To test whether the measurement of selected enzyme activities could be used to estimate more precisely the trophic shift of C isotopes,
Nile tilapia (Oreochromis niloticus) were fed semi-synthetic diets differing in their lipid contents (1.7%, 5.0%, 10.8% and 20.0%). The diets
were formulated to contain the same amount of nitrogen and metabolizable energy and were made from casein, wheat starch, corn germ oil
supplemented with vitamins, minerals and l-arginine. The influence of the different diets on the activity of two lipogenic enzymes, ATPcitrate lyase and malic enzyme, on d
13C values in the whole fish, the liver and their correlation was investigated. There was a strong positive
correlation between d
13C values in the lipids of whole fish and those of their livers. The activities of lipogenic enzymes increased
significantly with increasing trophic shift of C isotopes (Dd
13Cdiet-fish values) in the lipids. If the relationship between trophic shift and
enzyme activity can be confirmed in situations where feed quantity and quality are not known, the determination of enzyme activities would
enable better estimates of the trophic shift to be made thus significantly improving back-calculation of diets from stable isotope data.
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