CHANGES IN THE AQUATIC VEGETATION OF 2 GRAVEL PIT LAKES AFTER REDUCING THE FISH POPULATION-DENSITY

The effects of reducing fish density on the submerged macrophytes in two gravel pit lakes is described. In 1987-1988, 396 kg ha-1 of fish were removed from one lake and the aquatic vegetation cover increased from less than 1% to 93%. The mean standing crop of submerged macrophytes increased from 1.0 g m-2 in 1986 and 1987 (when fish were present) to 46.7 g m-2 in 1988 and 1989 (after fish removal). In 1990 fish were re-introduced into a sampling enclosure in the lake and here the macrophyte standing crop was reduced to 4.8 g m-2 from approximately 47 g m-2. In the second lake, macrophyte cover increased from 48% to 95% after the removal of 356 kg ha-1 of fish. The mean standing crop was 40.4 g m-2 in 1986 and 1987 and this increased to 99.4 g m-2 in 1988 and 1989. In 1990, in a re-stocked enclosure, standing crop was 36.4 g m-2. At the time of the fish removal, bream, Abramis brama (L.), comprised 48% of the fish biomass in the first lake and 19% in the second. This species was considered to be chiefly responsible for inhibiting initial establishment of submerged macrophyte seedlings and for reducing existing stands.