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Over the last 14 years, ichthyological and ecological parameters have been monitored in the Labudovo okno Ramsar site. This area is important for its biodiversity as it is home to many rare and endangered plants and animal species. A total of 3861 fish specimens were sampled and measured at six sampling sites four times during the sampling period. An analysis of biodiversity indexes, relative biomass (kg/ha), and relative annual production (kg/ha) was carried out to assess the effectiveness of existing conservation measures. The results obtained show a trend decline in biodiversity, relative biomass, and relative annual production. This indicates a biodiversity conservation problem that should be addressed through other mechanisms in addition to the principles of the Ramsar Convention.

Small run‐of‐river hydropower plants (RRHPs) have revealed strong harmful effects worldwide on stream habitats (e.g., fragmentation and destruction) and decreased fish species, especially anadromous species abundance. Recently, RRHPs have rapidly been installed in Serbia. Most were installed on montane streams because their steep slopes are most convenient for hydropower energy production at minimal costs. Brown trout Salmo trutta are prominent in the fish communities of this rarest type of aquatic ecosystems. Their native molecular diversity reflects a biodiversity hotspot in the Balkans, and they provide attractive fishing opportunities. Records from fishery management plans for highland stream fisheries where RRHPs were installed revealed reduced brown trout biomass compared with streams without RRHPs. Research on six streams with operational RRHPs revealed severe deterioration of habitat, for example, increased water temperature, reduced dissolved oxygen, and increased nutrient contents. Effects on fish communities were evident from a change in their structure. Resident, stream‐dwelling brown trout were the most affected species with steep declines in abundance, biomass, and productivity. Fragmentation owing to damming was also reflected by deterioration of their age structure. Unique native brown trout stocks susceptible to adverse effects are difficult to restore by stocking, and fish passages commonly supplied as compensation to overcome the disruption of habitats cannot mitigate the threat posed to the fish.

Streams’ order, their width and depth, as well as the water temperature, conductivity, dissolved oxygen contents and pH reaction were recorded for 18 streams of the Danube River basin in Serbia that home brown trout Salmo trutta. For each of them, the number of fish species was recorded, and for brown trout in them the number of cohorts, age-structure, productivity (biomass, annual production and average weight) and growth (von Betalanffy’s growth parameters L∞, t0, K and ø’) were calculated. They were examined in relation to stream’s habitat features. Stepwise multiple regression revealed strong interdependence between habitat variables. Size, i.e., width and depth of streams increased and their oxygen content dropped with increase in their order and rise of the water temperature, and water conductivity was strongly interrelated with order and depth of streams. Increase in streams’ order and their water’s temperature were accompanied with the increase of species number in them, number of brown trout cohorts raised with the water conductivity and dropped with the increase of the alkaline pH value of streams, whereas growth of brown trout was determined by width of streams and their water’s conductivity. However, when analyzed one at a time, very few of habitat and population traits, e.g., stream depth and L∞, varied clearly in streams of either various orders, or conductivity classes, respectively. The notable, but not yet significant variability of almost all other characters implicates their strong and complex acting in concert to determining number of fish species in syntopy, brown trout population, productivity and growth features.

Background. The diversity of wild trout stocks in Montenegro is compromised by insufficiently controlled stocking. Adriatic and Mediterranean areas show a high degree of endemism of the salmonid species, with numerous native Salmo taxa described. The invasive effect of brown trout, Salmo trutta Linnaeus, 1758, of the Atlantic haplogroup on the endemic Adriatic trout, Salmo farioides Karaman, 1938, in the drainage area of the Adriatic Sea basin is lower compared to the effect inflicted by S. trutta has on Salmo labrax Pallas, 1814 of the Danubian haplogroup. The presently reported study was intended to describe the genetic structure of the population of Adriatic trout, Salmo farioides, from the Mrtvica River, a right tributary of the Morača River, Montenegro, in order to get an insight into the spread of non-native strains and their introgression into the native Adriatic trout gene pool. Materials and methods. Trout specimens sampled from the lower section of the Mrtvica River from 2004 to 2007, and from its upper section in the spring of 2014, were analysed for their mtDNA haplotypes. Nuclear DNA markers (microsatellites, LDH-C1* gene) were also included in genetic analyses since mtDNA is inherited only maternally. After statistical analyses, relations between individuals from the upper and lower Mrtvica River were reconstructed. Results. Using both LDH-C1* and seven microsatellite loci with several alleles exclusive to S. trutta, an introgression was detected only in the upper reaches of the Mrtvica River, where only stream-dwelling trout form occurs. In the lower reaches no introgression was detected at all, as revealed by the absence of both the Atlantic mitochondrial haplotype and microsatellite alleles exclusive to S. trutta. Conclusion. The allelic frequency at seven microsatellite loci of Salmo spp. from the two river sections revealed very different population structure, as a consequence of a low rate of gene flow between them. In addition to the physical barrier that prevents Salmo sp. from the lower Mrtvica River to migrate to the upper river section, it seems that the opportunity to mate with the large pool of conspecifics, including lake-dwelling S. farioides (i.e., Salmo cf. dentex), prevents Salmo trutta from surviving at the lower stretch and spread to the rest of the Morača River and Lake Skadar systems.

