Search

Das Einzugsgebiet der 2.857 km langen Donau ist mit einer Gesamtfläche von 801.463 km² - rund 10% des europäischen Festlands - das zweitgrößte Flusseinzugsgebiet in Europa. Es erstreckt sich über 19 Länder und ist damit das internationalste Flusseinzugsgebiet der Welt. Über 100 Fischarten sind in der Donau nachgewiesen, darunter vier hochgradig gefährdete Störarten, die charakteristischen Flaggschiffarten der Donau (SCHIEMER ET AL. 2004). Die diadromen Donaufischarten (z. B. Störe (Acipenser spp.), Pontischer Maifisch (Alosa immaculata), Asowscher Maifisch (Alosa tanaica) und Schwarzmeerforelle (Salmo labrax)) sind zwingend darauf angewiesen ihren Lebenszyklus im Süß- und Salzwasser zu vollziehen. Einige Arten wandern bzw. wanderten dabei über sehr große Distanzen, beispielsweise der Beluga-Stör (Huso huso) vom Schwarzen Meer bis zu den Laichgründen >2.500 km ins Landesinnere (SCHMALL & FRIEDRICH 2014). Aber auch potamodrome Arten migrieren in der Donau über weite Strecken; für Döbel (Leuciscus cephalus), Barbe (Barbus barbus) und Nase (Chondrostoma nasus) sind z. B. Wanderungen über 169, 318 und 446 km nachgewiesen (STEINMANN ET AL. 1937, VAN TREECK ET AL. 2022). Die Fischartenzusammensetzung und der Fischbestand sind wichtige Indikatoren für die Gewässerqualität der Donau. Die Ergebnisse des jüngsten Joint Danube Survey 4 zeigen, dass die Fischartengemeinschaft entlang des gesamten Flusslaufs bedroht ist. Andererseits ist die Vielfalt der Fischtaxa immer noch auf einem Niveau, als dass wirksame Gewässerentwicklungsmaßnahmen dazu beitragen können den ökologischen Zustand bzw. das ökologische Potential zu verbessern und die Ziele der EG-WRRL zu erreichen (BĂNĂDUC ET AL. 2014, ICPDR 2021).

Despite great promise for understanding the impacts and extent of climate change and extreme weather events on aquatic animals, their species, and ecological communities, it is surprising that electronic tagging and tracking tools, like biotelemetry and biologging, have not been extensively used to understand climate change or develop and evaluate potential interventions that may help adapt to its impacts. In this review, we provide an overview of methodologies and study designs that leverage available electronic tracking tools to investigate aspects of climate change and extreme weather events in aquatic ecosystems. Key interventions to protect aquatic life from the impacts of climate change, including habitat restoration, protected areas, conservation translocations, mitigations against interactive effects of climate change, and simulation of future scenarios, can all be greatly facilitated by using electronic tagging and tracking. We anticipate that adopting animal tracking to identify phenotypes, species, or ecosystems that are vulnerable or resilient to climate change will help in applying management interventions such as fisheries management, habitat restoration, invasive species control, or enhancement measures that prevent extinction and strengthen the resilience of communities against the most damaging effects of climate change. Given the scalability and increasing accessibility of animal tracking tools for researchers, tracking individual organisms will hopefully also facilitate research into effective solutions and interventions against the most extreme and acute impacts on species, populations, and ecosystems.

Despite great promise for understanding the impacts and extent of climate change and extreme weather events on aquatic animals, their species, and ecological communities, it is surprising that electronic tagging and tracking tools, like biotelemetry and biologging, have not been extensively used to understand climate change or develop and evaluate potential interventions that may help adapt to its impacts. In this review, we provide an overview of methodologies and study designs that leverage available electronic tracking tools to investigate aspects of climate change and extreme weather events in aquatic ecosystems. Key interventions to protect aquatic life from the impacts of climate change, including habitat restoration, protected areas, conservation translocations, mitigations against interactive effects of climate change, and simulation of future scenarios, can all be greatly facilitated by using electronic tagging and tracking. We anticipate that adopting animal tracking to identify phenotypes, species, or ecosystems that are vulnerable or resilient to climate change will help in applying management interventions such as fisheries management, habitat restoration, invasive species control, or enhancement measures that prevent extinction and strengthen the resilience of communities against the most damaging effects of climate change. Given the scalability and increasing accessibility of animal tracking tools for researchers, tracking individual organisms will hopefully also facilitate research into effective solutions and interventions against the most extreme and acute impacts on species, populations, and ecosystems.

