L. Forró szerk.: Miscellanea Zoologica Hungarica 8. 1993 (Budapest, 1993)

Vida, A.: Threatened fishes of the Szigetköz

or later. Such ponds can be found everywhere in the Szigetköz. Their fish fauna is the result of artificial introduction. The very minor fraction of fishes that has colonized these ponds naturally are usually cyprinids and thus these ponds are not unique by any means, so their fish fauna is not discussed here. River branch "Mosoni-Duna" Mosoni-Duna forms the S-SW boundary of the Szigetköz. It is a slow flowing water as compared to the main channel. The rivers Rába (Raab), Rábca and Lajta (Leitha) enters into this branch, strongly influencing the water level and the fish fauna. Fishermen reported the occurrence of Gasterosteus aculeatus at the Halászibridge. From an area of 10 m

2 marginal water we subsequently could catch 50-80 specimens. Being a territorial-species such a population density is surprisingly high. The first record of this species in Hungary was that published by Behyna (1938). The first voucher specimen was collected in 1956 from Budapest (Sterbetz 1957). Since that time, a few other catches were reported from the Danube in Verőcemaros, Vác and Ercsi (Botta et al. 1980, 1984). According to Berinkey (1966) this species was not introduced here but a native. On the contrary, Pintér (1989) is convinced, that the above mentioned specimens have actually been released near Vienna. Intraspecific variation is quite considerable in Gasterosteus aculeatus. Pintér (1989) mentions only a single form from Hungary. We compared all preserved Hungarian specimens, which clearly belong to at least two forms. Thus, if they originate from an Austrian introduction, they must have come from two different ones. Just like the connected backwaters (branch-system) of the flood plain, the Mosoni-Duna also receives water from the main river. Seasonal changes of the water level are also typical here, but the inflowing rivers (Rába, Rábca, Lajta) have an important effect as well. The branch is also connected with the irrigation canal-system, determining its fauna, too. The Mosoni-Duna branch is similarly rich in fish species (n=54) as the flood plain. According to some verbal communication, the extremely rare cyprinid Pararutilus frissii meidingeri might also occur at the end of the river branch. Rutiluspigus virgo surely lives in the outlets of the river. Discussion Fish diversity of the Szigetköz is higher than that of neighbouring areas (Holtik et al. 1981, Schiemer & Spindler 1989, see also Table 1). Several migratory species reproduce and develop fry in this area. The broad range in water quality (physical, chemical and biological) provides habitats for many ecologically specialized stenoecious species. All these conditions are responsible for the high species number of the Szigetköz (Table 2). Comparing the number of fish species and the percentage of the protected or endangered species in the Szigetköz, to the fauna of the sub-montane region of the Raab River (the Raab flows into the Danube in Szigetköz, and is the largest river in the area with a sufficiently large sub-montane zone), it can be concluded that the Szigetköz fauna is more species-rich and possesses more unique species