Hubert Keckeis

Empirical evidence integrating multiple biological levels on freshwater biodiversity persistence in regulated river basins remains scarce for Neotropical systems. We evaluated fish assemblage structure, reproductive strategies, and life-cycle support for migratory species across lotic and lentic habitats in three tributaries of the Porto Primavera Reservoir, Upper Paraná basin, Brazil, over a 12-year monitoring period (2007–2018), encompassing 104 species and a Life-cycle Support Index (LSI) based on occurrence of eggs, larvae, juveniles, and adults. Marginal lagoons functioned as biodiversity hotspots, with higher richness and diversity than river channels; reproductive guilds were habitat-partitioned, with opportunistic and parental-care strategists in lentic environments and periodic/migratory strategists in lotic habitats. Tributary identity was the strongest predictor of assemblage composition: the Aguapeí River, partially protected, showed the highest LSI values, whereas the Verde River provided the strongest evidence of reproduction for conservation-priority taxa, including the critically endangered Brycon orbignyanus, whose adults occurred exclusively there, and the vulnerable Pseudoplatystoma corruscans, for which adults, eggs, and larvae co-occurred. Three proposed hydropower developments on the Verde River would fragment a 200-km free-flowing stretch, threatening these conditions. Undammed tributaries are irreplaceable for freshwater biodiversity and should be recognized as conservation priorities in hydropower licensing and basin governance.

Empirical evidence integrating multiple biological levels on freshwater biodiversity persistence in regulated river basins remains scarce for Neotropical systems. We evaluated fish assemblage structure, reproductive strategies, and life-cycle support for migratory species across lotic and lentic habitats in three tributaries of the Porto Primavera Reservoir, Upper Paraná basin, Brazil, over an 11-year monitoring period (2007–2018), encompassing 104 species and a Life-cycle Support Index (LSI) based on occurrence of eggs, larvae, juveniles, and adults. Marginal lagoons functioned as biodiversity hotspots, with higher richness and diversity than river channels; reproductive guilds were habitat-partitioned, with opportunistic and parental-care strategists in lentic environments and periodic/migratory strategists in lotic habitats. Tributary identity was the strongest predictor of assemblage composition: the Aguapeí River, partially protected, showed the highest LSI values, whereas the Verde River provided the strongest evidence of reproduction for conservation-priority taxa, including the critically endangered Brycon orbignyanus, whose adults occurred exclusively there, and the vulnerable Pseudoplatystoma corruscans, for which adults, eggs, and larvae co-occurred. Three proposed hydropower developments on the Verde River would fragment a 200-km free-flowing stretch, threatening these conditions. Undammed tributaries are irreplaceable for freshwater biodiversity and should be recognized as conservation priorities in hydro-power licensing and basin governance.

Introduction: River regulation schemes generally led to changes in riverine assemblage composition and their trophic status, which often is reflected in a general loss or decrease of characteristic species and a concomitant increase or dominance of generalist species. Objectives: This study examines changes of the hydrodynamic conditions, plankton biomass as well as changes in feeding guilds and the assemblages of macrozoobenthos and of fishes before and after the re‐connection of a sidearm of the main stem in a free‐flowing section of a large river (Danube in Austria). Methods: In order to analyze the effects in the sidearm, a before–after control‐impact design was applied; a river section in the main stem served as a reference site. Results: The higher and increased connectivity had sizable effects on hydraulic parameters, with the expected shift of prevailing lentic to year‐round lotic conditions in the sidearm. The composition of suspended solids changed significantly. Likewise, chlorophyll‐ a concentrations and zooplankton abundance decreased markedly. The new conditions in turn modulated the pathways and the structure of the food web. Conclusions: The permanent flow‐through in the sidearm decreased the importance of detritus and forced the one of periphyton and biofilm as an energy and nutrient source for insect larvae which also affected higher trophic levels, that is, fishes.

Seasonal drift patterns of fish eggs and larvae were analysed along the river section downstream the Iguaçu River Falls to the confluence with Paraná River, over three spawning seasons. This river section is situated in a narrow rocky canyon characterized by a steep slope, which leads to highly turbulent and fast flowing conditions in a series of rapids. Water transit time in this river stretch of 20 km was short (5–11 h), the flow‐velocity in rapids can reach values of more than 6 m s⁻¹. Densities of drifting eggs and larvae varied between three spawning seasons, and differences of eggs were more pronounced compared to larvae. Eggs and larvae were observed all along the river section. A spatial, longitudinal distribution pattern of larvae was found, with higher numbers in the upper section. A distinct spatial sequence of different embryonic stages was observed, with the highest abundance of early stages. Larval densities were low in comparison to egg densities, and preflexion and flexion stages predominated. High abundance of buoyant eggs in drift indicates fish spawning in rapid and turbulent waters of the Iguaçu River, particularly downstream from the Iguaçu Falls. Rainfall and turbidity may be the main trigger for spawning. Spawned eggs drifted quickly towards the confluence, most are likely to hatch in the Paraná River, while larvae disperse to nursery habitats even in the river section below the falls. Thus, inshore zones along the whole stretch provide nursery habitats for larval development of fish species exhibiting different reproductive strategies.

