Fattv acid content of some freshwater fish in lakes of different trophic levels – a bottom-up effect?

Abstract– Various sizes of roach, perch, pike, zander (pike-perch) and crucian carp were collected from lakes of different trophic levels in order to study the variation of lipids and fatty acids (FA) within and between species. Freeze-dried samples of the dorsal muscle were analysed quantitatively for total lipid content and FA content. The results indicate that total lipid and FA contents can vary considerably, both within and between species. In contrast to herbivorous fish, carnivorous-piscivorous fish FA patterns were more constant and independent of size - fry excepted. Lipid and FA contents of roach from two oligotrophic lakes were significantly higher than in roach from a eutrophic lake. Differences in basic food webs may be responsible for these results. In the oligotrophic lakes, the algal flora was dominated by species classified as high quality food for grazers, e. g. flagellates and diatoms. It was shown in earlier papers that these algal groups contain two long-chained FA of φ 3 type (eicosapentaenoic acid and docosahexaenoic acid) which are used as criteria indicating high nutritional value. Conversely, the eutrophic lake was dominated by blue-greens, a group of autotrophs lacking these long-chained φ 3 FA. Blue-greens have generally been classified as poor food for grazers. (The fatty acids are described by three numbers, x:yφz, where x=number of carbon atoms, y=number of double bonds, and z=position of the first double bond counted from the methyl end of the molecule.)