Salmonids are well known for their natal homing behaviour, meaning they return to breed in the same area where they originated. However, not all individuals return to their natal breeding grounds—a behavioural trait known as straying. The prevalence of straying is difficult to explore and therefore quantitative estimates for straying are seldom reported. In this study, otolith microchemistry and genetics were combined to investigate patterns of straying over ecological and evolutionary time, respectively, between neighbouring rivers flowing into Mariager fjord, Denmark. Otolith microchemistry was used to determine the river of origin for sea trout (Salmo trutta) upon their return to freshwater and 288 SNP markers were used to determine genetic structure among the rivers in the fjord. In this system, where the distance between rivers is short, otolith microchemistry achieved 80% accuracy in assigning juvenile brown trout to their natal river, thus allowing us to determine that approximately 43% of the adult sea trout had returned to non-natal rivers to spawn, with a similar proportion of strayers and natal homers in all of the rivers. Genetic analysis further supported that there was substantial gene flow among individuals originating from different rivers, indicating that sea trout in Mariager fjord make up one population. The findings obtained from otolith microchemistry and genetics complement each other and provide further evidence that sea trout in this system migrate to non-natal rivers and spawn there, which consequently affects the genetic structure of the population.

Geolocating aquatic animals with acoustic tags has been ongoing for decades, relying on the detection of acoustic signals at multiple receivers with known positions to calculate a 2D or 3D position, and ultimately recreate the path of an aquatic animal from detections at fixed stations. This method of underwater geolocation is evolving with new software and hardware options available to help investigators design studies and calculate positions using solvers based predominantly on time-difference-of-arrival and time-of-arrival. We provide an overview of the considerations necessary to implement positioning in aquatic acoustic telemetry studies, including how to design arrays of receivers, test performance, synchronize receiver clocks and calculate positions from the detection data. We additionally present some common positioning algorithms, including both the free open-source solvers and the ‘black-box’ methods provided by some manufacturers for calculating positions. This paper is the first to provide a comprehensive overview of methods and considerations for designing and implementing better positioning studies that will support users, and encourage further knowledge advances in aquatic systems

The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the development of bypass solutions, they are often constructed based on a limited knowledge of swimming behaviour. A bypass close to the stream bed is often recommended at fish passage facilities to accommodate downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. The current study used acoustic telemetry with depth sensors to explore the three-dimensional migratory behaviour of downstream-migrating silver eels. The eels were tracked as they approached a hydropower plant with a state-of-the-art angled bar rack and full-depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and using the bank areas to a greater extent. Downstream-moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full-depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100% when both bypass solutions were considered. The study provides knowledge of the swimming behaviour of silver eels, which is relevant for the design of bypass solutions for eels at migration barriers.

Geolocating aquatic animals with acoustic tags has been ongoing for decades, relying on the detection of acoustic signals at multiple receivers with known positions to calculate a 2D or 3D position, and ultimately recreate the path of an aquatic animal from detections at fixed stations. This method of underwater geolocation is evolving with new software and hardware options available to help investigators design studies and calculate positions using solvers based predominantly on time-difference-of-arrival and time-of-arrival. We provide an overview of the considerations necessary to implement positioning in aquatic acoustic telemetry studies, including how to design arrays of receivers, test performance, synchronize receiver clocks and calculate positions from the detection data. We additionally present some common positioning algorithms, including both the free open-source solvers and the ‘black-box’ methods provided by some manufacturers for calculating positions. This paper is the first to provide a comprehensive overview of methods and considerations for designing and implementing better positioning studies that will support users, and encourage further knowledge advances in aquatic systems.

De nyeste data fra kortlægningen af fiskebestandene i udvalgte danske vandløb er fra efteråret 2022, og de kan nu ses online på DTU Aqua's Ørredkort.

Den blåfinnede tun er vendt tilbage til vore farvande og mærkningsprojektet har tilmed mærket den største tun i projektet med en vurderet vægt på ca. 380 kg.

Tunmærkningen har igen haft besøg af H.K.H. Kronprins Frederik, som i går kom og hjalp med mærkningen og fik lejlighed til at høre om resultaterne fra netop den tun, han selv var med til at mærke i projektet i 2021.

