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.
The record is confirmed of a “Clariobranchus” hybrid Clarias gariepinus × Heterobranchus sp. specimen in the River Danube in Croatia. Clarias gariepinus was introduced to Europe for farming and research purposes. Because of their faster growth and short time to achieve market size, “Clariobranchus” hybrids have gained attention in European aquaculture. To date, no record has been found in the literature of “Clariobranchus” hybrid escapees in inland waters of Europe. The present finding suggests that the “Clariobranchus” hybrid escaped from an unprotected aquaculture system or open pond farm. The threat of establishment of a population of this hybrid in suitable environments, such as thermal ponds, cannot be ruled out.
The bleak, Alburnus alburnus, is an invasive fish in the Iberian Peninsula, where this species mainly disturbs the higly endemic fauna via competition and aggression. Despite this impact, information on bleak autecology is scarce in the Iberian Peninsula, with no data on growth and reproduction. The aim of the present study was to compare bleak populations across four Iberian streams: Muga, Fluviá, Cardener and Foix (northeastern Iberian Peninsula).These streams have similar enviromental conditions at the regional scale (e.g. Mediterranean climate, geomorphology). In Muga and Foix streams, bleak showed lower growth rate and back-calculated lenght at age 2. Body condition was lower in Foix streams, whereas lenght at maturity was higher. In Muga stream, the proportiun of females was lower. In Cardener stream, bleak showed higher back-calculated lenght at ages I and 2, growth rate, body condition and reproductive investment. Results showed that bleak populations are able to display wide phenotypic plasticity in small Mediterranean-type rivers. Specifically, bleak population "health" appears to be better in Cardener stream, whereas it is worse in Muga and Foix streams. Present findings suggest that inter-population plasticity allows bleak more successfully to invade Mediterranean fresh waters in the Iberian Peninsula.