We tested a novel approach for fish abundance and size estimates along a 3.6-km reach in the main stem of a free-flowing section of the Danube River east of Vienna. We deployed an adaptive resolution imaging sonar (ARIS) with horizontal beam orientation from a moving boat in order to assess spatial and temporal fish abundance distributions in different mesohabitats. To validate the results obtained from imaging sonar, hydroacoustic abundance and fish length estimates were compared with those received from boat electrofishing surveys conducted at the same time and place. Furthermore, fish occurrence was related to hydraulic conditions based on a three-dimensional hydrodynamic model in order to reveal patterns of habitat use. Hydroacoustic abundance expressed as catch per unit effort estimates deviated significantly from fish abundances assessed by boat electrofishing, but when comparing relative abundances both methods yielded a similar spatial and temporal pattern. Total fish length estimates obtained from the two methods did not differ significantly. The mobile application of the non-invasive ARIS technology offers the opportunity to quickly gain detailed information on fish abundance and position of individuals. This yields more specific information on temporal and spatial distribution patterns, habitat use and fish size distributions in large rivers.

Connectivity of nurseries and spawning habitats for young of the year life stage is essential for successful recruitment of fish populations and therefore provides a key indicator for river restoration measures. Models for dispersal offer the potential to draw conclusions regarding restoration scenarios and to fill knowledge gaps about possible implications for fish populations. A newly developed rheoreaction‐based correlated random walk model (RCRW), in combination with a three‐dimensional numerical model and a non‐steady‐state particle tracing model, was applied for nase carp larvae (Chondrostoma nasus) before and after a restoration project on the river Danube, Austria. Spatio‐temporal patterns of dispersal of virtual larvae, attached with rheoreactive behaviour, were analysed within both scenarios. In comparison to the heavily modified river reach, the restored reach enabled a greater amount of upstream movement from the release site and showed a generally higher variability of spatio‐temporal distribution patterns. In contrast, estimated total settlement of rheoreactive larvae was substantially higher for the situation prior to the restoration measure. By comparing model results with a previously field experiment it was found that model simulations including rheoreaction as a single behaviour for navigation could not explain the whole pattern of larval dispersal. Therefore it is highly recommended for future studies to develop larval dispersal models by considering other factors (i.e., behaviour, bio‐energetics and environmental factors) of existing and future individual‐based models, which could serve as a tool to analyse the effect of restoration measures for recruitment of riverine fish populations.

Invasive gobies can have severe detrimental effects on local fish communities, however, direct methods for population control are often insufficient (i.e. fishing) or not feasible (i.e. poisoning). Indirect methods, such as habitat modifications in the course of restoration programs, appear promising but are poorly studied. In this study, we investigate the effects of different restoration measures on the abundance and occurrence of non-native gobies in the main stem of a free-flowing section of the Danube and attempt to disentangle these measures from general large-scale trends by applying a Before-After-Control-Impact design. We found three invasive goby species (racer, bighead, and round goby) in the sampling area, partly with very high abundances. Four to six years after the installation, the measures had negative (riprap removal), neutral (side arm reconnection), or positive (groyne field adaptations) effects on goby abundances. We conclude that the impact of the measures depends on the type of intervention, is species-specific, and is largely related to substrate composition. Independent from the effect of the measures, abundances of bighead and round goby dropped in the project and reference sections after the pre-survey. This general decline probably indicates a stabilization phase of the goby populations on a lower level, but may also be influenced by a major flood event. Nevertheless, our results indicate a high potential of shoreline modifications for invasive species control, calling for considering and incorporating them in river restoration programs.

Non‐native fish species pose a major threat to local fish populations and aquatic ecosystems in general. Invasive gobies are a particular focus of research, but with partly inconsistent results. While some studies reported severe detrimental impacts on native species, others have concluded less serious or neutral effects. We provide results from a large‐scale, multi‐annual fish monitoring program on the occurrence and abundance of non‐native fishes in the main stem of a free‐flowing section of the Austrian Danube. Special emphasis was placed on identifying positive or negative interactions of invasive gobies with native species. Whereas most non‐native species occurred too sporadically or were too few in number to infer a direct threat on the local fish community, invasive gobies were among the most common fishes throughout all sampling years. Co‐occurrence analyses revealed species‐ and mesohabitat type‐specific associations of gobies with native species, which were primarily positive. Notably, native predators such as asp, burbot, or perch probably benefit from the ubiquitous gobies. Two characteristic fluvial fishes revealed negative associations with invasive gobies, namely barbel (Barbus barbus) and Danube whitefin gudgeon (Romanogobio vladykovi): they appear to avoid habitats occupied by gobies. Accordingly, high abundances of round and bighead goby most likely resulted in population losses of barbel and whitefin gudgeon, respectively. Overall, our results indicate a limited negative impact of non‐native species in the sampling area. This is because only two out of 51 occurring species were found to be adversely affected by gobies, the share of co‐occurrences with native species was high, and other non‐native species were generally rare. Nevertheless, invasions are highly dynamic, and new non‐native species are likely to occur in the Austrian Danube, calling for continued monitoring and awareness.

The special issue brings together selected contributions from the 39th Annual Larval Fish Conference hosted by the University of Vienna, Austria, and presents the latest research and understanding of dispersal patterns and processes of early life stages of fishes of various aquatic environments around the world (open ocean, coastal areas, estuaries, and rivers). An important component of this compendium is to indicate new approaches and to outline the importance of integration of information about movements and dispersal for recruitment, population dynamics, species conservation, and management issues.