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Specziar A.,Balaton Limnological Research Institute | Gyorgy A.I.,Management Research Institute | Gyorgy A.I.,Danube Research Institute | Erodouble acutes T.,Balaton Limnological Research Institute
Journal of Fish Biology

In this study, the relative role of spatio-temporal factors and associated environmental variables (water transparency and temperature) were quantified in relation to gillnet samples of fishes in a large and shallow lake (Lake Balaton, Hungary). Most of the variance (56·1%) in the relative abundance data (%) was related to the vertical segregation of fishes. This gradient substantially affected the catch per unit effort (CPUE) by number of the dominant species, the surface-oriented bleak Alburnus alburnus and the benthic common bream Abramis brama. It also influenced total CPUE, mean fish mass and species richness and diversity. At the lake level, horizontal habitat heterogeneity (i.e. littoral v. offshore) accounted for only 8·3% of the total variance in relative abundance data, but was important in structuring the CPUE of the ruffe Gymnocephalus cernua and the pikeperch Sander lucioperca. The longitudinal environmental gradient (i.e. lake basin), year and season of sampling, water transparency and temperature had significant effects on relative abundance only at the habitat level, but were also important components of variability of CPUE in some species at the lake level. As sampling schemes need to consider the main gradients in fish assemblage distributions, the use of surface and pelagic gillnets should be more intensively incorporated in the study and monitoring of fish assemblages in shallow lakes and lake habitats. Journal of Fish Biology. © 2013 The Fisheries Society of the British Isles. Source

Non-indigenous crayfish species have successfully invaded many European lentic and lotic ecosystems in the past 120 years. Orconectes limosus (Rafinesque, 1817) has one of the largest distribution areas from among these taxa. Its invasion front has recently reached the Lower Danube while it has also been gradually colonising Middle Danubian tributaries and adjacent areas in the Carpathian Basin. A further characteristic with invasion implications was observed along the Danube River at Szeremle, Hungary. On 28th September, 2011, at 3.15 pm an adult individual of O. limosus was observed walking from the Szeremle dead arm towards the Danube River. The river and the Szeremle arm are separated by a dike during low and medium water levels. The individual crossed successfully the dike and covered a distance of approximately 20 metres between the two separate water bodies. It is the first record of O. limosus in the Danube River catchment crossing on land from one water body into another. This behaviour definitely helps the colonisation of new water bodies when only small stretches of land separate inhabited and non-inhabited water bodies. It may also help the exchange of individuals between neighbouring populations, as well as to allow the use of terrestrial escape routes in case of desiccation or pollution of the water body. Source

Juvenile fish assemblages were surveyed by electrofishing once in each season along five sections at the lower reaches of the Ipoly River from Ipolytölgyes to the mouth, and five sections along both banks of the Danube River downstream of the mouth of the Ipoly River, in 2012 and 2013. A total of 6235 individuals of 41 fish species were caught, with 6023 individuals of 37 species being juveniles. Ten of the species were non-native. Bleak (Alburnus alburnus) and other juvenile cyprinid species (e.g. Abramis brama, Aspius aspius, Barbus barbus and Chondrostoma nasus), which are present in large populations in the Hungarian Danube stretch, are the most abundant and frequent in the lower reaches of the Ipoly River contributing to 61% of the total catch. In the mouth section of the Ipoly River, the invasive Ponto-Caspian goby species are the most abundant fishes (47%). Large woody debris and flooded terrestrial vegetation were identified as the most important habitat structures to the riverine cyprinids, while ripraps were preferred by gobies. These results demonstrate that not only the different habitats in a river estuary, but also the distant spawning and nursing zones of the tributaries are especially important for the fish community of the Danube River. Source

Guti G.,Danube Research Institute
Acta Zoologica Bulgarica

All anadromous sturgeon species are critically endangered in the Danube River. Action plans and conservation programs have been worked out since 2000 with the aim to draw attention to the critical situation of sturgeon populations and convey to the public and policy makers the urgency of some remedial measures. This paper reviews the major threats to anadromous sturgeons in the Middle Danube River, i.e. historical over-exploitation of populations, loss of spawning habitats, interruption of migratory routes between the key habitats, and pollution. The suggested in-situ conservation measures are critically evaluated with regard to the efficiency of fish pass facilities at hydropower dams, the estimation of survival rates of sturgeon populations, and the assessment of stock-recruitment from artificial propagation. Source

In recent decades the Ponto-Caspian mysids Limnomysis benedeni, Hemimysis anomala, and Katamysis warpachowskyi expanded their ranges throughout the North Atlantic region and proved to have profound ecological impacts in the invaded waters. The aim of this study was to (1) provide a comprehensive description about the life history of the previously least known K. warpachowskyi, (2) reveal the number of generations produced annually by the three invasive Ponto-Caspian mysids, and to (3) compare the life history traits of the three species directly for the first time based on a simultaneous sampling. To obtain a high-resolution picture about their body length-frequency distributions, a very intensive (approximately weekly) sampling was carried out in an artificial embayment of the Danube River (in Hungary), where the three species coexist. The relatively large L. benedeni had five generations per year and produced comparatively low numbers of young, while the similar sized H. anomala completed only four generations, but compensated for this with a higher fecundity. The smaller sized K. warpachowskyi was able to produce more than five (probably 6) generations per year owing to its short maturation time and long reproductive season, and attained brood sizes close to those of L. benedeni. The generation numbers revealed by the study can be regarded as extraordinarily high considering the body size of the animals and the temperate climatic conditions, which might contribute to their invasion success by increasing the chance of establishment, especially in the course of jump dispersal events at which Ponto-Caspian mysids have proved very successful. © 2013 Elsevier GmbH. Source

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