Individual study: Initial results of restoration of a man-made freshwater lake by fish stock reduction; Bleiswijkse Zoom, Zuid Holland, the Netherlands
Meijer M.L., Raat A.J.P. & Doef R.W. (1989) Restoration by biomanipulation of lake Bleiswijkse Zoom (The Netherlands): first results. Aquatic Ecology, 23, 49-57
In recent decades, many Dutch lakes have become eutrophic with resultant large increases in algae, turbid waters and declines in submerged macrophytes. In many, pike Esox lucius (the main piscivorous fish) have declined; large biomasses of planktivorous and benthivorous fish have developed. The main method of eutrophication management has been to reduce nutrient loading (mostly agricultural run-off), however, high fish stocks appear to hamper lake recovery. Biomanipulation, i.e. reduction of the biomass of planktivorous and benthivorous fish, may enhance recovery. As such, in 1987, the Bleiswijkse Zoom, a shallow eutrophic lake in The Netherlands, was divided into two compartments to investigate the use of fish stock reduction as a tool for restoring water quality.
Study site: Lake Bleiswijkse Zoom comprises three interconnected lakes (total length 2 km; width at widest point c. 200 m; area 14.4 ha; average depth 1.1 m) located in Zuid Holland, western Netherlands. Aquatic macrophytes were initially abundant. Subsequent to carp Cyprinus carpio stocking (1973-1974 and 1980-1982), during 1980-1987 the lake developed high phosphorus concentrations, had high summer chlorophyll-a concentrations, turbid water (Secchi value 0.2 m depth), and submerged plants disappeared.
The fish stock in autumn 1986 was about 650 kg/ha (mainly bream Abramis brama, white bream Blicca (Abramis) bjoerkna, carp and pikeperch Stizostedion lucioperca).
Fish manipulation: In April 1987, Galgje (3.1 ha; the southern lake compartment) was divided from Zeeltje (the 11.3 ha northern lake area used as a control) by a dam (with a 10 x 20 cm gauze-covered opening to allow water exchange but not fish movement). 2,000 kg fish (almost all planktivorous fish and about 85% of the benthivorous fish) were removed by seine- and electro-fishing.
In May and July, 800 small (3 cm long) pikeperch were introduced in Galgje to control young bream and carp. The rational for this was that a fish predator adapted to turbid water should be present to control young planktivorous fishes during macrophyte re-establishment.
Monitoring: Various water characteristics, including transparency (using a Secchi disk), concentrations of nutrients, suspended solids and chlorophyll-a were recorded at intervals. Species composition and density of zooplankton, phytoplankton and benthic fauna were monitored. In July and August the presence of macrophytes was recorded. O+ fish were caught monthly and measured. In the winter of 1987/1988, the fish stock in the two compartments was estimated.
The fish stock management in Galgje resulted in low algal biomass (low concentrations of chlorophyll-a), increased water clarity (Secchi disk transparency increased from 20 to 110 cm), reduced total phosphorus, nitrogen and suspended solids. Within two months, submerged macrophytes (mainly stoneworts, Characeae), became abundant. Until July 1987, high densities of large zooplankton (mainly Daphnia hyalina) caused low algal biomass. From June onwards, zooplankton densities decreased but the algal concentrations remained low, probably due to nutrient limitation and/or depression of growth by the colonising macrophytes. In Galgje, the number of perch Perca fluviatilis, rudd Scardinius erythrophthalmus and roach Rutilus rutilus (i.e. species associated with aquatic vegetation) was higher than in Zeeltje.
The survival of the young pikeperch was low, and pikeperch did not control young cyprinids.
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