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)
rock fills in the Danube. A further proof could be for its reophilous pattern that self-sustaining populations of this species have never been reported from lakes or other standing water bodies. It has been published only from the Danube and its side rivers (Holcik & Hensel 1974, Hensel 1979, Botta et al. 1984, Harka 1984, Guelmino & Harka 1985, Sivkov 1985, Keresztessy 1993, Harka 1992a, b). Also the body pattern serves as an adaption to this habitat. Its body shape, the robust and deep body, stronger fin rays, etc., can probably be related to its habits rather than directly to its habitat. G. baloni leads a solitary life, and is found in aggregations only when conditions are suboptimal. Botta et al. (1984) assumed that G. baloni was a territorial species spawning in pairs. This behaviour is common in the family Percidae. Since we observed the dispersion of the population during the breeding season, and that less favourable habitats without current were also occupied in this period, we can conclude that territorial behaviour is increasing at these times. Since G. baloni occurs in pairs at this time the spawning in pairs is also probable. Territorial behaviour, when existing, is less apparent in the vegetative period. Further studies are required in order to gain a better understanding of its habits, which have to include observations and tests of captive individuals as well. According to our investigations, the habitats of the two species differ considerably. Some overlapping could be observed in the Háros section in the breeding season only. In this period, G. baloni and G. cernuus co-occurred in the shallow water at the rock fills of the shore line, in a proportion of 11 to 2 on average (varying between 14:1 and 2:7), in favour of G. cernuus. No overlap was found in the Szigetköz area under concern. The situation observed at Háros can probably be explained by the lack of both suitable habitats as well as spawning areas. Such areas include the river sections regulated by transverse rode-fills, which have a muddy shore. The possibility of hybridisation in such a shared habitats is probably reduced by the considerably different social behaviour of the two species (territoriality - group-wise occurrence). For the occurrence of the two species in and outside the breeding season see Figs 1 and 2, respectively. (For a comparison of breeding biology in both species see the work of Basti [1988]). The growth of G. cernuus has been studied by many authors. In Hungary, its growth in Lake Balaton (Bíró 1971) is less intense than that observed for the Danube. Elsewhere, e.g. in certain areas of Siberia, it grows at a considerably faster rate (Kolomin 1977). Growing capacity of the two species as expressed in body length is probably similar. The differences observed have to be related to the characteristics of the two sampling sites. With the exception of age group 3 + , no significant difference could be found at Háros, where G. baloni proved to reach a larger size (p<0.02). In the Szigetköz area, however, the two species exhibited a different growth pattern. Here, growth of G. cernuus was more intense (p<0.001 for age group 0 +-4 + , and p<0.05 for 5 + ). In Hungary, the food of G. cernuus has exclusively been studied in Lake Balaton (Tölg 1960, Ponyi et al. 1972). According to Fedorova & Vetkasov (1974), and Kozlova & Panasenko (1977), older specimens started to lead a semi-predatory life, whereas Leszczynski (1963) observed a strong specialisation of adult individuals (these preyed upon mostly 2-3 species only), and consumed larger amounts of fish eggs (Fedorova & Vetkasov 1974, Pavlovskiy & Sterligova 1986). As regards food composition, specific differences can also be found besides those that may be explained by differences in habitat. This is more pronounced at Háros (CN = 0.53; p<0.001 in the % 2-test), and less so in the Szigetköz (CN = 0.76; p<0.001 in the % 2-test). Leeches, gastropods and crustaceans play a more important role in the food of G. baloni than in that of G. cernuus, particularly at Háros. G. baloni also consumed a larger amount of trichopterans
