The Role of Fish Communities in Water Quality Management of a Large Shallow Lake

Abstract

Management measures of Lake Balaton such as wetland reconstruction at the main inflow to the lake along with the "unplanned" commercial fishery led to great changes in the density and biomass of fish populations. There was no significant difference in CPUE data between the two, eastern and western, basins. Biomass of total fish stock in Lake Balaton has decreased substantially, 2-3 times between 1991-1999, and ranges between 120-194 kg ha -1. Bottom-up effects are more important than the topdown effects due to the impact of internal nutrient load. Changes in the biomass and thus the activity of omnivorous fish in the lake lowered the intensity of various indirect effects and feedback mechanisms causing changes in the nutrient metabolism of the lake. Intensified fishery effort in Lake Balaton did not result in an increased stock of piscivores. The ratio of piscivores and omnivores remained at 5% during the whole study period. Despite this low piscivores to omnivores ratio, the water quality has improved in all basins.

Ambiguous influences of omnivorous fish on trophic cascade and alternative states: Implications for biomanipulation from an ecological model

“…As the results, omnivorous fish may either increase or decrease the occurrence of alternative states, depending on the relative increase level of predation intensity (Figure 3). The two outcomes compatibly explain the coexistence of the two distinct expectations when applying biomanipulation: one (Outcome 1) queried the unpredictable long-term effect due to the reduced cascading effect (Lazzaro, 1997;Pace et al, 1999), and the other (Outcome 2) emphasized the urgent need to release high predation pressure on zooplankton (Tátrai et al, 2003;Beklioglu et al, 2007).…”

“…First, omnivorous fish biomanipulation is also necessary to release predation pressure (e.g. Tátrai et al, 2003;Beklioglu et al, 2007).…”