ALBERT

Notizen: SUMMARY 1. The development of bream populations, water transparency, chlorophyll-a concentration, extent of submerged vegetation and densities of the zebra mussel, Dreissena polymorpha, were analysed in three shallow eutrophic lake systems subject to different fish management.2. In Lake Veluwemeer, the bream population was reduced from c. 100 to 20 kg ha−1 after 5 years of fishing. The mortality caused by the fishery was estimated at 38% of bream 〉15 cm in addition to a 13% natural mortality of bream 〉17 cm. The decline was followed by an expansion of the Chara beds present in the shallow parts, an increase in water transparency in the open-water zone, an increase in the density of zebra mussels and a decrease in chlorophyll-a concentrations.3. The newly created Lake Volkerak showed trends opposite to those in Lake Veluwemeer. Bream colonised the lake in 1988 and reached a biomass of c. 140 kg ha−1 in 1998. The water transparency decreased from a maximum of 3 m to c. 1 m and the chlorophyll-a concentration increased from 5 to 45 μg L−1. Submerged vegetation colonised up to 20% of the total lake area in the first 5 years after creation of the lake in 1987 but decreased to 10% as turbidity increased.4. Seine fishery in the Frisian lake system did not appear to affect the bream population despite annual catches as high as 40–50 kg ha−1. The estimated natural mortality of fish 〉15 cm was 15% and mortality by fishery was 26%. The high loss was apparently compensated by good recruitment and high growth rates resulting from a c. 1 °C higher water temperature during the years when bream were removed by fishing. There was only a slight decrease in chlorophyll-a concentrations and a slight increase in water transparency.5. The results of this study suggest that the effects of bream exploitation in eutrophic lakes can vary depending on the efficiency of the fishery, recruitment success and temperature regime. In the absence of fishery, bream dominated the fish community in the study lakes and apparently prevented D. polymorpha and submerged vegetation from establishing because of physical disturbance, enhanced internal P-loading and resettling of resuspended sediments.