Changes in the aquatic vegetation of two gravel pit lakes after reducing the fish population density.

Abstract

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.