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Dendritic habitats, such as river ecosystems, promote the persistence of species by favouring spatial asynchronous dynamics among branches. Yet, our understanding of how network topology influences metapopulation synchrony in these ecosystems remains limited. Here, we introduce the concept of fluvial synchrogram to formulate and test expectations regarding the geography of metapopulation synchrony across watersheds. By combining theoretical simulations and an extensive fish population time-series dataset across Europe, we provide evidence that fish metapopulations can be buffered against synchronous dynamics as a direct consequence of network connectivity and branching complexity. Synchrony was higher between populations connected by direct water flow and decayed faster with distance over the Euclidean than the watercourse dimension. Likewise, synchrony decayed faster with distance in headwater than mainstem populations of the same basin. As network topology and flow directionality generate fundamental spatial patterns of synchrony in fish metapopulations, empirical synchrograms can aid knowledge advancement and inform conservation strategies in complex habitats.

The introduction and spread of alien fish species pose a major threat to native communities and ecosystem functioning in freshwaters. Black bullhead is one of the most successful invaders in European waters with several detrimental effects on native biota and ecosystems. In this study, we used stable isotope analysis to compare the body size and season-dependent diet, trophic position, isotopic niche size, and niche overlap of the invasive black bullhead with two native fish species (roach and European perch) in Lake Balaton, Hungary. We found that black bullhead could be characterized by invertivore-piscivorous feeding habit with a high rate of fish consumption. The rate of fish predation by invasive black bullhead increased with body size, while no seasonal differences were observed in fish consumption. Contrary to our hypothesis, little evidence of actual feeding competition was found between black bullhead and native fishes. Our results suggest that the studied species assimilate distinct energy resources in different proportions leading to a substantial amount of niche partitioning among them. We conclude that black bullhead may represent a threat for native, small-sized fishes primarily through predation and recommend urgent management actions (e.g. selective removal of the species) to minimize its adverse impacts on native communities. [ABSTRACT FROM AUTHOR]

A fish-based index is proposed to indicate the ecological status of Lake Balaton, Hungary in accordance with the standard of the European Water Framework Directive (WFD). The Balaton fish index (BFI) synthetises information of 13 lake-specific fish metrics including gillnetting and electric fishing data of species richness of native assemblages, relative abundance, biomass and age structure of native key species, representation of non-native species and general health status. The main anthropogenic pressures considered were the degradation of littoral habitats, invasion of non-native fish species, eutrophication and fishing/angling including stocking. Ecological quality ratio (EQR) is assessed by relating actual fish assemblage metrics to the supposed undisturbed reference status of Lake Balaton reconstructed by expert judgement based on recent and historic information on the fish fauna and its changes. Values of BFI were consistent and indicated good ecological status of Lake Balaton in the period of 2005–2018. This study provides an example on how an EQR assessment methodology might be established in unique habitats with no possibilities for statistical evaluation of pressure-respond relationships and exact determination of the reference status.

Biotic interactions exerted by invasive species have a strong effect on ecosystems. Intraguild predation may contribute to the decline in the distribution, abundance and population size of native species and may facilitate the spread of non-native taxa. In this study, we investigated the feeding ecology of the invasive fish Amur sleeper (Perccottus glenii Dybowski, 1877) in a lowland watercourse, where it co-exists with the threatened native fish European mudminnow (Umbra krameri Walbaum, 1792). We used two sampling protocols that differed in the frequency of sampling time (e.g. monthly samplings and samplings in 10- day intervals) to provide evidence of predation, an interaction that may lead to the decline of mudminnow populations with the spread of the Amur sleeper. Aquatic macroinvertebrates comprised a major part of the diet for both sampling intervals. However, finer temporal resolution revealed the importance of fish, especially mudminnow juveniles, as a periodically available food source in the Amur sleeper's diet. A high degree of dietary overlap was found between the different size groups of the Amur sleeper, but larger specimens tended to feed on a relatively higher proportion of fish. Our results suggested that temporal resolution of stomach content analyses may largely determine inferences on the importance of predation on juvenile mudminnow. Overall, we found that intraguild predation could contribute to the decline of European mudminnow populations, which underscores the importance of effective control measures to prevent the further spread of the invasive Amur sleeper. [ABSTRACT FROM AUTHOR]

Direct or indirect effects of nuclear power plants' (NPPs) warmwater effluents on the structure of biotic assemblages are poorly known in very large rivers. We examined changes in physical habitat structure, temperature condition and their possible effects on the structure of Danubian fish assemblages due to the outflow of the Paks NPP's warmwater channel, in Hungary. Seasonal surveys conducted both upstream and downstream from the outfall showed that its hydromorphological effects were generally local and comparable to natural or other anthropogenic hydromorphological changes. The effect of the returned cooling water was more apparent in the seasonally recorded surface water temperatures and depended highly on the spatial positioning of the sampling sites. However, environmental and spatial variables accounted only for a low amount of variance in case of both shoreline and offshore fish assemblage data. Overall, we found that the outflow exerted only a local scale effect on the structure of Danubian fish assemblages. Rather, fish assemblages varied largely both inshore and offshore, which dynamics overruled any effects of the artificially elevated temperature. Our study highlights the importance of the assessment of hydrogeomorphological variability of rivers and their influence on fish assemblage variability when examining spatial effects of thermal pollution. [ABSTRACT FROM AUTHOR]

Global ecosystems are facing a deepening biodiversity crisis, necessitating robust approaches to quantifying species extinction risk. The lower limit of the macroecological relationship between species range and body size has long been hypothesized as an estimate of the relationship between the minimum viable range size (MVRS) needed for species persistence and the organismal traits that affect space and resource requirements. Here, we perform the first explicit test of this assumption by confronting the MVRS predicted by the range‐body size relationship with an independent estimate based on the scale of synchrony in abundance among spatially separated populations of riverine fish. We provide clear evidence of a positive relationship between the scale of synchrony and species body size, and strong support for the MVRS set by the lower limit of the range‐body size macroecological relationship. This MVRS may help prioritize first evaluations for unassessed or data‐deficient taxa in global conservation assessments. [ABSTRACT FROM AUTHOR]

Environmental DNA (eDNA) metabarcoding is an effective method for studying fish communities but allows only an estimation of relative species abundance (density/biomass). Here, we combine metabarcoding with an estimation of the total abundance of eDNA amplified by our universal marker (teleo) using a quantitative (q)PCR approach to infer the absolute abundance of fish species. We carried out a 2850‐km eDNA survey within the Danube catchment using a spatial integrative sampling protocol coupled with traditional electrofishing for fish biomass and density estimation. Total fish eDNA concentrations and total fish abundance were highly correlated. The correlation between eDNA concentrations per taxon and absolute specific abundance was of comparable strength when all sites were pooled and remained significant when the sites were considered separately. Furthermore, a nonlinear mixed model showed that species richness was underestimated when the amount of teleo‐DNA extracted from a sample was below a threshold of 0.65 × 106 copies of eDNA. This result, combined with the decrease in teleo‐DNA concentration by several orders of magnitude with river size, highlights the need to increase sampling effort in large rivers. Our results provide a comprehensive description of longitudinal changes in fish communities and underline our combined metabarcoding/qPCR approach for biomonitoring and bioassessment surveys when a rough estimate of absolute species abundance is sufficient. [ABSTRACT FROM AUTHOR]