FISH NUTRITION
437
ratio is greater than 0.4 then the diet is deficient in w3 acids. Yu and
Sinnhuber (1971) have reported that 18 : 3u3 ethyl ester is as effective as
22 : 6w3 ethyl ester in supporting maximal growth of rainbow trout when
both acids were fed separately at 1 yo of the dry diet weight.
Thus the present situation is that w3 acids are the major essential
fatty acids of both freshwater and marine fish. Maximum growth
rates without deficiency symptons can be obtained with w3 acids alone
but not with w6 acids alone. Nonetheless marine fish and especially
freshwater fish have significant levels of w6 acid in their natural diets.
At a practical level we still do not know with certainty how much 0.16
acid can be safely incorporated into commercial diets without upsetting
the metabolism of w3 acids. This is important in the sense that most
readily available plant oils, e.g. corn oil, have high levels of 0 6 acids.
The problem in essence is that we have little information of the extent t o
which chain elongation of a member of one series, e.g. 18:3w3, is
inhibited by the presence of a member of the other series, e.g. 18 : 2w6
in fish. This situation, competitive inhibition of chain lengthening
between families of polyunsaturated fatty acids, is very well
documented in mammals (Holman, 1964). The situation in fish may well
prove to be more difficult to unravel than in mammals, due to the wider
range of polyunsaturated fatty acids present although biochemical
studies, especially with isolated enzyme systems, may provide a
relatively rapid solution. Excessive deposition of lipid, not necessarily
with pathological consequences, as well as the more extreme symptoms
of depigmentation, fin erosion and behavioural abnormalities, may
provide indications of inadequate diet in this respect. The use of fish
oils, wherever possible, as dietary supplements should be encouraged.
4. Positional distribution
The great bulk of fatty acids in the diet do not exist as such but are
esterified to form triglycerides and phospholipids that are quantitatively
the most important dietary lipids. Triglycerides and phospholipids
are both derivatives of the asymmetric molecule L-a-glycerophosphate.
Phospholipids are always asymmetric ; triglycerides are asymmetric
when different fatty acyl moieties are esterified on positions 1 and 3
(Fig. 8). In view of the wide range of fatty acid types found in fish
it is relevant to pose the questions (i) are individual fatty acids
preferentially located in the particular positions of phospholipids and
triglycerides, and (ii) is this location likely to be important in the
provision of fatty acids through the diet? These questions have been
examined by positional analysis of the fatty acids esterified to particu-
437
ratio is greater than 0.4 then the diet is deficient in w3 acids. Yu and
Sinnhuber (1971) have reported that 18 : 3u3 ethyl ester is as effective as
22 : 6w3 ethyl ester in supporting maximal growth of rainbow trout when
both acids were fed separately at 1 yo of the dry diet weight.
Thus the present situation is that w3 acids are the major essential
fatty acids of both freshwater and marine fish. Maximum growth
rates without deficiency symptons can be obtained with w3 acids alone
but not with w6 acids alone. Nonetheless marine fish and especially
freshwater fish have significant levels of w6 acid in their natural diets.
At a practical level we still do not know with certainty how much 0.16
acid can be safely incorporated into commercial diets without upsetting
the metabolism of w3 acids. This is important in the sense that most
readily available plant oils, e.g. corn oil, have high levels of 0 6 acids.
The problem in essence is that we have little information of the extent t o
which chain elongation of a member of one series, e.g. 18:3w3, is
inhibited by the presence of a member of the other series, e.g. 18 : 2w6
in fish. This situation, competitive inhibition of chain lengthening
between families of polyunsaturated fatty acids, is very well
documented in mammals (Holman, 1964). The situation in fish may well
prove to be more difficult to unravel than in mammals, due to the wider
range of polyunsaturated fatty acids present although biochemical
studies, especially with isolated enzyme systems, may provide a
relatively rapid solution. Excessive deposition of lipid, not necessarily
with pathological consequences, as well as the more extreme symptoms
of depigmentation, fin erosion and behavioural abnormalities, may
provide indications of inadequate diet in this respect. The use of fish
oils, wherever possible, as dietary supplements should be encouraged.
4. Positional distribution
The great bulk of fatty acids in the diet do not exist as such but are
esterified to form triglycerides and phospholipids that are quantitatively
the most important dietary lipids. Triglycerides and phospholipids
are both derivatives of the asymmetric molecule L-a-glycerophosphate.
Phospholipids are always asymmetric ; triglycerides are asymmetric
when different fatty acyl moieties are esterified on positions 1 and 3
(Fig. 8). In view of the wide range of fatty acid types found in fish
it is relevant to pose the questions (i) are individual fatty acids
preferentially located in the particular positions of phospholipids and
triglycerides, and (ii) is this location likely to be important in the
provision of fatty acids through the diet? These questions have been
examined by positional analysis of the fatty acids esterified to particu-