Hubert Keckeis

The Danube so colourful: a potpourri of plastic litter outnumbers fish larvae in Europe's second largest river.

Previous studies on plastic pollution of aquatic ecosystems focused on the worlds oceans. Large rivers as major pathways for land-based plastic litter, has received less attention so far. Here we report on plastic quantities in the Austrian Danube. A two year survey (2010, 2012) using stationary driftnets detected mean plastic abundance (n = 17,349; mean ± S.D: 316.8 ± 4664.6 items per 1000 m−3) and mass (4.8 ± 24.2 g per 1000 m−3) in the river to be higher than those of drifting larval fish (n = 24,049; 275.3 ± 745.0 individuals. 1000 m−3 and 3.2 ± 8.6 g 1000 m−3). Industrial raw material (pellets, flakes and spherules) accounted for substantial parts (79.4%) of the plastic debris. The plastic input via the Danube into the Black Sea was estimated to 4.2 t per day.

The early life history stages of riverine fish: ecophysiological and environmental bottlenecks.

Fish are good indicators of the environmental health of rivers and their catchments as well as important conservation targets. Bioindication has to be based on an understanding of the requirement of characteristic species with regard to: (a). The match/mismatch between reproductive strategies and environmental conditions. (b). The niche dimensions of critical stages vis-a-vis the key conditions. (c). The availability of microhabitats along the ontogenetic niche profiles, i.e. the connectivity from spawning substrates to early life history microhabitats. The main conditions for the embryonic period are temperature and oxygen supply which are responsible for embryonic mortality, the duration of the period, and size and condition of newly-hatched larvae. For the exogenously feeding larvae the functional of food acquisition, growth and bioenergetics to temperature, food availability and current velocity is decisive. Studies concentrated on Chondrostoma nasus, a target species for monitoring and conservation in large European river systems. Results obtained in experimental studies are compared with those from field studies in order to evaluate the match/mismatch between performances and microhabitat choice and population dynamics in the field. Discrepancies between requirements and field conditions in regulated rivers underline the significance of inshore zones as microhabitats (expressed in the Inshore Retention Concept) and the requirement for ecophysiological studies on target species for river restoration and conservation.

The effect of water current on foraging behaviour of the rheophilic cyprinid Chondrostoma nasus (L.) during ontogeny: evidence of a trade-off between energetic gain and swimming costs

The main aims of this study were to investigate (1) the relationship between the energy balance of young stages of a rheophilic cyprinid, Chondrostoma nasus, and water current and (2) the critical water velocities for this species during its early ontogeny. Experiments were conducted in a recirculating flume. Water velocity and type, size and density of prey were varied. The foraging behaviour of three size classes of nase (15, 20 and 25 mm TL) was filmed and analysed with an Image Analysis System. Capture success and consumption were determined and the following critical water velocities were measured: (1) nul benefit water velocity (NBWV); (2) nul consumption water velocity (NCWV); and (3) maximum sustainable water velocity (MSWV). The capture success drastically declined above a certain value of water velocity which increased with body size. Consumption varied with density and size of prey and with fish size. It was high within a range of water velocities which increased with fish size and then declined to the NCWV. The profitable range of water currents increased during ontogeny, and swimming costs in early larvae were higher in proportion to the maximal energy intake, compared with larger fish (positive net energy gain between 0 and 7 cm s−1 in 15 mm TL nase and between 0 and 15 cm s−1 in 20 and 25 mm TL nase). Although the three defined critical water velocities did not differ from each other in early larvae, MSWV in 20–25 mm TL nase were significantly higher than the NBWV and the NCWV. The MSWV of nase showed a linear increase with fish size. A comparison with values from the literature for other fish species shows that MSWV may depend more on body shape than on species-specific life history patterns.

