SummaryMorphometric-based condition indices are widely used to assess proximate body composition and collaterally, feeding and living conditions of fish. However, the exact relationship between condition indices and proximate body composition of fish and its relatedness to life history traits and seasonality have not been fully explored yet. Therefore, the aim of this study was to examine how the Fulton's condition factor (K-factor) is related to the chemical composition (i.e., lipid, protein, water and carbon content, and molar carbon: nitrogen ratio), length and gonadal development of fish, and how these relationships are influenced by gender and seasonality in three freshwater fish species: Amur sleeper (Perccottus glenii), pumpkinseed (Lepomis gibbosus) and rudd (Scardinius erythrophthalmus). We found that the strength and direction of association between the K-factor and proximate body composition can vary markedly among fish species. The K-factor correlated positively with gonadal development in pumpkinseed and Amur sleeper, while no such relationship existed in rudd. Condition factor can be a reliable measure of lipid content; however, the relationship was stronger in species with higher and more variable lipid contents.Moreover, we found striking and consistently negative linkage between the K-factor and water content of the fish body, which correspond with the findings of several other studies. In turn, we could not detect any relationship between the K-factor and protein content of fish. Gender seemed to exert negligible effect on the relationship between the K-factor and proximate body composition, while seasonal variance was obvious in most relationships.

Three species of small-sized rheophilic Barbus fishes are endemic to and widely distributed throughout the mountain regions in the Danube River basin. In Hungary, barbels referred to as B. petenyi occur in streams in the foothills of the Carpathians near the borders with Slovakia, Ukraine and Romania. However, up to now, no genetic investigations were carried out on rheophilic barbels in this region. This study aims to clarify the taxonomic identity and distribution of the rheophilic barbels in the Hungarian plain based on molecular and morphological analyses. Two mitochondrial genes (cytochrome b, ATPase 6/8) and one nuclear gene (beta-actin intron 2) were sequenced and several morphometric and meristic characters were recorded. Phylogenetic and morphological analyses revealed that there are four genetically distinct lineages among the rheophilic barbels in the Carpathian Basin. The results demonstrated that North-Hungarian Barbus populations belong to B. carpathicus and that B. petenyi presumably does not occur in Hungary. As expected, B. balcanicus was only recorded in samples from the Balkans analyzed for reference. A distinct species, new to science, was discovered to be present in Sebes-Körös River (Crişul Repede) in eastern Hungary and western Romania and is formally described here as B. biharicus Antal, László, Kotlík - sp. nov.