During the past 10 years world production of fish oil was decreasing from about 1.4 to 1.0 million tons (2007-08). Europe including Scandinavia accounted for about half of the world’s consumption of fish oil, Japan being the first world producer with 0.28 million tons. About 90000 tons are used for pharmaceutical applications but half of the world production is used for aquaculture feed. Fish oil is a by-product of industrial fishing and the fish meal industry.
In fatty fish, lipid amounts depend largely on the time of their capture around the year and are localized under the skin, around the intestines or in the white muscle. The oil content varies also from species to species. In fat fish, it can reach up to 21% (herring) and 18% (sardines).
In contrast with the other fats and oils, fish oils contain large amounts of eicosapentaenoic (20:5 n-3) and docosahexaenoic (22:6 n-3) acids, respectively 14-19% and 5-8%. The proportion of polyunsaturated fatty acids depends on the fish species and on the average temperature of the water where they are caught (the colder the water, the higher the amount of these components). Saturated fatty acids include C12 up to C24:0 components, and some branched chains (iso C16, iso C17..) are also found. Among monoenes, 16:1(n-7), 20:1(n-9) and 22:1(n-11) are present in various amounts, this last component being bioconverted from the corresponding fatty alcohol of crustacea (copepods) by the fish liver. More than 50 different fatty acids were described in marine fish oil, but 8 species frequently represent more than 80% of the total amount.
We give below the basic composition of two commercially important fish oils, that of menhaden representing about 80% of the production of north America. The composition of seal oil (Pagophilus groenlandicus) (Gregersen K et al., Int J Gen Med 2010, 3, 383) and krill oil (Euphausia superba) (Araujo P et al., Lipids 2014, 49, 163) are given as a comparison .
|
Menhaden |
Herring |
Seal |
Krill | |
| 14:0 |
7-12 |
5-8 |
4.5 |
10.9 |
| 16:0 |
15-26 |
10-19 |
8 |
24 |
| 16:1(n-7) |
9-16 |
6-12 |
14 |
5.5 |
| 18:0 |
2-4 |
1-2 |
1.2 |
1 |
| 18:1(n-9) |
8-14 |
9-25 |
14.9 |
10 |
| 20:1(n-9) |
– |
7-20 |
7.7 |
0.6 |
| 20:5(n-3) |
11-16 |
4-15 |
7.9 |
18.5 |
| 22:1(n-11) |
<1 |
7-30 |
1.8 |
0.1 |
| 22:6(n-3) |
5-14 |
2-8 |
8.6 |
9 |
The table below gives the stereospecific distribution of fatty acids in triacylglycerols of depot fats in man and some animals.
| sn | 14:0 | 16:0 | 16:1 | 18:0 | 18:1 | 18:2 | 20:1 | 22:1 | 20:5 | 22:5 | 22:6 | |
| Herring | 1 2 3 |
6 10 4 |
12 17 7 |
13 10 5 |
1 1 1 |
16 10 8 |
3 3 1 |
25 6 20 |
14 5 50 |
3 18 4 |
1 3 1 |
1 13 1 |
| Cod | 1 2 3 |
6 8 4 |
15 16 7 |
14 12 14 |
6 1 1 |
28 9 23 |
2 2 2 |
12 7 17 |
6 5 7 |
2 12 13 |
1 3 1 |
1 20 6 |
Menhaden |
1 2 3 |
12 10.5 5 |
24 20 7 |
18 10.5 9 |
1 2 4 |
13 7.5 13;5 |
1 0.5 3 |
1 0.5 2 |
3 17.5 16 |
1 3 2 |
4 17 6 |
|
Seal Blubber |
1 2 3 |
2 8 0 |
3 12 3 |
8 35 12 |
1 2 1 |
38 23 18 |
0 1 2 |
13 4 17 |
3 0 3 |
8 2 11 |
4 1 8 |
10 2 18 |
| Seal | 1 2 3 |
4 11 1 |
11 13 4 |
15 30 14 |
1 1 1 |
29 30 26 |
1 3 1 |
18 3 16 |
8 1 7 |
3 1 8 |
2 1 6 |
3 1 10 |
Seal blubber and menhaden : data from Wanasundara U et al., J Food Lipids 1997, 4, 51
Oils from fish and marine mammals are characterized by a large range of fatty acids from 12 to 26 carbon atoms and 0 to 6 double bonds. The bulk of the fatty chains is contributed by saturated (15-25%), monoenes (35-60%) and polyenes (25-40%). Among polyunsaturated, 20:5n-3 and 22:6n-3 are the most prevalent. The study of the structure of the triacylglycerol molecules of marine oils is complex because of their multiplicity and the low and differential hydrolysis by lipases of positions occupied by 20:5 or 22:6n-3, the use of the Grignard reagent being required.
From the above data obtained by chemical degradation as well as those obtained with nuclear magnetic resonance spectroscopy (Aursand M et al., JAOCS 1995, 72, 293), it may be inferred that fish and mammal oils have distinct positional distributions of w3 fatty acids. As an example, in Salmon, cod or herring, 22:5 and DHA are concentrated in the β-position (sn-2), while, in mammals, they are mainly found in the α-position (sn-1,3). This may be important in considering the bioavailability of these important fatty acids after ingestion of marine foods from various origins.
The distribution of unsaturated fatty acids at the Sn-2 position of triacylglycerols in several marine fishes and mammals living in cold places has been reported (Beppu F et al., Oleo Sci 2017, 66, 1217).
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