8. Lipids and Essential Fatty Acids in Aquatic Food Webs
189
spawn. Migration and spawning are highly energy-requiring activities, and it is
known that salmon hardly feed during migrations and that their lipid level when
caught in the rivers is considerably lower than that given in Figure 8.13 (3-4% of
fresh weight) (Olsen and Skjervold, 1991). The lipid level of adult salmon in the
sea is most likely higher than in most other species. The pattern of changes in
salmon lipids with age, including the reduction before spawning (Fig. 8.6), may be
general and valid also for freshwater fish, which store lipids in the flesh.
The absolute structural lipid content of salmon flesh, which mainly consists of
membrane PLs and other membrane constituents, is believed to be rather constant,
as was also shown for the rotifer (Fig. 8.9). Figure 8.13 suggests that the lower
threshold for total flesh lipids is in the range of 1-1.5% of fresh weight. This level
is of the same magnitude as the quantitative level of PLs found in salmon flesh,
some 1 % of fresh weight in adult individuals (0. Lie, personal communication).
This implies that the salmon juveniles transferred to seawater (see Fig. 8.13)
contained very low, although variable, contents of TAGs. Thus the energy stores
left after the energy-requiring processes of smoltification and adaptation to seawater were very low. The other implication, further elaborated below, is that the
fatty acid distribution in the salmon at the time of transfer to seawater reflected the
composition of the salmon membrane PLs. Tissue TAGs, however, were reflected
in older salmon in which the PLs constituted only a minor fraction of total lipids.
8.2.6.3.2. Fatty Acid Composition
When salmon are given the same food and dietary lipids throughout their life, as
much as 80% of the variation in their (03 fatty acid contents can be explained by
the variation in only two factors: the lipid content of the flesh and the body weight
of the fish, which relates to the age (Olsen and Skjervold, 1995). The effects of
light levels and seawater temperature on (03 fatty acid contents, as represented by
the latitude survey of salmon along the Norwegian coast, explained < 1 % of the
variation in (03 fatty acids. Although not further dealt with here, it should be noted
that salmon respond to changes in dietary lipid composition in a fashion similar to
that of B. plicatilis.
The pattern of variation found for the quantitative content of the dominant
EFAs (EPA and DHA) through the salmon life cycle are, as expected, similar to
the pattern of variation found for total lipids (Fig. 8.14A). DHA is most dominant
during the early life stages in fresh water and during the first part of the seawater
stage. The average quantitative contents of EPA and DHA at the time of slaughtering (weight >3 kg) are about 7 and 12 mg' g-l fresh weight, respectively. The
corresponding total (03 and (06 fatty acid contents are 22 and 5.5 mg . g-l fresh
weight, respectively, with linoleic acid as the dominant (06 fatty acid. Thus, DHA
and EPA presumably make up 80-90% of the (03 fatty acids in salmon flesh, and
(03 fatty acids dominate over m6 fatty acids by a factor of 4.
The percentage content of EPA and DHA in salmon flesh during the life cycle
clearly demonstrates the strong dominance of DHA through the early developmental stages (Fig. 8.14B). In fact, DHA makes up 20-50% oftota1 fatty acids all
through the juvenile stage in fresh water and through the first few months of
189
spawn. Migration and spawning are highly energy-requiring activities, and it is
known that salmon hardly feed during migrations and that their lipid level when
caught in the rivers is considerably lower than that given in Figure 8.13 (3-4% of
fresh weight) (Olsen and Skjervold, 1991). The lipid level of adult salmon in the
sea is most likely higher than in most other species. The pattern of changes in
salmon lipids with age, including the reduction before spawning (Fig. 8.6), may be
general and valid also for freshwater fish, which store lipids in the flesh.
The absolute structural lipid content of salmon flesh, which mainly consists of
membrane PLs and other membrane constituents, is believed to be rather constant,
as was also shown for the rotifer (Fig. 8.9). Figure 8.13 suggests that the lower
threshold for total flesh lipids is in the range of 1-1.5% of fresh weight. This level
is of the same magnitude as the quantitative level of PLs found in salmon flesh,
some 1 % of fresh weight in adult individuals (0. Lie, personal communication).
This implies that the salmon juveniles transferred to seawater (see Fig. 8.13)
contained very low, although variable, contents of TAGs. Thus the energy stores
left after the energy-requiring processes of smoltification and adaptation to seawater were very low. The other implication, further elaborated below, is that the
fatty acid distribution in the salmon at the time of transfer to seawater reflected the
composition of the salmon membrane PLs. Tissue TAGs, however, were reflected
in older salmon in which the PLs constituted only a minor fraction of total lipids.
8.2.6.3.2. Fatty Acid Composition
When salmon are given the same food and dietary lipids throughout their life, as
much as 80% of the variation in their (03 fatty acid contents can be explained by
the variation in only two factors: the lipid content of the flesh and the body weight
of the fish, which relates to the age (Olsen and Skjervold, 1995). The effects of
light levels and seawater temperature on (03 fatty acid contents, as represented by
the latitude survey of salmon along the Norwegian coast, explained < 1 % of the
variation in (03 fatty acids. Although not further dealt with here, it should be noted
that salmon respond to changes in dietary lipid composition in a fashion similar to
that of B. plicatilis.
The pattern of variation found for the quantitative content of the dominant
EFAs (EPA and DHA) through the salmon life cycle are, as expected, similar to
the pattern of variation found for total lipids (Fig. 8.14A). DHA is most dominant
during the early life stages in fresh water and during the first part of the seawater
stage. The average quantitative contents of EPA and DHA at the time of slaughtering (weight >3 kg) are about 7 and 12 mg' g-l fresh weight, respectively. The
corresponding total (03 and (06 fatty acid contents are 22 and 5.5 mg . g-l fresh
weight, respectively, with linoleic acid as the dominant (06 fatty acid. Thus, DHA
and EPA presumably make up 80-90% of the (03 fatty acids in salmon flesh, and
(03 fatty acids dominate over m6 fatty acids by a factor of 4.
The percentage content of EPA and DHA in salmon flesh during the life cycle
clearly demonstrates the strong dominance of DHA through the early developmental stages (Fig. 8.14B). In fact, DHA makes up 20-50% oftota1 fatty acids all
through the juvenile stage in fresh water and through the first few months of