0+ perch predation on 0+ bream: a case study in a eutrophic gravel pit lake

1. We studied the population dynamics of 0+ fish in a eutrophic gravel pit lake in which predation of 0+ Eurasian perch on other 0+ fish was suspected to influence perch growth and the structure of the fish community, with effects on the lake food web. 2. The adult fish community was dominated by piscivorous species, especially perch, and the 0+ fish community was dominated by perch and bream. Bream grew to a total length of 80 mm between May and the end of August, gradually decreased in numbers during the summer, and completely disappeared in autumn. Stomach analysis revealed that 0+ perch greater than or equal to28 mm fed on 0+ bream. 3. The initially unimodal cohort of 0+ perch gradually broadened and became bimodal by the end of July. Fish of the larger, piscivorous cohort grew faster (1.4 mm day(-1)) than the smaller, zooplankton-consuming fish (0.6 mm day(-1)). Although individuals of both cohorts later consumed zooplankton and grew at similar rates (0.5 mm day(-1)), only perch of the large cohort (mean TL 125 mm) were found by mid-October. Intraspecific competition for food or cannibalism of older perch may have contributed to the disappearance of the smaller perch. 4. Early piscivory of the fast-growing 0+ perch apparently resulted in the disappearance of 0+ bream by the end of the growing season and precocious maturation of male perch. In contrast to findings in other studies, these large 0+ perch thus avoided the juvenile bottleneck by switching to piscivory early during their ontogenetic development. 5. The observations of this study suggest that early piscivory of 0+ perch can have a long-lasting impact on fish communities in eutrophic lakes, particularly if prey fish are abundant and the structural complexity of the lake is low. Furthermore, early piscivory of 0+ perch may help prevent the expected increase in 0+ cyprinids following reduction of adult cyprinids, which is considered important to ensure the long-term success of biomanipulation experiments.