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)

The spawning of this species is most intense from mid-April to the end of May. From the end of May until early June some spawning could be observed, though only in flowing areas. Besides the so far published habitats, G. baloni was also found in the rivers Szamos, Mura and Túr. Gymnoceplialus cernuus The occurrence of G. cernuus was characteristic in areas with an open bottom. This habitat consisted of the area between and beyond the two transverse rode-fills in the Háros section, and of the open sections between the cross dams in the Szigetköz. Some exceptions were seen during and before the breeding season. In this period, from March to April, G. cernuus individuals could be observed in large numbers in the shallow littoral region with no water current. Both in the vegetative, as well as in the generative season, the occurrence of this species was group-wise. In an individual school, three to four age classes were present, but usually there was one dominant age group. G. cernuus could mostly been observed in areas with no water current (Figs 1, 2). However, in some occasions, feeding schools moved into whirly areas behind rode-fills. As regards substrate type, this species could be found both on gravel, sand and clay bottom, as well as on muddy substrate. In the Háros section, it preferred muddy, sandy bottom to other types. The spawning of G. cernuus lasted from early April to the beginning of June in the areas under concern, and took place primarily in the shallow water at the rock fills of the shore line. Growth Gymnocephalus baloni Studies of the growth of G. baloni have not yet been published. According to our investigations, it is a slowly growing species, reaching 12-13 cm average length at the age of 7 years. Growth is the most intense in the first year (31-32 mm), increases evenly during the second to fourth summer (15.5-18.2 mm/year), and slowly decreases afterwards, amounting 7-12 mm annually in the sixth and seventh year of age (Table 3, Figs 3, 4). The growth of the G. baloni populations is statistically similar in both habitats investigated with the exception of age group 0 (p<0.05 in favour of the Háros population^. Growth of G. baloni at Háros can be described by Bertalanffy's curve Lt (cm) = 19.4 (1-e" 133 ^ + °297

))

5 and in the Szigetköz by L

t (cm) = 23.4 (l-e"°'

0% (t + a5I4)

). Body length/body mass relations were determined on the basis of samples collected in 1993. Significant differences were found between the two habitats (see also Specziár 1995): lgw = 3.48 IgL - 5.51 (R

2 = 0.96, n = 50) (Háros) lgw = 2.91 IgL - 4.43 (R

2 = 0.97, n = 31) (Szigetköz). No statistically significant differences could be found between the growth of males and females. However, males reached a somewhat larger size in all age groups in both habitats. Gymnocephalus cernuus This species is also slowly growing, reaching 32-36 mm body length in the first year of age in the Danube. 6-8 years are needed to attain 12-13 cm body length. Growth slows after the first year. It is more or less even in the second to fifth year (14.8-19.7 mm/year), but strongly