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

Specziár, A.; Vida, A.: Comparative study of Gymnocephalus cernuus (Linnaeus, 1758) and G. baloni Holcik and Hensel, 1974 (Pisces, Percidae)

3. Number of opercular spines is: * 2 = G. baloni * 1 = G. cernuus (after Holcik & Hensel 1974, Sivkov 1985). This character is extremely useful, as it can be seen even when investigating stomach contents of predatory fish. 4. Body depth (after Holcik & Hensel 1974, Sivkov 1985). This characteristics usually separates the two species very well (G. baloni has a larger relative body depth) with a reliability of 90-96% (CD = 1.28-1.75). 5. Number of praeopercular spines is: * 12-13 = G. baloni * 8-11 = G. cernuus (after Botta et al. 1984). This characteristic is of no use, as our results show a great overlapping: 9-14 in G. baloni and 8-11 in G. cernuus (see Holcik & Hensel 1974, Sivkov 1985). 6. The form of the soft membrane connecting two rays in the anal fin is: * deeply notched = G. baloni * barely notched = G. cernuus (after Botta et al. 1984, Pintér 1989). This peculiarity is of no use when working with improperly stored specimens, in which the membrane separates itself, and with fluid-preserved materials. In other instances it is a reliable character. According to our studies, the difference in notching at the AU (its extent as expressed in % of the All-length): * G. baloni = 40.93±8.21 (Háros); 38.5O±6.08 (Szigetköz) * G. cernuus = 12.61±7.74 (Háros); 13.28±5.93 (Szigetköz) t = 13.75; p<0.001; CD = 1.78 (Háros) t = 16.13; p<0.001; CD = 2.10 (Szigetköz). So this key character can be used with 97-99% reliability. Habitat Gymnocephalus baloni The occurrence of G. baloni in the area under concern was, with one or two exceptions, restricted to the littoral zone. Eight individuals were found among submerged tree trunks, six juvenile and two adult specimens were recovered on a sandy or gravel bottom. The littoral region in the Háros section consisted of two transverse rode-fills and additional rock fills on the shore line with a length of 30 and 100 m, respectively. The same littoral habitat in the Szigetköz area consisted of cross dams isolating the side arms from the main arm. From our collectings we can assume that this species prefers extensive cover. G. baloni occurs either solitarily, or in pairs during the breeding season. In the vegetative period only occasional aggregations could be observed (for instance during critically low water levels). The species exhibits a dual adaptation to flow characteristics. In the vegetative period, it requires high water velocity. Maximal occurrence was observed at 8.9+4.0 cm/s velocity. In this period it could not be observed in areas without current (Fig. 2). In and prior to the generative period (mostly from the end of March to mid-May) a balanced distribution of 1-2