Individual study: Initial results of a reduction in fish stocks to restore the Duinigermeer, Overijssel, the Netherlands
Van Berkum J.A., Klinge M. & Grimm M.P. (1995) Biomanipulation in the Duinigermeer; first results. Aquatic Ecology, 21, 81-90
In the Netherlands, after decreasing nutrient inputs, fish stock reduction has been used in several cases as a follow-up measure in lake restoration attempts. Such an experiment was undertaken at a freshwater lake, the Duinigermeer. In the paper summarized here, the results of the first growing season after fish stock reduction are compared with those of the prior season.
Study site: The Duinigermeer (surface area 28 ha; average depth 1 m), is situated in northwest Overijssel province. It is connected to numerous dykes (total length 5 km; 2 ha surface area). Over the previous 20 years, improvement of water quality (external phosphorus loading much reduced) and water management to reduce the impact of drought, had been achieved.
Up to the 1960s the lake had about 50% submerged vegetation cover (mostly pondweed Potamogeton and stonewort Chara spp.). During the 1960s the area started to dry up and eutrophication resulted in turbid waters, submerged macrophytes and bulrush Scirpus (Schoenoplectus) lacustris (previously present in some areas) disappeared, and cyanobacteria started to dominate until the beginning of the fish removal experiment.
Fish reduction The fish stock was assessed in June 1992 by a combination of trawl-netting and electro-fishing, including in the adjacent dykes. The standing stock was estimated at 130 kg/ha (mostly roach Rutilus rutilus and bream Abramis brama). The target was to remove 80% of the stock.
Prior to stock reduction the lake was isolated from the adjacent (much larger) Giethoornse Meer with nets (later replaced with a wooden dam). The reduction was undertaken between November 1992-June 1993 using seine-netting, followed by fyke netting and electro-fishing, including in the dykes.
A total of 3,582 kg fish (119.4 kg/ha) was removed, mainly bream (>25 cm) and small roach. All fish except pike Esox lucius <30 cm, tench Tinca tinca, rudd Scardinius erytrophtalmus and relatively rare species (spined loach Cobitis taenia, bullhead Cottus gobio and burbot Lota Iota) were removed. After the reduction in June 1993, the fish stock was estimated at 26-40 kg/ha, thus the reduction target of 80% was achieved.
Monitoring: Water samples were taken in the lake during winter and summer for analysis. The development of the emergent vegetation, and cover of floating-leaved and submerged vegetation (by aerial surveys) was monitored. The fish stock was re-assessed in December 1993.
After fish stock reduction, by the end of April 1993 the entire lake bottom had become visible as turbidity decreased. A month later, rapid development of submerged vegetation occurred; by the end of June, 50% of the lake bottom was covered by four species of stonewort. At the end of April 1993, densities of cladoceran zooplankton increased but then declined 3 months later following a decline in phytoplankton (concentrations of chlorophyll-a dropped considerably).
The long-term stability of the new situation in the lake is considered to depend largely on the persistence of the submerged vegetation and the recovery of a characteristic tench-rudd-pike community, with the piscivorous pike regulating planktivorous fish numbers and thus maintaining suitable zooplankton grazing pressure on the phytoplankton.
Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.springerlink.com/content/kj60175281m54n23/fulltext.pdf