Predation by piscivorous fish on age-0 fish: spatial and temporal variability in a biomanipulated lake (Bautzen reservoir, Germany)

Predation by perch (Perca fluviatilisL.) and zander (Stizostedion luciopercaL.) on their new-hatched fish was studied in the littoral and pelagic zone in the biomanipulated Bautzen reservoir (Germany). Sampling was conducted from the middle of May to the middle of August in 1997. To investigate predation, we compared the littoral and pelagic zone, because the littoral zone could provide a refuge area for age-0 fish. This study focused on: 1. timing of predation, 2. selectivity of predation and 3. feeding pressures. In the littoral zone, age-1 perch fed first on age-0 percids at the beginning of June, when age-0 percids reached a mean length of 20 mm. Age-2 and older perch and zander started feeding on age-0 percids in the middle of June in both habitats. At this time, age-0 percids reached a mean length of 30 mm. In both habitats, species selective feeding on age-0 perch and age-0 zander was found. Most of the sampling time zander fed selectively on their own descendants. In general, mean lengths of age-0 fish found in the predator stomachs were smaller than those in situ. The strongest decrease of age-0 fish abundance started in both areas before percids began to feed on age-0 fish. In the middle of June, predation pressure on age-0 fish was higher in the pelagic zone, whereas in July and August it was higher in the littoral zone. After a strong initial decline due to unknown factors, age-0 fish abundance was controlled by piscivores. Predation started at the time when age-0 fish gape sizes were big enough to feed on large daphnids. As age-1 perch started first feeding on age-0 fish, at high densities they could be an important source of early mortality on age-0 zooplanktivores. This would be advantageous for the daphnid population in two ways, because the efficiency of two different predators would be reduced. Age-0 fish are consumed by age-1 perch, and age-1 perch feed on age-0 fish and not on daphnids. Consequently, the feeding behaviour of age-1 perch could be a key to achieve a stable long-term biomanipulation.