Changes in the abundance of planktivorous fish in Lake Skanderborg during the past two centuries - a palaeoecological approach

Fish have a profound effect on food-web structure and water quality in lake s. Quantitative historical data for fish are however scarce, which render i t difficult to interpret palaeoecological records in the sediment. To recon struct the historical changes of the fish community structure and abundance of Lake Skanderborg during the past 150 yr we conducted a stratigraphic an alysis of selected zooplankton remains in the lake sediment. From these dat a we inferred the abundance of planktivorous (expressed as catch net(-1) ni ght(-1) in multi mesh-sized gillnets, CPUE) and percentage piscivores using a previously developed transfer functions for Danish lakes. Inferred CPUE was high throughout the study period, being highest before 1900 after which it declined following introduction of pikeperch (Stizostedion lucioperca) in 1903-1904. Further reductions occurred after a major reduction in extern al P-loading in 1974 and fish removal in 1983-1985. The inferred CPUE in th e surface sediment is in accordance with recent monitoring data. Good corre spondence was also obtained between measured zooplankton:phytoplankton biom ass ratio and Daphnia mean body weight versus that calculated from inferred CPUE using equations established earlier from contemporary data on Danish lakes. At present, the fish stock is numerically dominated by roach (Rutilu s rutilus) bream (Abramis brama) and small perch (Perca fluviatilis), while pikeperch (Stizostedion lucioperca) is the most important piscivore. In ac cordance with the inferred data, earlier fish investigations and angler des criptions dated as far back as to 1927 indicate little changes in fish comm unity structure. The early dominance of plankti-benthivorous fish most like ly reflects an early eutrophication of the lake due to early urban developm ent in the lake catchment. The development of a transfer function for plank tivores fish have im roved the possibilities of discovering changes in trop hic structure and ecological state of lakes in the past. (C) 2001 Elsevier Science B.V. All rights reserved.