L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Specziár, A.: Effects of the diversion of the Danube on two ruffe species in the Szigetköz area

In the year 1994, some improvement in the condition of G. cernuus could be experienced (CFi994=2.19±0.13, p 0.001), but this was still very much below the level of 1992. As revealed by our observations, similar tendencies could be experienced in the case of breams, but due to the lack of earlier data, no direct comparison could be made. Additionally, G. baloni could be found only in limited numbers in 1993, and subsequently it could no longer be collected in 1994, even though it was abundant in the area until the end of 1992 (e.g. at the Gombócos, Halrekesztő and Arvai cross dams). The impoverishment of the condition of the ruffes can possibly be explained by the food scarcity that resulted from habitat diminution. In the case of G. baloni, another reason could be the reduction of the current velocity (parallel with the reduction of discharge) and the silting-up connected herewith. (Before the diversion, seasons of limited discharge occurred e.g. during winter, but these were soon to be followed by seasons of higher discharge, which thoroughly flushed the branches. In contrary, after diversion the water flow in most of the branches was reduced or became stagnant) Conclusions 1. The diversion of the Danube most strongly affected the older specimens of the two ruffe species. 2. The condition of surviving ruffe populations strongly impoverished to 1993, which could be explained by the food scarcity occurring after the dwindling of habitats. 3. The impoverished condition of the populations and the absence of older individuals will on the long run negatively affect both ruffe species, as the above tendencies possibly influenced spawning in 1993 and 1994. 4. The regeneration of the G cernuus population started already in 1994, and within a few years the population is expected to reach a state of balance on a possibly reduced level. 5. G. baloni could not be found in the Szigetköz in 1994 in spite of intensive searching, which, unfortunately, may indicate that this fish species (protected in Hungary since 1993) is in danger of becoming extinct in the area under concern. References Guti, G. (1993): Fisheries ecology of the Danube in the Szigetköz floodplain. - Oposc. Zool., Budapest 26: 67-75. Holcik, J., Basti, I., Ertl, M. & Vranovsky, M. (1981): Hydrobiology and ichthyology of the Czechoslovak Danube in relation to predicted changes after the construction of the Gabcikovo-Nagymaros river barrage system. - Works Lab. Fish. Res. Hydrobiol. 3: 19-158. Jancsó, K. & Tóth, J. (1987): A kisalföldi Duna-szakasz és a kapcsolódó mellékvizek halai és halászata (Fishes of, and fishery in the Danube section of the Kisalföld area and its side-branches). - In: Dvihally, Zs. (ed.) A kisalföldi Duna-szakasz ökológiája (The ecology of the Danube section of the Kisalföld), Veszprém. Ricker, W. E. (1975): Computation and interpretation of biological statistics of fish populations. - Bull. Fish Res. Bd Can. 191. 382 pp. Tóth, J. (1983): A Bős-Nagymarosi vízlépcsőrendszer környezeti hatásairól és néhány várható ökológiai problémáról (On environmental effects of, and some expected ecological problems related to the Bős-Nagymaros dam-system). - Földr. Közi. 31: 1-11.