Pike (Esox lucius L.) stocking as a biomanipulation tool. 2. Effects on lower trophic levels in Lake Lyng, Denmark

Abstract

In order to study how pike stocking affects trophic structure pike fingerlings (0-3600 ha-1) were stocked during six years in eutrophic Lake Lyng (lake area 10 ha), Denmark. Subsequently, marked changes were recorded in the abundance of zooplanktivorous fish, catch per unit effort 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 from 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 low fish density. Additionally, cyclopoid copepod and rotifer abundance correlated significantly (p<0.05, r2 = 0.70 and 0.83, respectively) and positively with fish abundance. Mean summer chlorophyll a correlated significantly (p = 0.04, r2 = 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 zooplanktivorous fish abundance, which is hypothesized to be due to lower sedimentation of organic material and lower phosphorus release from the sediment in years with high zooplankton grazing on phytoplankton. It is concluded that pike stocking can be used as a lake restoration tool to increase lake water transparency by creating a trophic cascade. The effect of stocking, however, seems to last only during the season in which it has been undertaken, the impact being most significant at high stocking densities. The method is therefore regarded to be most useful in shallow, turbid lakes in which the nutrient loading has been sufficiently reduced to allow a substantial and permanent macrophyte coverage if clearwater conditions are established.