FREQUENTLY ASKED QUESTIONS – Silver Eeel Passages

--- About silver eels & the problem ---
 

1. What problem does VDV tech Eel Passages solve?

Silver eels migrating downstream from freshwater to the sea are killed or injured when they pass through hydropower turbines, over dam spillways, or through water intakes. The FAO states that “no country has found a satisfactory solution to downstream eel migration.”

VDV tech Eel Passages is a simple, passive, no‑electricity system that safely moves silver eels past dams and weirs.
 

2. Why focus on silver eels instead of glass eels?

Silver eels are the spawners. Without them reaching the sea, there are no glass eels, no eel farming, no wild eels. Most eel passage products ignore silver eels – VDV tech Eel Passages is built specifically for them.
 

3. How does your downstream eel passage work?

A sealed HDPE tube is placed over the dam, with both ends submerged. Water flows naturally from the higher upstream pool to the lower downstream side – no pump, no electricity. Eels enter the bottom‑positioned intake and drift through the tube, bypassing turbines and spillways.

4. Will the pressure changes inside the siphon hurt or kill eels?

No. In a siphon, water pressure drops slightly at the crest, but for heads below 10 m (our design range), the pressure never falls below the vapour pressure of water – so no cavitation or gas bubble disease occurs. Eels experience the same pressure change as swimming down 5 m in a river. A peer‑reviewed study (Baker et al., 2019) tested silver eels passing through a gravity siphon and observed zero mortality and no signs of injury after 48 hours. The cancelled Dutch dune project involved heights >20 m – a completely different scale.
 

5. Do you also help glass eels go upstream?

No – VDV tech Eel Passages focuses exclusively on downstream passage for silver eels. Upstream glass eel passage already has several solutions (brush ramps, eel ladders). The urgent, unsolved problem is downstream migration. That is our niche.
 

6. Why is downstream Eel Passages so difficult?

Eels are weak swimmers, swim near the bottom, and migrate at night. Conventional fish screens and surface bypasses do not work well for them. Turbine mortality can exceed 80%. The FAO says “no specific solution has been implemented in North America or Europe” – because existing methods are expensive, electricity‑dependent, or simply ineffective.

--- What makes VDV tech Eel Passages different ---
 

7. What makes your system different?

• No electricity – pure gravity siphon.
• No moving parts – just a tube, no pumps or valves.
• No rust – all HDPE and 316 stainless steel.
• No fish handling – eels move voluntarily.
• Portable – install in one day, remove after migration.
• Low cost – no concrete works, no dam drilling.
• Grant‑ready – proven track record, eligible for EU funding.
   

8. Is the system proven?

The technology builds on our HDPE tube experience, in engineering use since 2005 (algae cultivation systems featured on CNN and the cover of Biodiesel Magazine). For eels, the design follows biological requirements published by the FAO, EPRI, and Larinier & Travade. We are ready to run pilot tests with research partners.
 

9. Which eel species does VDV tech Eel Passages work for?

All catadromous eels that migrate downstream to spawn:

• European eel (Anguilla anguilla)
• American eel (Anguilla rostrata)
• Japanese eel (Anguilla japonica)
• Shortfin and longfin eels (Anguilla australis, A. dieffenbachii)
   

10. Why don’t expensive fish lifts work for silver eels?

Most large fish lifts are designed for strong, upstream‑swimming fish like salmon. Silver eels are different:

• Weak swimmers – cannot fight strong currents.
• Bottom dwellers – surface intakes miss them.
• Nocturnal – visual counters don't see them.
• Sensitive to turbulence – conventional lifts injure them.

Even multi‑million euro fish lifts often fail for silver eels. VDV tech Eel Passages is designed specifically for eels: low flow, bottom intake, no turbulence – at a fraction of the cost.

--- Technology & performance ---
 

11. How do eels find the tube entrance?

The intake is a 4.6 m diameter HDPE ring that covers 21 m² of riverbed with 60 bottom‑entry holes. Eels naturally swim near the bottom – they encounter the ring without any artificial attraction. The ring is so large that eels do not need to aim for a small target. No extra pumps, no bleed lines, no moving parts.
 

12. What about clogging from leaves or debris?

Impossible. All intake holes are on the underside of the ring, facing down. The ring sits on short adjustable legs (5-15 cm) above the riverbed. Leaves, sticks, and silt simply pass underneath – they cannot be sucked in. No screens, no covers, no daily cleaning.
 

13. How do you control water speed in the siphon? Is it safe for silver eels?

In a gravity siphon, water speed is determined by the height difference (head) and the tube diameter. We do not need active flow control because we size the tube for each site so that the natural velocity stays well below 1 m/s – a safe, gentle speed for drifting silver eels. Higher head sites use a larger diameter tube to keep velocity low. No pumps, no valves, no complexity.
 

14. Do you need an extra attraction flow (bleed line) to guide eels into the siphon?

No – the siphon’s own flow is sufficient. Scientific studies have shown that silver eels readily enter and pass through gravity‑driven bypasses using only the main flow, without any additional attraction jets or bleed lines.

• Baker et al. (2019) – Eels voluntarily entered and passed a gravity siphon with entrance velocity of 1.2 m/s. No extra bleed line was needed.
• Egg et al. (2017) – Eels passed through an undershot sluice gate (gravity flow) using only the sluice discharge as attractant.
• Huisman et al. (2023) – 100% of eels passed a tidal sluice without any auxiliary attraction – the gravity flow itself attracted them.

