Influence of zooplankton and phytoplankton on the fatty acid composition of digesta and tissue lipids of silver carp: mesocosm experiment

Zooplankton appeared to be the major contributor to the diet of 1+ silver carp, whereas 3+ fishes exhibited a more evenly balanced spectrum between zooplankton and phytoplankton. The fatty acids profiles of digesta were influenced by zooplankton, particularly for 1+ silver carp. Together, Fatty acid profiles of tank zooplankton and digesta were characterized by high proportion of 20:5 omega 3 and 20:6 omega 3. The fatty acids composition of the phytoplankton reflected the dominance of cyanobacteria and chlorophycea, with high quantities of 18:2 omega 6 and 18:3 omega 3. Although cyanobacteria accounted for >70% of the phytoplankton biomass ingested by the carp, fatty acids profiles of digesta were not influenced by phytoplankton Fatty acids composition. The low digestive and conversion efficiency of Microcystis aeruginosa explain this absence of relation. The neutral lipids in silver carp tissues reflected poorly the fatty acids profiles in the diet, the semi-natural conditions and the diet dominated throughout the study by zooplankton, led to little variation in tissues fatty acids. The phospholipids in the muscle, liver and peri-intestinal fat were characterized by a rather low proportion of polyunsaturated fatty acids (PUFA) in both 1+ and 3+ fish. From a qualitative view point, cryptophycea, diatoms, and especially zooplankton are much more valuable food for the silver carp than cyanobacteria and desmid chlorophycea which are poor in long-chain PUFA. (C) 2000 The Fisheries Society of the British Isles.