The Bednja River is the longest river flowing with its full course exclusively through Croatia and one of the largest right tributaries of the Drava River, which belongs to the Danube River basin. Due to the variety of habitats present within the course of the Bednja River, as well as older literature reports, it can be expected that this river harbors a high number of freshwater fish species and would likely benefit from conservation and preservation efforts. We compiled and analyzed the existing literature data on the Bednja River fish communities and performed field investigations at various localities on this river in order to describe the current structure of its freshwater fish fauna and monitor changes induced by human activities. Our results corroborate the presence of a rich and diverse fish community. Moreover, with 36 native species, the Bednja River harbors one of the richest fish communities in Croatia as well as in the Danube basin. Unfortunately, modifications of the native fish community were evident in the form of local extinctions, reductions in the population abundance of several native species and the presence of non-native species. Habitat degradation and fragmentation were identified as the most serious threats provoking negative effects on the native fish populations, followed by predatory and competitive effects of invasive species.

Determination of the ecological state of any water body is crucial for its adequate conservation and is, moreover, required by the Water Framework Directive. Employment of multimetric indices, which integrate various anthropogenic threats to water systems with the responses of different elements of fish communities, improve our understanding and allow us to monitor the ecological state of freshwater bodies. However, design of multimetric indices that describe the ecological state of water bodies based on fish communities has already proved difficult for the Mediterranean region for several reasons, including the frequent presence of species-poor and ecologically tolerant fish communities, high endemicity and a serious lack of localities with undisturbed fish communities. The development of an ecological state index for natural water bodies in Croatia based on fish as a biological element which we present in this paper was also hindered by similar obstacles, particularly due to exceptionally high endemicity present in rivers of the Adriatic watershed, the great number of distinct water types, and significant anthropogenic pressures. Nevertheless, based on comprehensive sampling of fish communities throughout Croatia and following appropriate statistical procedures, we were able to identify stressors acting on certain water bodies, as well as responses of fish communities to them, and, finally, describe the ecological state of natural water bodies throughout Croatia. We also propose measures that will most likely help in achieving improvement and/or maintenance of the ecological states of these water bodies.

Gene structure and diversity emerged from evolutionary history of species represent a valuable and fragile gene reservoir crucial for dealing with environmental changes, as well as handy tool for discovering one species evolutionary history. Complete sequencing of the mitochondrial cytochrome b gene (cyt b) of Black Sea trout collected among Adriatic and Danube watershed in Croatia was used to describe phylogenetic and intraspecies diversity, population structure and viability, and consequently to describe colonization of this species in Croatia. In this paper we report on analysis based on cyt b gene of 414 different individuals collected on 92 different localities in order to define geographic distribution of this species in Croatia along with its phylogenetic status and populations structure. Phylogenetic reconstruction was performed using MP (maximum parsimony) and ML (maximum likelihood) phylogenetic methods, along with MJ (median-joining) algorithm which resulted in phylogenetic network. To adequately describe status and stability of populations, effective population sizes and migration rates were estimated using MIGRATE 3.2.1, intraspecific genetic diversity was approximated with DnaSP 5.10 Software as well as viability of populations, simulated with Vortex 10 software. Finally, evolutionary history of Black Sea trout was analyzed using BEAST software. Sequencing of cytochrome b gene revealed 43 different haplotypes and an interesting evolutionary pattern which was deeply affected by alternation between glacial and interglacial periods. Also, intraspecific genetic diversity indicated high variety of this species, making this area a “gene pool hot spot”, with numerous localities containing pure populations of Salmo labrax, even though some of them did not stay immune to either natural or anthropological pressures. All in all, considering mtDNA and population analyses, it seems like this particular area played a major role in present species distribution and evolution, most probably owing it to its specific geology.