,
2004). The chum salmon fry forms school during daytime; after sunset, the
schools break up, and the fry move to the water surface possibly because
of poor vision in darkness and are carried downstream by the surface fl ow
(Hoar, 1951).
