Previous studies on plastic pollution of aquatic ecosystems focused on the world's 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 dispersal patterns of marked larvae of the nase carp (Chondrostoma nasus L.) were observed alongside dissimilar shoreline configurations in the main channel of the free-flowing Austrian Danube and compared with those of floating particles to investigate the mode of dispersal (active–passive). Individuals of different larval stages and floats at similar densities were released at an artificial rip-rap with groynes and a rehabilitated gravel bar. In both habitats, marked individuals were recaptured during the sampling period of 4 d after release. Relevant shoreline attributes for larval dispersal, such as the accessibility of nursery habitats, connectivity between adjacent habitats, and retention potential, were more pronounced at the gravel bar than at the rip-rap. At the gravel bar, larvae moved upstream and downstream within the connected bankside nurseries and displayed longer residence times. Larvae settled in groyne fields along the rip-rap as well; however, longitudinal dispersal was disrupted by groynes, forcing larvae to enter the main channel. Rather than settling in subsequent groyne fields, we assume that these larvae are displaced downstream and potentially lost from the local population.
Abstract:We investigated the mode (active versus passive) of larval downstream dispersal and its influencing factors in the nase carp (Chondrostoma nasus). Marked larvae (early and later stages), together with equivalent numbers of passive particles, were released in the main channel of the River Danube (Austria) at different flow (low, high) and current (over-critical, under-critical) conditions. Larvae and particles were recaptured with stationary nets at varying distances from release. We assumed that differences in the spatial dispersal patterns between larvae and particles were due to fish activity. We hypothesized that river discharge, developmental stage, current speed, and distance from release would influence these differences. We found that activity was independent of developmental stage or current speed at release, although activity was higher during low flow conditions. It may be that larvae deliberately enter the current during low flow, because the hydraulic conditions facilitate active dispersal. Furthermore, activity was greatest near the release site. This might be due to an intrinsically greater activity when fish are placed into novel surroundings or a result of rheoreaction. The discharge-dependent dispersal patterns observed represent an important ecological link between flow and recruitment and demonstrate the importance of inshore conditions for the early life stages of fish in large rivers, especially with regard to river modification and restoration schemes.Résumé : Nous avons étudié le mode (actif ou passif) de dispersion des larves vers l'aval et les facteurs qui l'influencent chez le hotu (Chondrostoma nasus). Des larves marquées (stades précoces et plus avancés) et un nombre équivalent de particules passives ont été relâchées dans le chenal principal du fleuve Danube (Autriche) dans différentes conditions d'écoulement (débit faible, débit élevé) et de courant (supercritique, sous-critique). Des larves et des particules ont été capturées dans des filets stationnaires à différentes distances du lieu de lâcher. Nous sommes partis de l'hypothèse que les différences des motifs de dispersion spatiale entre les larves et les particules étaient dues à l'activité des poissons. Nous avons postulé que le débit du fleuve, le stade de développement, la vitesse du courant et la distance par rapport au lieu de lâcher influenceraient ces différences. Nous avons constaté que l'activité était indépendante du stade de développement ou de la vitesse du courant au lieu de lâcher, l'activité étant toutefois plus forte durant des conditions de faible débit. Il est possible que les larves entrent délibérément dans le courant quand le débit est faible, parce que ces conditions hydrauliques facilitent la dispersion active. En outre, l'activité était la plus forte près du lieu de lâcher. Cela pourrait être dû à une activité intrinsèquement plus forte quand les poissons sont placés dans un environnement nouveau ou en raison d'une rhéoréaction. Les motifs de dispersion dépendant du débit observés constituent u...
Abstract:The dispersal of fish larvae in rivers might result from water movement but also from larval behaviour. Although potentially crucial for dispersion, knowledge of the role of behaviour is still fragmentary. This study intends to contribute to the question of how riverine fish larvae drift or move. All dispersal-relevant movement patterns of larvae of a characteristic rheophilic species were analyzed based on the parameters (i) swimming activity, (ii) direction of movement, and (iii) the orientation towards the current vector. Experiments were conducted in a novel flume mesocosm at three different flow scenarios covering the current velocity range of natural habitats. Mean current velocities in these scenarios were under, near, and over the "critical current velocity", above which fish larvae are not able to constantly hold their position in the water column. Three consecutive larval stages were tested to account for possible ontogenetic shifts in movement behaviour, both during the day and at night. Our results strongly suggest that the assumption of mainly passively drifting larvae has to be refused; in total, 92.6% of all observed movement events were characterized by swimming activity and directed orientation, whereas only 7.4% could be assigned to passive drift. During downstream movement, a significant portion of movement events (57.1%) was attributed to larvae that orientated in an upstream direction and performed active swimming movements.Résumé : La dispersion des larves de poisson dans les rivières peut être le résultat du mouvement de l'eau, mais également du comportement des larves. Les connaissances sur le rôle du comportement demeurent fragmentaires, malgré l'importance potentiellement cruciale de ce dernier pour la dispersion. L'étude s'intéresse à savoir comment les larves de poissons de rivière dérivent ou se déplacent. Tous les motifs de déplacement pertinents pour la dispersion des larves d'une espèce rhéophile caractéristique ont été analysés en fonction des paramètres suivants : (i) l'activité natatoire, (ii) la direction des déplacements et (iii) l'orientation par rapport au vecteur de courant. Des expériences ont été menées dans un mésocosme en canal novateur pour trois scénarios d'écoulement différents couvrant la fourchette de vitesses du courant dans les habitats naturels. Les vitesses moyennes du courant dans ces scénarios étaient inférieures, semblables ou supérieures à la « vitesse critique du courant » au-delà de laquelle les larves de poisson ne peuvent maintenir constamment leur position dans la colonne d'eau. Trois étapes larvaires consécutives ont été étudiées pour tenir compte de possibles changements ontogéniques du comportement de déplacement, tant durant le jour que la nuit. Nos résultats donnent fortement à penser que l'hypothèse des larves dérivant principalement de manière passive doit être rejetée; au total, 92,6 % de tous les évènements de déplacement observés étaient caractérisés par une activité natatoire et une orientation dirigée, alors que seuls 7,4 % de ces...
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