Typification of vessel-induced waves and their interaction with different bank types, including management implications for river restoration projects

Restoration projects often aim to initiate side erosion processes in order to improve the ecological situation, leading to banks of gentle slopes which positively affect biotic habitats. However, within navigable reaches such measures have to be planned under consideration of the impact of vessel-generated waves on YOY-fish. Hence an extensive study was conducted at different bank types to analyze and describe waves, induced by passing vessels. A high resolution wave gauge was exposed, to obtain wave data of different vessel types over a broad discharge range. Simultaneously vessel passage was recorded using an automatic identification system. Attributed to the ships, different wave types could be distinguished and their specific parameters (e.g., wave height) were described. Additionally we analyzed by which of these parameters wave height is primarily affected. For passenger ships a speed reduction of 5xa0kmxa0h−1 results in a reduction of wave height of 14xa0cm at the Danube. A morphological analysis was accomplished to identify habitat loss and shift of the water’s edge based on ship induced maximum drawdown. Habitat loss, expressed as retrogression of the water’s edge, ranged between 0.88 and 35xa0m for the highest drawdown recorded.

Patterns and processes in the drift of early developmental stages of fish in rivers: a review

Current-mediated downstream dispersal by the early developmental stages of fish in rivers is a common phenomenon. Knowledge of patterns and processes in the dispersal, or ‘drift’, of young fishes provides important information on spawning location and spawning success, habitat use, movement paths and flow-ecology relationships more generally, all of which are critical for effective river conservation and management. But despite the importance of such information, our understanding of the patterns and processes of the drift of the early life stages of riverine fishes is limited. Furthermore, riverine fish drift research has tended to occur in isolation from movement studies of other organisms, limiting its integration with higher level concepts and theory. This manuscript reviews the literature on the dispersal of young fishes in running waters. Relevant studies from all climatic zones and geographical regions are investigated, with particular attention given to the types and life history stages of fishes that drift and the seasonal and diel patterns of drifting. We then consider how fish enter the drift and their mode of drifting, attempting to reconcile a long-running discussion, under what we call the ‘active–passive conundrum’. We argue that, aside from eggs, the early stages of fish are not exclusively either passive or active drifters, but usually a mixture of the two, which we term ‘actipassive’ drift. Finally, we evaluate existing knowledge in the context of a general conceptual framework for movement ecology, identifying gaps in our understanding of the roles of internal state, navigation capacity, motion capacity, external factors and internal factors in influencing the dispersal process.

Early development of the postcranial skeleton of the pikeperch Sander lucioperca (Teleostei: Percidae) relating to developmental stages and growth

The early development of the postcranial skeleton (pectoral girdle, pelvic girdle, vertebral column and fins) in pikeperch (Sander lucioperca (L.)) was studied from hatching to days 47 and 43 post fertilization (dpf) at two different rearing temperatures, 15.5 and 18.0°C. Four embryonic and six larval stages were described, ranging from 3.4 ± 0.3 mm to 21.8 ± 2.1 mm in total length. The crucial point in larval development is swimbladder inflation, which enables larvae to swim energy efficiently. Until this time point, only the most essential skeletal elements to enable swimming movements have developed. As the larvae become neutrally buoyant, they grow and differentiate postcranial elements rapidly. Concurrently, swimming performance and foraging success seems to improve. A specific size is correlated with a distinct developmental stage defined by a set of traits that includes the skeletal elements. The developmental sequence of skeletal structures is temperature independent, although growth is slower and the individual developmental stages are reached later at 15.5°C than at 18.0°C. J. Morphol. 2012.

Effects of vessel-induced waves on the YOY-fish assemblage at two different habitat types in the main stem of a large river (Danube, Austria)

The effects of navigation on young-of-the-year (YOY) fish were investigated for the Danube River based on an integrative approach. During YOY growing season, wave height, wave frequency and water currents were recorded. Synchronously, fish drift and fish abundance of YOY were monitored in two distinct inshore nursery habitats—a gravel bar and a groyne field. The characteristics of vessel-induced waves were correlated to YOY-drift. In both habitats, an increase in drift densities during ship passages was observed. At the same time, assemblage composition and drift densities differed between habitat types. At the gravel bar, drift densities of larval cyprinids—representing many keystone species of the Danube—were higher during ship passages. In the groyne field, drift was low and percids dominated the YOY-assemblage along the shore. Banks with gentle slopes in the main channel seem to be particularly affected by vessel-induced wave wash. Precisely such habitats are often established within the scope of modern river restoration projects because they provide suitable nursery habitats for riverine fish species. Beside the positive effects on biota, the establishment of these habitats in navigable reaches may create problems for YOY-fish due to displacement effects induced by ship-generated waves.

