360 Part III: Muscle Foods
during late spring shortly after spawning, while it
can have about 24% fat during late November due to
active feeding (Leu et al. 1981; Fig. 16.1).
Different parts of the aquatic animal will also
have different fat contents. Ohshima et al. (1993a)
reported the following descending order for the fat
content of mackerel and sardines:
- Skin (including subcutaneous lipids),
- Viscera,
- Dark muscle, and
- White muscle.
Some species, most notably salmonids, have larg-
er fat deposits in the belly flaps than in other parts of
the fish. Distribution of fat in many fish seems to be
in descending fat levels from the head to the tail
(Icekson et al. 1998, Kolstad et al. 2004). Diet will
have a significant impact on the fat content and fatty
acid profiles of aquatic animals. Aquacultured fish
on a controlled diet, for example, do not show the
same seasonal variation in fat content as wild spe-
cies. It is also possible to modify the fat content and
fat type via diet. For example, adding fish oil to fish
feed will increase the fat content of fish (Lovell and
Mohammed 1988), usually proportionately to the
level of fat/oil in the diet (Solberg 2004). It has also
been shown with many species (salmon being the
most investigated) that the fatty acid composition is
a reflection of the fatty acid composition in the fish
diet (Jobling 2004). It is therefore possible to selec-
tively increase the nutritional value of aquacultured
fish by increasing the level and improving the ratio
of nutritionally beneficial fatty acids (Lovell and
Mohammed 1988). Conversely, one can decrease
the level of unstable fatty acids to increase the shelf
life of the product. A good example is catfish (Table
16.6), which in the wild has more unstable (but more
nutritionally beneficial) fatty acids than grain-fed
aquacultured catfish (St. Angelo 1996).
Although varying levels of total fat content in
fish can influence quality, fatty acid type rather than
quantity appears to be more important. The major
classes of lipids in fish are triglycerides and phos-
pholipids. Triglycerides, the majority of lipids in
fish (except very lean fish), are neutral lipids. They
exist as either large droplets within the adipose tis-
sue or smaller droplets between or within muscle
cells (Undeland 1997). These lipids are deposited
and stored as an energy source for fish (Huss 1994).
The phospholipids, which are polar lipids, are one of
the main building blocks of muscle cell membranes
and are usually present only at low levels, 0.5–1.1%
(O’Keefe 2000). Some muscles of very lean species
such as cod, pollock, and whiting can contain less
than 1% lipids, most of which are in the form of
membrane phospholipids.
Compared to other animals, aquatic animals are
especially rich in long-chain polyunsaturated fatty
acids (PUFAs), which are found in higher numbers
in the membrane phospholipids than in storage tri-
glycerides (Hultin and Kelleher 2000). Aquatic ani-
mals adapted to cold water also have higher levels of
PUFAs than those adapted to warmer waters, since
more unsaturation is essential to maintain mem-
brane fluidity and function at low temperatures. It
has been reported that about half of the membrane
phospholipids in cold-water fish are the omega-3
fatty acids eicosapentoic acid (20:53) and docosa-Table 16.6.Fatty Acid Composition of
Extracted Lipids from Wild and Pond—
Reared Channel CatfishFatty Pond–
Acids Wild (%) Reared (%)
16:0 20.63 19.62
16:1 7 7.48 4.15
18:0 8.04 6.67
18:1 23.72 42.45
18:2 6 2.9 12.35
18:3 3 2.78 1.66
18:4 3 0.6 0.96
20:1 9 0.83 0.97
20:3 6 0.36 0.98
20:4 6 6.82 1.88
20:4 3 0.62 0.16
20:5 3 7.02 1.49
24:1 9 0.34 0.16
22:4 6 0.68 0.14
22:5 6 1.34 0.51
22:5 3 3.05 0.98
22:6 6 13.56 4.97Saturated 28.67 26.28
Monounsaturated 31.64 47.64
Polyunsaturated 39.77 26.07
-6 PUFA 12.13 15.85
-3 PUFA 27.64 10.22
3/ 6 2.54 0.62
Source:Adapted from Chanmugam et al. 1986.