GROWTH AND DIET OF INTRODUCED COREGONID FISH COREGONUS PELED (GMELIN) AND COREGONUS LAVARETUS (L.) IN TWO BELGIAN RESERVOIR LAKES

Introduced Coregonus lavaretus L. and Coregonus peled Gmelin populations in the Robertville and Bütgenbach Reservoir lakes (Belgium) were studied in order to assess their adaptation to their new habitat.We considered the evolution of abundance, biomass, size distribution structure, growth parameters and diet.Maximal temperature values are acceptable for C. peled but slightly too high for an optimal development of C. lavaretus.In order to remain in their acceptable range for dissolved oxygen, the coregonids must stay in surface water during stratification, although water temperature is largely above their optimum in these water layers.Length frequency diagrams established for C. peled in both lakes show only older cohorts, which indicate that there is no natural reproduction.Slope coefficients for C. peled in both lakes indicate an allometric growth (b<3), length increasing proportionally more rapidly than weight.C. lavaretus shows a broad diet spectrum with 14 categories whereas C. peled seems to have a more narrow diet spectrum.In our case C. peled shows an essentially zooplankton oriented diet in both lakes.In conclusion, the Bütgenbach and Robertville reservoir lakes seem to meet the habitat requirements of C. lavaretus and C. peled in terms of growth capacities and diet composition.On the other hand, the conditions necessary for the natural reproduction of these two species do not seem to be totally fulfilled, probably due to the trophic status and the fluctuations of the water level as consequence of reservoir operations.