Abstract

In this study the role of perch within the top-down manipulated food web of the Bautzen Reservoir was analysed. To analyse the development of the manipulated fish stock fish were sampled with standard fleets of gill-nets and gill-nets of various mesh sizes at different times of the year during the investigation period from 1986 to 1999. Samplings were additionally conducted by trawling in the littoral and pelagic areas from May to August in 1997 and 1998. Other factors (e.g. age-0 fish and zooplankton abundance) were determined during the study period and analysed with regard to their influence on the feeding behaviour of perch.

The results of this study show that interactions between predator and prey cause shifts in the composition of fish stock. Size and species composition of the piscivorous fish stock were decisive for the stability of the fish community. An abundant piscivorous perch stock can, in addition to regular stocking with other piscivores, effectively control other species by predation and superior competitive abilities.

Age-1 perch were exclusively caught in the littoral zone, whereas older perch and zander differed in patterns of habitat use. In the littoral zone age-1 perch fed first on age-0 percids, when age-0 percids reached a mean length of 13.4 mm. Older perch and zander (age-1 and older) started at a mean prey length of 19.3 mm feeding on age-0 percids in both habitats at the beginning of June. Both age-1 and adult perch exhibited species-selective feeding on age-0 perch and age-0 zander, but not on age-0 roach. Predation started at the time when age-0 fish gape sizes were big enough to feed on large daphnids. In general, mean lengths of age-0 perch and age-0 zander found in the perch stomachs were smaller than those in situ. The maximum and average lengths and body depths of all prey fish species increased with increasing predator size

Feeding behaviour of age-1 perch was strongly determined by the abundance of daphnids. There was a clear functional relationship between diet composition of age-1 perch and availability of D. galeata, but age-1 perch could not be described as important predators of daphnids compared to the age-0 fish group and older planktivores (e.g. roach). Age-1 perch were at least partly piscivorous in all the years analysed. Age-2 perch fed on D. galeata only at a low level. They were mainly piscivorous.

This study indicates that the above average growth of perch in response to the high proportion of piscivores in the biomanipulated Bautzen Reservoir contributes to the strong overall piscivory of the perch population. This study provides for the first time the basis for the management of the key stone species perch by indirect factors, which will lead to a higher stability of biomanipulation.

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You can find my thesis at http://www.shaker.de/Online-Gesamtkatalog/Details.idc?ISBN=3-8265-9962-4

e-mail address: doerner@igb-berlin.de