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)

caddisflies and crustaceans (Isopoda, Corophiidae, Gammaridae) in the food of this species is considerable. In the Szigetköz area, on the contrary, the role of other groups in the food other than chironomid larvae is much less significant (CN= 0.56; p<0.001 in the % -test). Only clams (Pisidium sp.) and Corophium curvispinum can be mentioned in this respect. The food contained plant matter in very small quantities in both areas under concern. In spring, occasionally fish eggs, and in one instance in the Szigetköz area even a fish (Proterorhinus marmoratus) could be found. The composition of the food changed strongly during the year in both areas. While a gradual reduction in the proportion of chironomid larvae (from 63.5% to 39.1%) in favour of amphipods (from 14.4% to 42.5%) could be observed in the Háros section, an increase in the proportion of chironomid larvae (59.0% to 91.9%) became apparent in the Szigetköz. Gymnocephalus cernuus The food composition in G. cernuus is shown in Figs 5c and 5d. Significant differences were found between the two habitats also in this species (CN= 0.43; p<in the %

2-test). The food consumed was more varied (D= 1.45) in the Háros section. It consisted mainly of chironomid larvae and pupae, and gammarids. Besides, in smaller amounts also Corophium curvispinum, ephemeropteran and trichopteran larvae were found. Although the number of cyclopid specimens in the samples was considerable, their overall role in the food mass is negligible. The proportion of leeches is similarly insignificant; segmented worms have been found in a few instances only, while gastropods are lacking entirely. By contrast, the main component in the food of G. cernuus in the Szigetköz area were chironomid larvae and pupae (D = 1.01). The proportion of other food components is well-balanced, but extremely small. This species consumes also fish eggs in spring. The changes observed in food components were similar at both sampling sites. The proportion of chironomid larvae strongly increased in summer, that of Corophium curvispinum in spring and autumn. With the exception of the summer period, when a significant dwindling of the food basis (D = 0.50) could be observed, the results of our investigations in the Szigetköz during 1993 were similar to those described by Nagy (1985) for the Csallóköz area. Elsewhere, the food of G. cernuus may be of entirely different composition. Discussion We found several ecological and ethological differences between G. baloni and G. cernuus originally separated on a morphological basis. Most important of these is the divergence in habitat, as this probably explains most of the differences found. For further investigation, a more detailed study of the deep- and shallow-bodied morphs of G cernuus described from the Dneiper (Aleksandrova 1974) could be of possible help. From the determinating characters, we suggest to use at least one more character (e.g. the degree of notching at the All) in addition to coloration features, when working in the field. When working in the laboratory, we recommend to use as many reliable characteristics as possible. Recently conducted morphological studies (Kovac 1993a, b) may serve as a basis for separating the two species in their early stages of ontogeny. Based on our investigations, we concur with Holcik & Hensel (1974) and Botta etal. (1984) that G. baloni is bound to hiding-places located in flowing waters, so it is mostly found at