Background. The molecular diversity of brown trout, Salmo trutta Linnaeus, 1758, has been poorly studied in Croatia. The control region of mitochondrial DNA (CR mtDNA) is in addition to other molecular markers a reliable for identifying phylogenetic lineages (haplogroups) and haplotypes of brown trout. Based on analyses of the control region of mitochondrial DNA several major brown trout phylogenetic lineages were identified of which the Danubian (DA) haplotypes, though not all, are considered native to Croatian rivers belonging to the Danube basin. The introduction of allochthonous haplotypes into natural streams seriously threatens the genetic diversity of this species. Therefore, the aim of this study was to map brown trout populations inhabiting Croatian rivers of the Danube River basin and to investigate their molecular diversity and phylogeographic patterns of the established haplotypes. Materials and methods. Anal fin tissue was taken from 141 specimens of brown trout in 14 localities in the protected areas of Croatia, situated in the mountainous regions of Gorski Kotar, Žumberak, as well as Mountain Papuk in the western Slavonia. The total DNA was extracted and then the amplification of the mtDNA control region was carried out using primers Trutta-mt-F and HN20. Amplification of the 440 bp long region of the LDH-C1 gene locus was done using primers Ldhxon3F and Ldhxon4R. Amplified LDH-C* fragments were used for Restriction Fragment Length Polymorphism (RFLP) analysis using BselI restriction enzyme. Results. Analysis of the CR mtDNA revealed the presence of two phylogenetic lineages, the DA and the Atlantic (AT). Haplotypes Da1, Da2, and Da22 were recorded within the DA lineage and At1 was recorded within the AT haplogroup. Two new haplotypes were described for the first time in this study and are named Da1f and Da1g. Restriction analysis of the lactate dehydrogenase gene locus revealed a high degree of hybridization between brown trout of DA and AT haplogroups. Conclusion. The results of this study confirmed the complex molecular diversity of brown trout and the high degree of the introduction of non-native haplogroups into rivers of the Danube basin in Croatia. Conservation of native brown trout populations has become evident, as introduced allochthonous DA and AT haplogroups severely disrupt the indigenous brown trout stock.

This study examined the relationship between the sagittal otolith morphometric variables (length, height and weight) and body growth of the European eel. Eels that were studied ranged in total length from 11.2 to 79.5 cm. The relationships between the sagittal otolith variables and fish somatic growth were described with a non-linear function. The resulting coefficients of determination ((r^2)) ranged from 0.782 to 0.914. The variable most strongly related to fish size was found to be the sagittal otolith length (OL) with 91.4 % of the variability. The results of this study provide the first comprehensive data regarding the relationship between the sagittal otolith morphometric variables with the body length of Anguilla anguilla., U ovom radu se opisuje odnos između morfometrijskih varijabli sagitalnog otolita (duljina, visina i težina) i rasta tijela europske jegulje. Proučavane jegulje imale su raspon ukupne dužine od 11,2 do 79,5 cm. Odnos između sagitalnih otolitskih varijabli i somatskog rasta ribe opisani su nelinearnom funkcijom. Dobiveni koeficijenti određivanja ((r^2)) kretali su se od 0,782 do 0,914. Varijabla koja je najsnažnije povezana s veličinom riba utvrđena je na temelju sagitalne duljine otolita (OL) s 91,4% varijabilnosti. Rezultati ove studije pružaju prve sveobuhvatne podatke o odnosu između morfometrijskih varijabli sagitalnog otolita sa dužinom tijela Anguilla anguilla.

Brown trout is a salmonid fish with a natural range extending throughout western Eurasia and North Africa. Due to its commercial value, it has also been introduced worldwide. In continental Croatia, introduced trout of the Atlantic lineage hybridizes with native trout of the Danubian lineage, threatening the native genetic diversity. The geometric morphometrics approach was used in this study to analyse changes in shape between native trout, introduced trout and their hybrids, classified a priori by molecular phylogenetic analyses. A total of 19 landmarks and semi-landmarks were used to capture the shape of 92 trout individuals belonging to two lineages and their hybrids. Canonical variate analysis and discriminant function analysis were used to analyse and describe shape variation. A significant difference was found between the shape of the Atlantic lineage trout and both Danubian lineage trout and hybrids, with the most prominent differences in body depth, head length and eye size. No statistically significant shape differences were observed between Danubian lineage trout and the hybrids. The observed significant differences in shape could be the result of genetic diversity or trout phenotypic plasticity. Further studies are needed to clarify the origin of this variation in shape.