FOOD-WEB MANIPULATION IN A SMALL, EUTROPHIC LAKE-WIRBEL, POLAND - LONG-TERM CHANGES IN FISH BIOMASS AND BASIC MEASURES OF WATER-QUALITY - ACASE-STUDY

A whole-lake experiment was carried out for seven years (1988-1994) in a shallow, eutrophic lake. In the first phase (1989-1991), repeated i ntroductions of young-of-the-year pike (Esox lucius) were used to cont rol the density of juvenile stages of the dominant planktivores (roach Rutilus rutilus, white bream Blicca bjoerkna and Leucaspius delineatu s). The successive introductions of juvenile pike were accompanied by selective removal of large pike, roach, bream Abramis brama and white bream. No visible improvement in water quality was registered in the f irst three years. Assessments made after the lake was treated with rot enone, revealed the high efficiency of juvenile pike in controlling pr ey of vulnerable size. Though the first three age-classes were nearly exterminated, older fish (3+ to 6+) remained abundant enough to keep t heir planktonic prey in check. Indeed, the non-altered density of domi nant cladocerans indicated that they were still heavily preyed upon. A lgal biomass remained high. In October 1991, rotenone was applied to r emove all fish. Immigrating fish were gradually recolonizing the lake from spring 1992 onwards. However, the process was kept under control by continued stocking with juvenile pike and intensive fishing. The ye ars following the rotenone treatment witnessed a considerable improvem ent in water quality, with water transparency 30% higher, a significan t 2.2-fold decrease in seston dry weight and 2.8-fold decrease in alga l biomass, averaged for the summer months. These events can largely be attributed to substantial changes in the herbivorous zooplankton, abo ve all increased density of Lake Wirbel's largest cladoceran, Daphnia cucullata.