At the test case Altusried, FIThydro partners investigate fish swimming tracks and model migration habitats with the help of the CASiMiR-migration software to assess the effectiveness of the attraction flow downstream of the bypass. The models of migration corridors and fish swimming tracks are used to evaluate different mitigation scenarios for improving the findability of the fishway entrance.
Factors influencing successful fish migration
Hindered fish migration is one of the main impacts of hydropower on river ecology. One of the key aspects for successful fish migration is the guidance of fish to the entrance of bypass installations. Within FIThydro investigations are performed on physical and chemical factors such as smell, temperature, oxygen concentration, noise that might affect the probability of fish finding a migration facility, e.g. at the test case Freudenau, Austria.
However, hydraulic factors such as flow velocity magnitude and direction, as well as morphological parameters (e.g. shape and structure of the riverbed) are assumed to be main triggers that influence the so called “attraction flow” leading fish to migration facilities. But how to assess this “attraction flow”?
Upstream migration at the test case Altusried
In the Upper Iller river in South Germany there are five hydropower installations with upstream impoundments operated by the FIThydro partner LEW. All of them are equipped with bypass channels for upstream fish migration. The lower part of the bypass channel is constructed as a nature-like fish ladder with boulders and vertical slots. The outlet of the bypass channel at the HPP Altusried is located about 257 m downstream of the weir that represents a barrier for migrating fish (see Figure 1).
Target species in the test case are barbel (Barbus barbus) and grayling (Thymallus thymallus), which both migrate large distances upstream to their spawning grounds in spring and early summer.
Investigating how fish find the fishway entrance
To gain knowledge about the movement of fish approaching the fish ladder, an acoustic telemetry system was installed by INBO (Research Institute Nature and Forest) in the area of the fish ladder outlet. The system had to be adapted specifically to the situation since acoustic telemetry is usually performed in deeper waters with less sources of noise (turbulent flow, turbines).
After extensive system testing, with extenscive support of local fisheries experts and LEW, 25 graylings and 22 barbels were tagged and introduced into the system. The tags emit an unique acoustic signal for each individual fish that is detected by hydrophones (Figure 2). Using this information fish positions are derived and can be assembled to show the fish tracks.
Fish tracks and analysis
Over a period of about 4 months almost 8 million fish positions were recorded. In a first step, different filtering methods were applied to reduce the amount of data and to eliminate false detections. In a second step, a hydrodynamic model of the study area was setup by flussbau IC and the tracks were analyzed and categorized to find correlations between the fish movements, hydraulics and river morphology.
Habitat model and agent-based approach
Using the information from the tracks and results of a hydrodynamic model, FIThydro partner SJE developed an approach to determine migration habitat suitability and set up a migration corridor map (see example for barbel in Figure 4).
Basis for the model are the preferred hydraulic conditions in the last period before entering the fish ladder. Migration suitability is highly dependent on flow rate. With increasing flow, the highly suitable areas (blue and green in Figure 4) are shifted from the middle of the river towards the river banks.
In a second step, fish agents are placed into the virtual habitat model environment. They move according to behavioral rules that depend on migration suitability, thresholds for velocity, swim direction in relation to flow direction, tendency to keep the swim direction from the preceding time step and “jump criteria” for overcoming “dead end” zones. The final movement in every step is calculated using a weighted random algorithm.
First comparisons indicate that modelled tracks are close to the observed for some individuals. For other individuals, the observed tracks are more diverse as the modelled ones. Research work on the integration of additional parameters and additional random factors is going on.
The following two videos show animations for barbel 46857 of a) the observed track and b) the modelled fish agent track (simulation). The red box indicates the fish pass entrance area where the simulation stops as soon as the fish agent has found it.
Different scenarios for improved fishway findability
The CASiMiR migration model developed and applied at the test case Altusried allows a testing of different scenarios for improving the findability of the fishway entrance. The software can be used during the planning of new or assessment of existing fishways. At the test case Altusried the model is used to e.g. evaluate whether an additional water supply at the fishway entrance would improve the attraction flow or if morphological enhancements such as guiding structures or boulder placement could increase the attraction of the fish pass outlet.
Matthias Schneider & Ianina Kopecki (SJE)
picture rights: top picture © flussbau iC; videos © SJE