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Résumé :
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The zander (Stizostedion lucioperca, L.), an alien piscivore, was introudced into the Great Ouse System of rivers in 1963; following this introduction a decline in the cyprinid populations (mainly roach Rutilus rutilus L. and common bream Abramis brama L.) was seen. The Anglian Water Authority in an attempt to bring about a recovery in the fishstocks implemented a cull of the two main piscivores pike and zander in the Middle Level Systems of the Cambridgeshire Fens. This study concentrated on the changes in the fish community following the cull. By 1983 the total fish biomass had recovered from its previous low of 44,6 kg/ha in 1981 to a biomass of 195,1 kg/ha. This recovery occurred due to a succession of strong year-classes since 1979; the cull was shown to have influenced the patterns of recruitment and growth, along with environmental conditions. Quantitative and Qualitative population sampling coupled with age and growth studies of zander, pike, roach and common bream enabled the response of the various year-classes to be investigated, so that the effect of management on the community could be guaged. A comparative study of the feeding ecology of zander and pike, via stomach contents analysis, showed differences with implications for the fish community. The zander predates predominantly on juvenile fish and by concentrating its predation pressure in this way under certain habitats may cause population declines. The way in which environment and management can influence the impact of zander in a habitat are discussed.
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