The restoration of fish ponds in agricultural landscapes

Fish ponds in central and eastern Europe are artificial lakes of less than I ha to several 100 ha in size. Some of them were created around 900 years ago. These fish ponds form an important part of the hydrological system in the landscape; they also serve as water purification systems and sediment traps, provide habitat for many plant and animal species, as well as being used for recreation purposes-but above all, they are dedicated to fish production. The original oligo- or mesotrophic character of these fish ponds has been altered to one of eutrophic or even hypertrophic conditions due to the nature of the intensive management for high fish production, as well as agriculture practices within the catchment area. It can also be due to other uses of fish ponds, such as wastewater recipients. A thick layer of black anaerobic sediment invariably develops and serves as an uncontrolled internal source of nutrients. In order to stop the development of water blooms and restore the lake to reasonable healthy conditions, both the external and internal loading has to be reduced. An example of such a fish pond restoration within the Czech Republic is described. The phosphorus budget of the Vajgar fish pond (40 ha, S Bohemia) was evaluated and the dry matter, organic matter, nutrients, heavy metals and other metals content were estimated in the vertical profile of the fish pond's sediment. A precision suction dredger was designed, constructed, and used for the selective removal of 330 000 m(3) black sediment from the Vajgar fish pond. The sediment was pumped to settling ponds at a distance of 2.5 km. Later, the suction dredger was used for sediment removal from other fish ponds. In one case, 40000 m(3) Of eutrophic sediment was directly applied on an agriculture field. In another case, sediment containing oil products was decontaminated by using a biocatalyst and bacterial culture. Sediment removal from Vajgar fish pond resulted in a temporary negative phosphorus budget (lower concentrations in the outflow than the inflow). The temporary absence of cyanobacteria (Microcystis sp.) blooms was the result of the removal of the black sediment layer. The improvement of water quality after sediment removal was, however, temporary. A more holistic approach, aimed at the reduction of matter losses from the fish pond's catchment area, is therefore being adopted. The principles of such an holistic approach are described. (C) 2002 Elsevier Science B.V. All rights reserved.