The eel is a catadromous species, that is it spawns at Sea (the Sargasso) and enters coastal and transitional waters at the glass eel stage, then maturing through the (pigmented) elver and ‘yellow’ eel stages in rivers and coastal waters, before returning to sea to spawn at the ‘silver’ eel stage. The whole life cycle can take up to 20 years. During concerted migrations when they are following tides and water currents, these migratory fish are at particular risk of becoming drawn into water intakes and damaged or destroyed.
The European Commission’s Eel Recovery Plan (Council Regulation No. 1100/2007) requires EU Member States to ensure that at least 40% of the adult silver eel run escapes to sea to spawn enabling them to complete the life cycle. In England and Wales this requirement is brought into law through The Eel (England & Wales) Regulations 2009 (known as “The Eel Regulations”). The Eel Regulations require, among other things, effective eel screens to be installed at any water intake capable of abstracting >20 m3/day from a water body where eels may be present. In the Regulations an eel screen is defined as any device, moving or stationary, that is designed to impede the passage of eels through a diversion structure. For England and Wales, the Environment Agency has published guidance on suitable screening techniques and how they should be applied . Physical screening methods require mesh sizes or bar-spacings that are not viable in many industrial or water supply applications owing to the risk of blockage. For example, where glass eels or elver may be present, a mesh size of 1-2 mm is required, increasing to 3 12.5 mm for yellow eels and 15-20 mm for silver eels. Therefore non-physical (behavioural) screening techniques are commonly used to deter eels from water intakes.
Of the various behavioural screening methods used for fish protection, Environment Agency guidance1 (see graph) shows that strobe lights are the only method that has proved successful against eel, acoustic deterrents being ineffective owing to their poor hearing ability. Older strobe light fish deterrent systems used powerful but unreliable xenon flash tubes that have limited lifespan (typical 106 flashes at maximum output levels) and require voltage levels in the kilovolt range. Fish Guidance Systems’ High Intensity Light (HIL) strobe units use modern low-voltage light-emitting diode (LED) technology with a 10 year lifespan. Not all LEDs are suitable for this purpose. The spectral FGS HIL systems are matched to the eel’s visual pigments and the light pulse characteristics have been optimised in laboratory flume fish reaction trials. The trials show that FGS HIL strobes provide effective deflection, even in waters with turbidity levels of up to 70 NTU, with significant enhancement of effect in waters of moderate turbidity (20 NTU) compared with clear water conditions (0 NTU).
It should be noted that the distance that fish are repelled in laboratory trials is constrained by the size of the tank and the aim in real applications is to deflect fish into flow streamlines that will sweep them away from a water intake.
1. Screening at intakes and outfalls: measures to protect eel. The Eel Manual – GEHO0411BTQD-E-E. The Environment Agency, 2011.