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

Fishes in the Kis-Balaton Reservoir 9 thalmus), the carp, the bleak and the pumpkinseed were the dominant species among the adults. The abundance class values of cyprinids among juveniles indicate the increment of the population size, too. The occurrence of the bitterling (Rhodeus sericeus amants), the asp (Aspius aspius),the eel (Anguilla anguilla), the weatherfish (Misgurnus fossilis), the pumpkinseed (Lepomis gibbosus) and the perch (Perca fluviatilis) was registered for the first time in 1986. Between 1986 and 1988 the contracts for fish studies were lacking and the next survey has been started only in 1989. In the mean time the Balaton Fishing Cooperative received the licence for fishing in the Kis-Balaton Reservoir in 1988 with certain restrictions of space, time and fish size. The detailed catch data of the Cooperative are not available this way the further evaluation of the fish stock development has to be based on our sampling data collected during 1989. Three of the non-indigenous species became the most common: the crucian carp, the Pseudorasbora parva and the pumpkinseed. The relative frequency of the crucian carp exceeds 50 % of the total catch. The stock of the bitterling has increased also and the tench still has a considerable frequency. All of the other taxa have shown drastic decrease in their population size compared to the former values. The lack of information on impacts of legal and illegal fishing activities taking place in the reservoir makes the complex evaluation difficult. On the other hand, the effect of selective fishing of the most valuable species (carp, pike) on the overall community structure was demonstrated by the recent changes of the abundance pattern experienced in the Kis-Balaton Reservoir. The relative frequencies of these most valuable species having the strongest fishing impact were very small in 1989. At the same time, the overall dominancy of the crucian carp became characteristic in all of the basins of the reservoir. This species, having advantages in its reproduction strategy, is not the target of the official fishing actually. Table 2. shows the relative frequency data collected by trapping and gill nets illustrating the characteristic changes of the fish assemblages in the last five years. Table 2. Relative frequency values (yearly averages) of fish species captured by trapping and gill nets in the Kis-Balaton Reservoir NOTE: Data of small-sized species are not included. TAXA 1985 1989 (Relative Frequency, % ) Esox lucius 4.5 1.5 Cyprinus carpio 20.9 0.8 Abramis brama 2.8 1.5 Aspius aspius 0 1.3 Carassius carassius 0.8 0.2 Carassius auratus gibelio 43.9 83.0 Rutilus rutilus 2.0 0.8 Scardinius erythrophthalmus 3.1 2.0 Tinea tinea 8.6 7.9 Perca fluviatilis 0.2 0.6 Silurus gianis 0 0.1 Total number of individuals 212 810