Havørreder vandrer på forskellige tidspunkter i deres livscyklus. Selvom de kan vandre flere hundrede kilometer ude i havet, er de kendte for med høj præcision at vende tilbage til deres oprindelsesvandløb for at gyde. DTU Aquas seneste undersøgelser viser dog et andet billede fra tilløb til Mariager Fjord.

The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the development of bypass solutions, they are often constructed based on a limited knowledge on swimming behaviour. A bypass close to the stream bed is often recommended at fish passage solutions for downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. This study used acoustic telemetry with depth sensors to explore the three-dimensional migratory behaviour of downstream-migrating silver eels. The eels were tracked as they approached a hydropower plant with a state-of-the-art angled bar rack and full-depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and also using the bank areas to a greater extent. Downstream-moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full-depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100%, when both bypass solutions were considered. The study provides knowledge on swimming behaviour of silver eels, relevant for design of bypass solutions for eels at migration barriers.

European rivers are obstructed by more than one million barriers that have resulted in excessive loss of river continuity. On the main course of the Danube River there are 83 longitudinal continuity interruptions, out of which 65 dams are used for hydropower (ICPDR 2022a). The Iron Gate Hydropower and Navigation System is one of the largest river engineering projects undertaken in Europe, with the dams mainly built to provide hydropower and flood protection, and to facilitate navigation along the Danube. These infrastructures represent introduced barriers to fish migration. Hence, ensuring passage opportunities for fish at the Iron Gate dams is considered to be of major importance for the conservation of migratory fish populations in the Danube River basin. Restoration of river continuity at these sites would reopen an additional 900 km for migration up to the Gabčikovo dam, providing suitable habitats and spawning grounds along the Danube and its tributaries. Knowledge about fish behavior and movements in the vicinity of these river infrastructures is required to build effective up- and downstream passage facilities to allow the migration of fish species. To gain insight in the approach routes and aggregation areas a refined approach to acoustic telemetry is employed to support migration facilitation.

Natal homing, a strategy in which individuals return to breed in the same river as they were born, is prevalent among salmonids. However, some individuals may not return to their natal river, and instead stray to non-natal rivers. To date, there is limited documentation on patterns of straying among iteroparous salmonids that have returned to spawn over multiple years. In this study, 21538 out-migrating juvenile anadromous brown trout (Salmo trutta L.) were tagged over two years with passive integrated transponder (PIT) tags, and followed as they returned to either their natal or three non-natal rivers in a fjord system, over multiple years. In total, 206 individuals were detected returning for more than one year, of which 43% were strayers. A divergence in migratory strategies among strayers was documented: a smaller proportion of strayers were only detected in non-natal rivers, while the majority (67%) displayed high variability in their migratory patterns as they were detected in both their natal and non-natal rivers, sometimes within the same reproductive season. This study documents high variability in the migratory patterns of repeat spawners and high probability for them to stray, and suggests that straying is an important life-history strategy possibly affecting individual fitness.

1. Internal seiches are common in stratified lakes, with significant effects on stratification patterns, hydrodynamics and vertical nutrient transport. In particular, seiches can change the vertical distribution of the thermocline and the cold hypolimnetic and warm epilimnetic water masses by several metres on a timescale of a few hours, leading to rapid and strong changes in temperature profiles and oxygen availability, with profound effects on mobile and sessile organisms. This could affect fish communities directly, through physiological stress and elevated mortality, and indirectly, through prey distribution. 2. The aim of this study was to analyse the effects of internal seiche dynamics on lacustrine fish behaviour, and to characterise fish reaction patterns, with the main focus on vertical movement of fish in the vicinity of a shifting thermocline, and avoidance of cold hypolimnetic water. 3. The analysis was based on acoustic telemetry data from Lake Milada, a post‐mining lake in the Czech Republic, with a total of 61 tracked individuals of four species: northern pike (Esox lucius), wels catfish (Silurus glanis), tench (Tinca tinca) and rudd (Scardinius erythropthalmus). 4. The effects of seiche dynamics on the four species studied were weak but significant during the day, while at night they affected only rudd. Upward seiches elicited stronger responses in fish than downward seiches, and the impacts occurred only during the strongest seiche events. 5. Thermocline shifting during seiche events may induce a transient reduction in habitat for seiche‐reacting species, and thus affect predation and other inter‐ and intra‐specific interactions, as well as fish community dynamics.