Method-integrated fish assemblage structure at two spatial scales along a free-flowing stretch of the Austrian Danube

Fish assemblages in large rivers are highly diverse communities. However, a single method to describe these complex systems adequately is not available as of yet. In this paper we propose a combination of normalized catch-per-unit-effort data of three fish sampling methods, thus providing a more inclusive representation of the overall assemblage. We investigate fish species occurrence and abundance in a survey at three consecutive sites in the free-flowing main channel of the Austrian Danube east of Vienna. Using boat and wading electrofishing, as well as longline fishing we collected data in four mesohabitats at each site. We analysed richness, assemblage structure and abundance of single species and tested site and mesohabitat as differentiating forces on the fish assemblage structure, applying PERMANOVA and NMDS. The investigated sites are populated by a species-rich fish assemblage, of which we collected a total of 36 species. The composition of the assemblage was rather consistent except for the occasional occurrence of rare species. At the smaller mesohabitat scale, however, the assemblages differed considerably and hydromorphological conditions were apparently the major structuring force. Overall, the combination of methods considerably improves the assessment across sites and mesohabitats and contributes to our understanding of large river fish assemblages.

Short-term effects of inshore restoration measures on early stages, benthic species, and the sublittoral fish assemblage in a large river (Danube, Austria)

The rip-rap along an approx. 3-km-long shore in the main channel of the River Danube in Austria was completely removed to initiate the formation of a river bank through natural erosional and depositional processes. This study aimed to determine the short-term effect of this restoration measure on species composition and abundance of different developmental stages of fish at two spatial scales (micro- and mesohabitat). For this purpose, changes in abundance of early stages, as well as assemblage structure and species diversity of the benthic and the sub-littoral fish community were studied before and after restoration. No significant effects of restoration measures on the benthic fish assemblage were found. Significant and contradictory effects of the measures on the early stages and on the sub-littoral assemblage were observed. The abundance of fish larvae decreased after restoration, and this change could be attributed to the generally higher flow velocities in the new inshore microhabitats. Simultaneously, the species number and the abundance of fishes from the sub-littoral assemblage increased after restoration. This study has found that the main channel of the Danube still contain a high fish species diversity, and potentially plays a crucial role in recruitment of characteristic fluvial fishes.

Intercalibration of hydroacoustic and mark-recapture methods for assessing the spawning population size of a threatened fish species.

Hydroacoustic counting and a three-year mark-recapture study with passive integrated transponders (PIT tags) were used to estimate the size of a spawning population of nase Chondrostoma nasus, a threatened potamodromous cyprinid that undertakes annual spawning migrations into a tributary of the Danube River. In 2005, the estimates of the size of the spawning population from the hydroacoustic counts (N = 2234, 95% CL 1929-2538) and from the Jolly-Seber model (N = 1198, 95% CL 461-5842) corresponded well. Estimates from the jackknife-estimator based on the hydroacoustic counts yielded slightly higher values (N = 2783, 95% CL 2529-3037), but were still in the same order of magnitude as those from the hydroacoustic and mark-recapture approach. At low run-size, hydroacoustic counting was more time consuming and technically demanding than mark-recapture studies. At the same time, it was non-invasive, provided real-time data on a fine temporal scale, and estimates showed less variability than the Jolly-Seber model. Mark-recapture of fish in spawning streams involved substantial disturbance at a sensitive stage of the life cycle. Hence, hydroacoustics is highly suited for population estimates of threatened potamodromous fishes, where interference needs to be minimized.