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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 | Year: 2013

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.

Bodis E.,Danube Research Institute | Toth B.,Danube Research Institute | Sousa R.,University of Minho | Sousa R.,University of Porto
Biological Invasions | Year: 2014

The impact of Dreissena fouling on unionids has hardly been studied in Europe, despite the fact that in some ecosystems (e.g. Lake Balaton, Hungary) infestations of several hundreds to a thousand individuals per unionid have been observed. At present, the zebra mussel Dreissena polymorpha is a dominant species in Lake Balaton and in the last decade three other invasive bivalves were introduced, potentially increasing the pressure on native unionid survival. We examined whether the fouling of dreissenids (zebra and quagga (D. rostriformis bugensis) mussels) has a negative impact on native (Anodonta anatina, Unio pictorum and U. tumidus) and invasive (Corbicula fluminea and Sinanodonta woodiana) bivalves and whether there are any interspecific and temporal variations in fouling intensity and physiological condition measured by standard condition index and glycogen content. A significant negative impact was detected on native unionids only in July and September (no impact was detected in May), when the fouling rate was high. For invasive species, a significant negative impact was detected on S. woodiana with a high level of dressenid infestation; whereas no significant impact was detected on C. fluminea. Overall, this study confirms that Dreissena may threaten unionid species including the invasive S. woodiana, although high interspecific and temporal variations were observed. This situation should be taken into account in future ecological and conservational assessments because species respond differently to Dreissena fouling and effects seem to be more pronounced in late summer/early autumn. In addition, this study provides the first evidence that the invasive C. fluminea appear to be less vulnerable to dressenid fouling. © 2013 Springer Science+Business Media Dordrecht.

Bodis E.,Danube Research Institute | Toth B.,Danube Research Institute | Sousa R.,University of Minho | Sousa R.,CIIMAR – Interdisciplinary Center of Marine and Environmental
Hydrobiologia | Year: 2014

Large-scale mortality of invasive bivalves was observed in the River Danube basin in the autumn of 2011 due to a particularly low water discharge. The aim of this study was to quantify and compare the biomass of invasive and native bivalve die-offs amongst eight different sites and to assess the potential role of invasive bivalve die-offs as a resource subsidy for the adjacent terrestrial food web. Invasive bivalve die-offs dominated half of the study sites and their highest density and biomass were recorded at the warm water effluent. The density and biomass values recorded in this study are amongst the highest values recorded for aquatic ecosystems and show that a habitat affected by heated water can sustain an extremely high biomass of invasive bivalves. These mortalities highlight invasive bivalves as a major resource subsidy, possibly contributing remarkable amounts of nutrients and energy to the adjacent terrestrial ecosystem. Given the widespread occurrence of these invasive bivalves and the predicted increase in the frequency and intensity of extreme climatic events, the ecological impacts generated by their massive mortalities should be taken into account in other geographical areas as well. © 2013 Springer Science+Business Media Dordrecht.

Guti G.,Danube Research Institute
Acta Zoologica Bulgarica | Year: 2014

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.

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.

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.

Potyo I.,Danube Research Institute | Guti G.,Danube Research Institute
Acta Zoologica Bulgarica | Year: 2014

Consideration of diurnal changes of fish distribution is one of the basic requirements of standard monitoring of fluvial fish assemblages. Sampling results of day and night electrofishing were evaluated in different types of watercourses, in a very large river, that is the Danube River, as well as in a tributary of a large stream (Ipel River) and in a tributary of a small stream (Szodrákosi stream). A total of 4859 individuals belonging to 39 species were caught during the period 2012-2013. Species richness and fish abundance in the night samples was significantly higher than in the day samples at low water level in the Danube River. Diurnal variability of fish samples in the large stream was less than in the very large river. Difference between the day and the night samples was not significant in the small stream. Preliminary observations indicate an impact of running water quantity on diurnal changes in riparian fish assemblages.

Farkas-Ivanyi K.,Danube Research Institute | Guti G.,Danube Research Institute
Acta Zoologica Bulgarica | Year: 2014

Impacts of river engineering, particularly effects of channelisation were studied by analysis of historical maps from the early 19th century in the Szigetköz floodplain of the Danube River. Different hydrogeomorphological situations were investigated at a study site (Bodaki-branch system), where several deficiencies of altered river ecosystems (areal decline of aquatic habitats, change of bed load transport, decrease of lateral river-floodplain connectivity, etc.) can be recognised. Prior to extensive regulations, the Szigetköz floodplain was a highly dynamic braided and anabranching channel system. Alteration of the hydrogeomorphological processes by engineering lead to aggradation of river-floodplain ecosystem, formation of several abandoned channels and significant decline of the ecological rejuvenation of the aquatic habitats. Our results indicate long-term changes of landscape dynamics and fragmentation of the river ecosystem from the pre-regulation period up to the present situation.

Egri A.,Optics 1 | Blaho M.,Optics 1 | Kriska G.,Eötvös Loránd University | Kriska G.,Danube Research Institute | And 4 more authors.
Journal of Experimental Biology | Year: 2012

The characteristic striped appearance of zebras has provoked much speculation about its function and why the pattern has evolved, but experimental evidence is scarce. Here, we demonstrate that a zebra-striped horse model attracts far fewer horseflies (tabanids) than either homogeneous black, brown, grey or white equivalents. Such biting flies are prevalent across Africa and have considerable fitness impact on potential mammalian hosts. Besides brightness, one of the likely mechanisms underlying this protection is the polarization of reflected light from the host animal. We show that the attractiveness of striped patterns to tabanids is also reduced if only polarization modulations (parallel stripes with alternating orthogonal directions of polarization) occur in horizontal or vertical homogeneous grey surfaces. Tabanids have been shown to respond strongly to linearly polarized light, and we demonstrate here that the light and dark stripes of a zebra's coat reflect very different polarizations of light in a way that disrupts the attractiveness to tabanids. We show that the attractiveness to tabanids decreases with decreasing stripe width, and that stripes below a certain size are effective in not attracting tabanids. Further, we demonstrate that the stripe widths of zebra coats fall in a range where the striped pattern is most disruptive to tabanids. The striped coat patterns of several other large mammals may also function in reducing exposure to tabanids by similar mechanisms of differential brightness and polarization of reflected light. This work provides an experimentally supported explanation for the underlying mechanism leading to the selective advantage of a black-and-white striped coat pattern. © 2012. Published by The Company of Biologists Ltd.

Borza P.,Danube Research Institute
Limnologica | Year: 2014

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.

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