L. Forró - É. Murai szerk.: Miscellanea Zoologica Hungarica 6. 1991 (Budapest, 1991)

Csányi, B.: Changes of the fish assemblages in the Kis-Balaton Reservoir between 1985 and 1989

MISCELLANEA ZOOLOGICA HUNGARICA Tomus 6. 1991. p. 5-12 Changes of the fish assemblages in the Kis-Balaton Reservoir between 1985 and 1989 by B. CSÁNYI (Received June 25,1991) Abstract: Faunistical survey and community structure (relative frequency) data collection were carried out to follow the structural composition of fish populations. Considerable change was revealed in the abundance pattern of the fish stock during the study due to the intensive fishing impact being continuous in the reservoir since 1988. The overall dominancy of non-indigenous species has been detected (Carassius auratus gibelio, Pseudorasbora parva, Lepomis gibbosus) together with the important decrease of the carp and the pike stock. The future fish stock management needs obviously regulations concerning selective fishing, replacement or other biomanipulatory techniques. Key words: fish fauna, colonization, relative frequency, fish stock management INTRODUCTION The Kis-Balaton Reservoir (Fig. 1) connected directly to the River Zala has been established in 1985, having approximately 20 km surface area and an average depth of 1.4 m. The river water enters the flooded area through the broken section of the former protecting embankment and leaves the reservoir through sluices that can be passed by fishes occasionally. Functioning as a prereservoir, the establishment has an important role in the eutrophication control of the Lake Balaton. Large amount of the inorganic P- and N-compounds is taken up by the biota, starting with primary producers. A wide range of biological mass transport possibilities are realized in the reservoir, where the fish assemblage has a considerable importance. Since the beginning of the operation the spatial heterogeneity of the reservoir was observed taking into consideration the routine physical and chemical analyses done by the District Water Authority (Keszthely) and the community structure of different biological compartments (plankton, benthon, nekton) described by the Research Centre for Water Resources (VITUKI, Budapest). Multivariate methods were applied (PODANI 1988) in order to reveal relationships between water quality and aquatic communities (CSÁNYI et al. 1989). Three main objectives were formulated during the research program: 1. Description of the initial fish fauna after inundation; 2. Observation of the colonization and the important structural changes of fish communities in time; 3. Elaboration of a fish stock management strategy that is depending on the primary function of the reservoir concerning water quality management.