Selective removal fishing — water quality and practical viewpoints

Removal fishing can be an effective method for restoration of eutrophied lakes (J EPPESEN & SAMMALKORPI 2002, COOKE et al. 2005). Costs may become very high in large lakes, however, which makes it difficult to maintain efficient fishing effort to achieve permanent results. In large, mesotrophic Lake Pyhãjãrvi in southwest Finland (Table l), a profitable commercial fishery targeting planktivorous vendace ( Coregonus albula) has for decades acted as unplanned biomanipulation, and the existing fishery infrastructure has made it possible to develop large-scale intentional and intensive fish removal operations (VENTELÃ et al. 2007). As a specialized zooplankton feeder, vendace i s akeystone species in the pelagic system, controlling zooplankton an d indirectlyphytoplankton (HELMINEN & SARVALA 1997). In the 1980s and early 1990s there was a positive correlation in Pyhãjãrvi between vendace year-class strength and the Iate summer values of chlorophyll a concentration, while zooplankton biomass was negatively correlated with both vendace year-class and chlorophyll a concentration, indicating an intermediary role of zooplankton between vendace and phytoplankton (HELMINEN & SARVALA 1997). Fishing of vendace has been very efficient; nearly 90 % o f the youngest age group is fished out yearly (SARVALA et al. 1998). Fishing methods include mainly winter seines (usually 8 seining groups in the 1980s, but mostly 4 since 1995) and f)rke nets in spring and autumn. In the 1990s, water quality in Lake Pyhãjãrvi deteriorated in spite of continued commercial fishing and weak vendace stock: phosphorus concentrations and phytoplankton biomass increased, and cyanobacterial blooms became more frequent, harming the recreational uses of the lake and even causing taste problems in the fish (VENTELÃ et al. 2007). The vendace population collapsed because of failed recruitment in 2 successive years due to unfavourable weather conditions and excessive predation pressure and remained small due to temporary overfishing (SARVALA & HELMINEN 2002). As a result of competitive release, the other fish species became more abundant and caused adverse water quality effects (SARVALA et al. 1998). The eutrophication of Pyhãjãrvi is connected to high extemal nutrient load from the catchment. The lake received industria! and community waste waters Table l. Description ofLake Pyhãjãrvi. Nutrient and chlorophyll a values represent averages ± SD during the open water periods (May-Oct) during 2000-2005.