PIKE (ESOX-LUCIUS L) STOCKING AS A BIOMANIPULATION TOOL .2. EFFECTS ON LOWER TROPHIC LEVELS IN LAKE-LYNG, DENMARK

In order to study how pike stocking affects trophic structure pike fin gerlings (0-3600 ha(-1)) were stocked during six years in eutrophic La ke Lyng (lake area 10 ha), Denmark. Subsequently, marked changes were recorded in the abundance of zooplanktivorous fish, catch per unit eff ort of roach, which was the dominant fish species, thus varied from 17 to 272. Simultaneously, marked changes were recorded in the abundance and relative composition of zooplankton. Daphnia abundance ranged fro m 7 to 36 ind. l(-1) and Bosmina from 2 to 99 ind. l(-1), mean summer abundance of Daphnia being highest and Bosmina lowest in years with lo w fish density. Additionally, cyclopoid copepod and rotifer abundance correlated significantly (p < 0.05, r(2) = 0.70 and 0.83, respectively ) and positively with fish abundance. Mean summer chlorophyll a correl ated significantly (p = 0.04, r(2) = 0.70) with the abundance of roach , suggesting an impact on phytoplankton level also, disregarding that the phytoplankton biomass was dominated by filamentous cyanobacteria. Total phosphorus concentration was lowest in years with low zooplankti vorous fish abundance, which is hypothesized to be due to lower sedime ntation of organic material and lower phosphorus release from the sedi ment in years with high zooplankton grazing on phytoplankton. It is co ncluded that pike stocking can be used as a lake restoration tool to i ncrease lake water transparency by creating a trophic cascade. The eff ect of stocking, however, seems to last only during the season in whic h it has been undertaken, the impact being most significant at high st ocking densities. The method is therefore regarded to be most useful i n shallow, turbid lakes in which the nutrient loading has been suffici ently reduced to allow a substantial and permanent macrophyte coverage if clearwater conditions are established.