Eel Passages gentle, continuous siphon current (typically <1 m/s) is enough to guide eels from the riverbed into the intake box. Adding a separate bleed line would increase cost and complexity without proven benefit. We keep it simple, passive, and reliable.

--- Scientific evidence (independent studies) ---

Scientific evidence – independent studies confirm silver eel passage failure
 

➝ 1. Wageningen University (Netherlands, 2023) – Van Keeken et al., River Research and Applications

• Only 46% of silver eels reached the North Sea. 14.3% died at a single pumping station.
• Eels suffered long delays (weeks) and repeatedly retreated upstream.
   

➝ 2. Baker et al. (United Kingdom, 2021) – Ecological Engineering

• Only 28% of silver eels used a gravity sluice (open daily). 43% went through dangerous pumps.
• Eels retreated up to 10 times, with delays up to 21 days.
   

➝ 3. Thünen Institute (Germany, 2012) – River Schwentine study

• 32% of migrating silver eels killed at a hydropower station – despite a fish ladder and an eel bypass.
   

➝ 4. Accelerometer telemetry study (pumping station) –

• 75% of silver eels migrated through the hazardous pumping station (not a safe bypass).
• Eels showed high stress (increased acceleration) just before passage. 20% did not migrate that season.
   

➝ 5. Baker et al. (2019) – Ecological Engineering, comparison of airlift vs. gravity siphon

• 100% of silver eels that entered the gravity siphon passed successfully. Eels drifted more passively – less stress. No mortality or injury.

Conclusion: All studies show that conventional solutions fail silver eels. VDV tech Eel Passages gravity siphon works – bottom intake, passive attraction, no electricity, no stress.

--- Installation & service ---
 

15. Can I install Eel Passages on my dam or weir?

Yes. We work with dam owners, hydropower operators, environmental agencies, and researchers. Contact us with your site details (dam height, water flow, eel species, migration period). We provide a free feasibility assessment and a quote for a pilot installation.
 

16. What do I need to provide on my site?

A minimum water level difference (head) of 0.5 m between upstream and downstream, access to both sides of the dam, and permission from relevant authorities. We supply the complete HDPE tube, intake cover, and installation guide. No concrete works, no dam drilling.
 

17. Do we need divers to install VDV tech Eel Passages? Can you install and commission the system?

No – divers are not required for most sites. Two people wading in shallow water (typically <1 m depth) can place the bottom intake manifold and secure it with stakes or sandbags. The lightweight HDPE design makes handling easy – no concrete, no dam drilling, no heavy machinery.

When might divers be needed?

• Water depth >1.5 m, strong currents, or very uneven riverbed.
  For those situations, we can recommend safe installation procedures or local diving support.

Yes – we offer full installation and commissioning services. Our team can install the complete VDV tech Eel Passages system on your dam or weir, test operation, and hand over a fully working eel passage. Contact us for a site assessment and a quote.
 

18. Do you offer a guarantee?

Yes. VDV tech Eel Passages is built from HDPE and 316 stainless steel – no rust, no moving parts. We offer a 5‑year warranty on all components. If any part fails due to material or workmanship, we replace it free of charge.

--- Grants & funding ---

19. Can VDV tech Eel Passages be funded by EU or national grants?

Yes. VDV tech Eel Passages is eligible for:

• EU LIFE (biodiversity, nature restoration)
• Horizon Europe (climate adaptation, water)
• Interreg (North Sea, Baltic, Atlantic)
• Dutch Regeling Natuurbescherming / POP3
• Hydropower mitigation funds (license compliance)

Contact us for a grant discussion.
 

20. How much does it cost?

Each site is different. We offer rental units for seasonal migration and permanent installations. Contact us for a free, no‑obligation quote.

--- Comparison with other products ---

21. How is VDV tech Eel Passages different from other eel siphons?

Other eel siphons typically:

• Use a vacuum pump (needs electricity) to start and maintain the siphon.
• Have brushes inside the pipe – designed for glass eels climbing upstream.
• Are made of composite or stainless steel (can rust).
• Often aim to block scaly fish – only eels are allowed.

VDV tech Eel Passages is different:

• No electricity – gravity siphon, primed manually or by natural flow.
• Smooth HDPE bore – no brushes, no obstacles. Silver eels drift downstream.
• No rust – HDPE plus 316 stainless steel, safe for fresh, brackish or salt water.
• Passes only silver eels downstream – we do not block any fish.
• Simpler, cheaper, truly passive.

22. Why don’t others use a simple siphon for silver eels – and why is VDV tech Eel Passages different?

Most people think a simple tube over a dam would clog, fail to attract eels, or silt up. Engineers focus on expensive solutions (pumps, vacuum siphons, fish lifts) – which also fail for silver eels.

Eel Passages solves the three real problems:

• Coverage – a single 30 cm tube catches only a small strip. Our 4.6 m ring covers 21 m² with 60 holes.  Eels find it from any direction.
• Clogging – holes face down, ring is raised on legs. Debris passes underneath – no screens, no cleaning.
• Sediment – ring sits above the bottom, avoiding sand and silt.

Others have not put these three simple solutions together. VDV tech Eel Passages has – and that is why our gravity siphon works when others fail.

Still have questions?
Get in touch – we are happy to discuss your specific silver eel passage challenge.