FISH NUTRITION
435
fish is supported by recent experiments in this laboratory. Plaice were
kept on a diet containing corn-oil and lipids present in cod muscle
(added as dietary protein) from a very early age. The dietary ratio
0 3 1 ~ 6 was 0.36. At 12 months old some of these fish were subjected t o
a fat-free diet for a further six months and subsequently compared with
fish continued on the corn oil-cod muscle diet for the same period, and
also with wild fish of the same size. Table XVI shows the levels of
fatty acids in triglycerides and phospholipids from both the liver and the
remaining extrahepatic tissues. It is emphasized that in these tables
data are expressed as mg of a fatty acid component in the liver or
extrahepatic tissue of fish normalized to 100 g body weight. It is clear
that fish on a fat-free diet lose their polyenoic acids readily, although
phospholipids retain these acids much more tightly than triglycerides.
With fish continued on the corn oil-cod muscle diet the levels of 22 : 6w3
acid are very similar to those in the wild fish. This applies to triglycerides and to a lesser extent to phospholipids in liver and extrahepatic
tissues. However, the levels of 16:0, 16:1, 18:0, 1 8 : l and 18:2w6
acids are very much elevated in the cultured fish. Examination of the
total triglyceride shows that elevated levels are found in both liver and
the extrahepatic tissues, although the levels of phospholipid remain
essentially constant. It would appear from these experiments that the
cultured plaice are depositing large levels of triglycerides in their livers
and extrahepatic tissues possibly in order to obtain normal levels of
22:6w3 acid. It is important to note that the cultured fish in these
experiments showed no pathological conditions and good growth rates
were obtained.
It is of intmest that excessive deposits of fat have been observed
along the lateral line of intensively farmed plaice (Roberts, 1970) fed
a non-defined diet. Poston (1968b) has presented evidence suggesting
that accumulation of lipid in the liver of trout fed a diet containing
hydrogenated vegetable oil is a manifestation of an essential fatty acid
deficiency. Both these conditions may well be a direct consequence of
an inadequate w3/w6 ratio in dietary fatty acids.
Very recently Cmtell(l971) has shown that rainbow trout achieved
better growth rates and feed conversion efficiencies on diets containing
linolenio acid ethyl ester (w3) than on diets containing Iinoleic acid
ethyl ester (w6). The optimal level of linolenate was 1% of the dry
weight of the diet or about 2% of the total dietary calories. Linolenate
reversed all symptoms of essential fatty acid deficiency but linoleate
only reversed some of these symptons. Efficient conversion of linolenate
to higher polyunsaturated acids occurred. The ratio 20 : 3 ~ 9 1 2 2 : 60.13
was suggested as an index of essential fatty acid deficiency in that if the
435
fish is supported by recent experiments in this laboratory. Plaice were
kept on a diet containing corn-oil and lipids present in cod muscle
(added as dietary protein) from a very early age. The dietary ratio
0 3 1 ~ 6 was 0.36. At 12 months old some of these fish were subjected t o
a fat-free diet for a further six months and subsequently compared with
fish continued on the corn oil-cod muscle diet for the same period, and
also with wild fish of the same size. Table XVI shows the levels of
fatty acids in triglycerides and phospholipids from both the liver and the
remaining extrahepatic tissues. It is emphasized that in these tables
data are expressed as mg of a fatty acid component in the liver or
extrahepatic tissue of fish normalized to 100 g body weight. It is clear
that fish on a fat-free diet lose their polyenoic acids readily, although
phospholipids retain these acids much more tightly than triglycerides.
With fish continued on the corn oil-cod muscle diet the levels of 22 : 6w3
acid are very similar to those in the wild fish. This applies to triglycerides and to a lesser extent to phospholipids in liver and extrahepatic
tissues. However, the levels of 16:0, 16:1, 18:0, 1 8 : l and 18:2w6
acids are very much elevated in the cultured fish. Examination of the
total triglyceride shows that elevated levels are found in both liver and
the extrahepatic tissues, although the levels of phospholipid remain
essentially constant. It would appear from these experiments that the
cultured plaice are depositing large levels of triglycerides in their livers
and extrahepatic tissues possibly in order to obtain normal levels of
22:6w3 acid. It is important to note that the cultured fish in these
experiments showed no pathological conditions and good growth rates
were obtained.
It is of intmest that excessive deposits of fat have been observed
along the lateral line of intensively farmed plaice (Roberts, 1970) fed
a non-defined diet. Poston (1968b) has presented evidence suggesting
that accumulation of lipid in the liver of trout fed a diet containing
hydrogenated vegetable oil is a manifestation of an essential fatty acid
deficiency. Both these conditions may well be a direct consequence of
an inadequate w3/w6 ratio in dietary fatty acids.
Very recently Cmtell(l971) has shown that rainbow trout achieved
better growth rates and feed conversion efficiencies on diets containing
linolenio acid ethyl ester (w3) than on diets containing Iinoleic acid
ethyl ester (w6). The optimal level of linolenate was 1% of the dry
weight of the diet or about 2% of the total dietary calories. Linolenate
reversed all symptoms of essential fatty acid deficiency but linoleate
only reversed some of these symptons. Efficient conversion of linolenate
to higher polyunsaturated acids occurred. The ratio 20 : 3 ~ 9 1 2 2 : 60.13
was suggested as an index of essential fatty acid deficiency in that if the