8. Lipids and Essential Fatty Acids in Aquatic Food Webs
193
salmon (Fig. 8.16A). Values obtained for juvenile salmon in the freshwater stage
(see inset, Fig. 8.16A) fit well with values obtained for older stages in seawater.
The inverse relationship between percentage 003 EFA and lipid content is also
quite evident (Fig. 8.16B). The percentage content of 22:5003 (DPA) is almost
constant and independent of the lipid content of the salmon, the percentage EPA
shows a minor increase in very lean salmon (less than two times), and relative
DHA abundance increases more than four times in a pronounced and nonlinear
fashion as the lipid content of the salmon decreases, particularly when the lipid
content is <50 mg . g-l fresh weight. Again, values for juveniles fit well into the
general relationship for older stages (see inset, Fig. 8.16B).
The pattern of variation shown in Figure 8.16B, characterized by an inverse
relationship between percentage EFA and lipid content (cf. Ahlgren et aI., 1994),
can be understood on the basis of the compartment scheme for TAG-zooplankton
(Fig. 8.8). TAGs and PLs are the main lipid compartments in salmon flesh. The
PLs contain higher fractions of PUFA than the TAGs, and their composition is, as
discussed above, to some extent genetically controlled. Assuming that salmon
PLs constitute 10 mg . g - I fresh weight, it may be deduced that PLs will be the
dominant lipids in salmon with very low total lipid content (Fig. 8.13; see also
lean TAG-zooplankton case, Fig. 8.8). The fatty acid composition obtained for
very lean salmon (Fig. 8.16B) will then primarily reflect the fatty acid composition of its PLs. This again implies that DHA is the dominant PUFA of the salmon
membranes and that 30-40% of the individual fatty acids of the cell membranes
are DHA. This also explains why percentage DHA tends to increase in starving
marine fish, fish larvae, and zooplankton (i.e., in animals with high EFA requirements).
In contrast to lean salmon, fat salmon may be characterized by a dominance of
TAGs (Fig. 8.13; see also fat TAG-zooplankton case, Fig. 8.8), and the fatty acid
composition of fat salmon will therefore mainly reflect the fatty acid composition
of the salmon TAGs. The impact of PLs on total flesh fatty acid composition will
be 4-8% in salmon >3 kg. The slightly enhanced DHA levels in large salmon,
compared with the percentage content in the food (Figs. 8.14 and 8.15), can
therefore be explained solely by the DHA contribution of the cell membrane PLs.
This implies that the percentage DHA of salmon TAGs becomes almost identical
to that of the feed.
The flesh lipid contents of marine fish differ strongly, primarily as a result of
how the fish store fat. Species that accumulate lipid in the muscle, such as salmon,
may show very high contents of flesh lipids after periods of active food consumption. However, species that store lipid in the liver exhibit flesh lipid levels of the
same magnitude as the typical PL levels of their muscles. Table 8.2 reviews lipid
and EFA contents of some marine fish species. The ranges of variation in lipids as
well as in percentage fatty acids are quite pronounced. If the percentage EFA
content of the diverse fish species presented in Table 8.2, among which not all are
recognized as cold-water species, is plotted as a function of their lipid content
(Fig. 8.17), we obtain more or less the same relationship as that for salmon (Fig.
8.16).
193
salmon (Fig. 8.16A). Values obtained for juvenile salmon in the freshwater stage
(see inset, Fig. 8.16A) fit well with values obtained for older stages in seawater.
The inverse relationship between percentage 003 EFA and lipid content is also
quite evident (Fig. 8.16B). The percentage content of 22:5003 (DPA) is almost
constant and independent of the lipid content of the salmon, the percentage EPA
shows a minor increase in very lean salmon (less than two times), and relative
DHA abundance increases more than four times in a pronounced and nonlinear
fashion as the lipid content of the salmon decreases, particularly when the lipid
content is <50 mg . g-l fresh weight. Again, values for juveniles fit well into the
general relationship for older stages (see inset, Fig. 8.16B).
The pattern of variation shown in Figure 8.16B, characterized by an inverse
relationship between percentage EFA and lipid content (cf. Ahlgren et aI., 1994),
can be understood on the basis of the compartment scheme for TAG-zooplankton
(Fig. 8.8). TAGs and PLs are the main lipid compartments in salmon flesh. The
PLs contain higher fractions of PUFA than the TAGs, and their composition is, as
discussed above, to some extent genetically controlled. Assuming that salmon
PLs constitute 10 mg . g - I fresh weight, it may be deduced that PLs will be the
dominant lipids in salmon with very low total lipid content (Fig. 8.13; see also
lean TAG-zooplankton case, Fig. 8.8). The fatty acid composition obtained for
very lean salmon (Fig. 8.16B) will then primarily reflect the fatty acid composition of its PLs. This again implies that DHA is the dominant PUFA of the salmon
membranes and that 30-40% of the individual fatty acids of the cell membranes
are DHA. This also explains why percentage DHA tends to increase in starving
marine fish, fish larvae, and zooplankton (i.e., in animals with high EFA requirements).
In contrast to lean salmon, fat salmon may be characterized by a dominance of
TAGs (Fig. 8.13; see also fat TAG-zooplankton case, Fig. 8.8), and the fatty acid
composition of fat salmon will therefore mainly reflect the fatty acid composition
of the salmon TAGs. The impact of PLs on total flesh fatty acid composition will
be 4-8% in salmon >3 kg. The slightly enhanced DHA levels in large salmon,
compared with the percentage content in the food (Figs. 8.14 and 8.15), can
therefore be explained solely by the DHA contribution of the cell membrane PLs.
This implies that the percentage DHA of salmon TAGs becomes almost identical
to that of the feed.
The flesh lipid contents of marine fish differ strongly, primarily as a result of
how the fish store fat. Species that accumulate lipid in the muscle, such as salmon,
may show very high contents of flesh lipids after periods of active food consumption. However, species that store lipid in the liver exhibit flesh lipid levels of the
same magnitude as the typical PL levels of their muscles. Table 8.2 reviews lipid
and EFA contents of some marine fish species. The ranges of variation in lipids as
well as in percentage fatty acids are quite pronounced. If the percentage EFA
content of the diverse fish species presented in Table 8.2, among which not all are
recognized as cold-water species, is plotted as a function of their lipid content
(Fig. 8.17), we obtain more or less the same relationship as that for salmon (Fig.
8.16).