To address the dearth of information on the biology of white-eye bream in its novel range of Europe, we examined the morphology and growth rate of specimens inhabiting a dam reservoir on the lower Vistula River and compared the results with those from other waters of native and non-native regions of this species. Altogether, 24 mensural and 15 qualitative characters of 93 individuals (45 females and 48 males) as well as age of 108 specimens were determined. Body length (l) of the fish ranged from 142 to 277 mm. To examine the patterns of relative growth, relationships between 21 mensural characters (raw data) and l were tested using a non-linear regression analysis: 5 characters were best described by a linear function, 5 by a power one, 10 by a quadratic and 1 by a split-linear regression. There were significant differences in morphometric characters among the studied population and those from the Zegrze Reservoir (Vistula River catchment) and the Dnieper River, suggesting substantial morphological plasticity of white-eye bream. Meristic characters (the basic characters are expressed by the following formula: l.l. 50-57; D II-III 7-8; A III-IV 36-43; P I-III 15-18; V I-III 8-9; C IV-VII 16-18; sp.br. 18-25; 5-5) showed considerable overlap with the literature data except the number of unbranched fin rays, which revealed greater variability. The growth rate of the white-bream from the Włocławek Reservoir was moderate in the first year of life, and then it was faster than in many waters from its native range. This marked increase of growth rate seemed to be site-specific rather than region-specific, and associated with particularly rich feeding resources (abundant bottom fauna) of the eutrophic and strongly flowing dam reservoir.
Almost after a century, the occurrence of Zingel streber (Siebold, 1863) was recorded again in the area of confluence of the rivers Morava and Dyje. The population consisted of fish 0+ to 5+, with 0+ group predominating in the sample, indicating that the species had successfully reproduced in 2003. Analysis was made of their karyotype (n=5) and meristic characters (n=10). The following was the result of a study of growth of standard lengths (n=16): SL1 – 81 mm, SL2 – 114 mm, SL3 – 130 mm, SL4 – 146 mm, SL5 – 166 mm. The biggest individual was a female 5+ of age, TL 200 mm, SL 180 mm. The habitat types preferred by individuals 0+ were the rapidly flowing sections with gravel bottom, in which the stream velocity was 0.2–0.6 m.s-1. Re-occurrence of this species was facilitated by the marked improvement of water quality after 1990 as well as by the barrier-free connection of sections of the rivers Morava and Dyje with the Danube via the Slovakian-Austrian part of the River Morava. Further dispersal of this species is limited by the weir in r.km 26.7 on the Dyje, and six weirs between r.km 74.1 and r.km 101.8 on the Morava. Z. streber is protected by national law and the area mentioned above has been proposed as a pSCI for the NATURA 2000 system.