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

Vida, A.: Threatened fishes of the Szigetköz

salmoides is mentioned here, too. The co-occurrence of the two subspecies of Cobitis (Sabanejewia) aurata (i.e. ssp. balcanica and bulgarica) is also reported. Since there is no obvious geographical isolation in the habitats of the two subspecies, their sympatric coexistence is questionable. Gymnocephalus baloni, Gobio albipinnatus and Eudontomyzon mariae are not mentioned in their list. The main channel This is the 56 km long navigation line of the Danube, with artificial shore lines (rock fills). The frontier between Hungary and Slovakia is the main axis of the flow in the river (Thalweg). The channel gradient is the highest here in the Hungarian part of the Danube: the elevation is reduced by 19 m in 56 km. The running water of the upper part of the main channel, having increased water velocity up to 2 m/s, is the habitat of many rheophilous species that are otherwise very rare in Hungary because of the absence of similar conditions. That is the reason why the Szigetköz area is so precious for Hungarian ichthyologists. The moderately rheophilous species can only find suitable conditions downstream of the groins (transverse rode-fills) and behind the moving banks. Occasionally species characteristic to the connected backwaters also turn up here. This is also the main route of migratory fishes. Consequently, the number of fish species in the main river is extremely high (57) as compared with other water bodies in Hungary (see Table 1). Many of these are known to live exclusively here in the country (e.g. Hucho h. hucho). This is one of the rarest endemic species of the Danube System. This migratory salmonid fish has a very broad ecological tolerance. Natural reproduction of Hucho h. hucho in the Hungarian Danube cannot be excluded, but unfortunately, it is more probable that occasional catches of this species originated from artificial fry releases in Austria or Slovakia. According to Holclk (1982), this species migrates only 10 to 30 km following its prey populations. Since the river upstream the Danube-Bend is rich in Chondrostoma nasus, the main prey of Hucho h. hucho, the suitable habitat is available. Nevertheless, its protection is necessary. A ban to catch any salmonid fish in the river would be the first step towards conservation. Another species, Cottus gobio, was reported in the 1960s from the DanubeBend between Vác and Verőcemaros (Botta pers. comm.). There was no other natural habitat known of this species in Hungary. In our study between 1989-1992 an abundant Cottus gobio population was observed along the elevated current sides of the river. According to the literature (Lelek 1987), this species is generally regarded as characteristic for the salmonid zone. StraSkraba et al. (1966) have shown that competition between Cottus gobio and the brown trout is almost negligible despite earlier reports. The reproductive biology of Cottus gobio populations and their role in the food web have not been studied in Hungary so far. The side arm system of the flood plain area The 6000 ha area between the main channel and the flood-control dike in Szigetköz includes 77 river branches with a total length of 137 km. They belong to the five respective side arm systems, as follows: - Side Arm System (SAS) of Tej falusziget; - SAS of Cikolasziget; - Double SAS of Bodak-Dunaremete;