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A helping hand to get past river barriers: optimising eel pass design to aid upstream migration

Updated: Sep 27, 2022

European eels start their life in the Sargasso Sea. After hatching from eggs, the leaf-shaped larvae called ‘leptocephali’ drift in currents to the European continent where they change into glass eels, before beginning their upstream migration into freshwater. This is where they spend the majority of their lives, feeding and growing, before migrating over 5000 km back to the Sargasso Sea to spawn.

This migratory life history makes them susceptible to numerous threats, including river infrastructure such as weirs, dams, hydropower facilities, pumping stations and tide gates, which act as barriers to migration and prevent access to important freshwater habitats. Following a steep decline in recruitment, the European eel is now classed as Critically Endangered on the IUCN Red List of Threatened Species and many European countries are implementing measures designed to protect and improve stocks.

Improving passage for eels at riverine barriers is a key focus of conservation efforts. Many different designs of eel pass have been installed on river structures, but in the majority of cases robust tests on their effectiveness are lacking and there is concern that some passes function poorly. Installing fish passes can be costly, so research is vital to determine which solutions are the most fit for purpose and cost effective.

Learning from historic mistakes

ZSL, in collaboration with our Environment Agency partners, are using the information gathered from good and bad real-world examples of pass installations as a foundation for developing better solutions. Flume-based studies at the newly renovated Institute of Zoology animal research facilities are enabling us to both optimise existing eel pass designs, and to try out some new promising concepts (Fig. 1). These facilities offer the great benefit of being able to control, measure and replicate environmental variables such as water temperature, light levels, flow characteristics etc..., while simulating numerous pass and barrier scenarios.

Fig 1. Test flumes at the ZSL Institute of Zoology

Essentially, the purpose-built test flumes recirculate water and allow us to control and measure flow characteristics as water flows down the ascent section of the simulated eel pass (Fig 2.). Juvenile eels respond strongly to these flow cues, which motivate them to swim and climb up and over obstacles during their migration.

Fig 2. Recirculated water provides the flow cues that motivate eels to ascend the pass

Eels predominantly migrate in darkness, so the trials are conducted under infrared light which is outside the spectral sensitivity of eels. Multiple cameras above the flumes enable us to track the eels’ movement and quantify a wide range of passage metrics including success rate, speed and route of ascent. Trials with larger juvenile eel employ Passive Integrated Transponder (PIT) telemetry to automatically detect when individuals move through key locations in the pass (Figure 3.).

Fig 3. PIT telemetry used to track eel movements in the test flumes

The design of the crest of an eel pass is particularly important because it determines whether the eel successfully advances over the top, and thereby passes the barrier, or whether it swims or gets washed back down the pass. In pumped passes, water supply is also mainly delivered to this section, so trials have been investigating both the effect of crest shape and the influence of different flow patterns at the crest (Fig. 4).

Fig 4. Testing different crest designs and flow delivery options

The next stage will use Computational Fluid Dynamic (CFD) modelling, so we can interrogate eel tracks in relation to the underlying hydrodynamic conditions such as velocity and turbulence intensity (Fig. 5). This is important because previous research has usually focussed on the ‘what’, rather than the ‘why’. Looking solely at how many eels successfully ascended is useful, but this study will allow us to understand what causes the good or poor results, which is important to inform better passage design.

Fig 5. Using Computational Fluid Dynamic (CFD) modelling to quantify hydrodynamic conditions on the eel pass (credit: Chris Grzesiok)

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