L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)
Specziár, A.: Effects of the diversion of the Danube on two ruffe species in the Szigetköz area
are based on studies conducted at the Háros section of the main channel of the Danube (1635 to 1634 rkm) in 1993 (50 specimens of G. baloni and 49 specimens of G. cernuus). The changes observed in the populations of ruffes in the Szigetköz area were described by using Fulton's condition factor (Fulton 1911, cit. Ricker 1975): CF=100 W71 3 (where "w" stands for body mass in g, "1" for standard length in cm [resulting in CF±s, where "s" stands for standard deviation]), and by the age structure of the adult population (ageing as made on the basis of scale rings). To create comparable conditions, the samples used included spring, summer and winter collectings at the same rate. To have samples with similar length distribution the older specimens (5 + - 9 + ) were excluded from the condition analysis because of their uneven presence in the catches. (The average body lengths were 84.3±16.8 mm, 85.9±8.3 mm and 87.5±10.3 mm at G. baloni. The same were 87.6±1 1.5 mm, 91.5± 14.8 mm, 93.8±9.2 mm and 93.6±10.7 mm at G. cernuus). The sex ratios of the samples were also similar (statistically not differing from 1:1). To demonstrate differences, t-test (condition) and chi-square- (%'-) tests ( a g e distribution) were performed, and the significance levels (p) of these were given. Results In the year 1993, the age structure of the adult populations of both G. cernuus and G. baloni inhabiting the branch system in the Szigetköz area underwent a change. As of G. cernuus, individuals older than 4 years in 1992 constituted 28.6% of the adult population, whereas the same was only 10.8% (p<0.05) for 1993. A similar, but even more pronounced tendency could be observed in the case of G. baloni, in which the ratio of specimens older than 4 years fell from 25% experienced between 1989 and 1992 to 3.2% (p). The reason for this decline is probably the worse adaptability of adult individuals to habitat alterations. After the diversion of the Danube, changes in the condition of the surviving populations were equally evident. For 1993 the condition of both ruffe species greatly impoverished (Fig. 1.). In G. cernuus this reduction was 17.1% (CFi992=2.45+0.31, against CFi993=2.03±0.17), whereas it was 8.6% (CFi989-92=2.78±0.28, against CFi993=2.54±0.22) in G. baloni (p<0.001 in both cases). 3.5 G cernuus G baloni Fig. 1.: The conditions (Fulton's condition factor) of Gymnocephalus baloni and G. cernuus in the Szigetköz side branch system (1989-1994) and at the Háros section of the Danube (in 1993).
