Articles & Case Studies

Keeping Eels off a Slippery Slope: How Filter Screens Can Help Prevent Population Decline

Posted: Friday 17th February 2017

The eel population is declining across Europe and environmental agencies are looking for new ways to protect the species. Here, Mark Burns, director of leading manufacturer, Croft Filters Ltd, outlines the issues involved in eel preservation and how filter screens can help grow the population.

Eels are one of a small number of species that are catadromous, meaning they migrate back and forth from sea water to fresh water throughout their life-cycles. However, over the last 20 years, there has been a dramatic decline in the number of eels in European river systems.

The International Council for Exploration of the Seas has stated that, in some areas, the number of elvers (young eels) being born has dropped to around one per cent of traditional levels. In 2017, the European eel is a critically endangered species and the European Union has been forced to implement a recovery plan to help increase their numbers across the continent.

Experts suggest that many are being killed after being drawn into freshwater intake systems - the pipework used to obtain water from lakes, rivers or reservoirs for hydroelectric power generation, water supply or irrigation.

These intake systems are typically fitted with screens at the inlet to protect wildlife from entering them. However, unlike other water creatures, eels respond tactically rather than visually to water intake screens. This means they will try and squeeze through a screen mesh even when it is smaller than the diameter of their body. Therefore it's important that water intake systems have screens that eels cannot pass through yet will still allow for the passage of water.

To increase the eel population in Britain, the Eel Management Plans for England and Wales have suggested the roll out of appropriate eel-proof screen filters to allow them to pass through fresh water safely. These wedge wire, fine woven wire or perforated plate screens work by ensuring particles smaller than the pore size will pass through the filter while larger particles will be blocked.

The aim of these screen filters is to prevent injury and death to both adult and juvenile eels in upstream and downstream migration and their design will depend on the flow rate and pressure within each filtration process. The Environment Agency publication, The Eel Manual Screening, suggests that the design of the screen must also take the angle of the bank and the direction of the flow into account. More demanding water intakes with angles of 20 degrees or more will likely require a narrower mesh or bar spacing in order to prevent the eels from passing through.

However it is not only the angle that needs to be considered, different sized eels - depending on the stage of their lifecycle - will react to different screens and therefore must be accounted for when determining the right screen to use.

Users of the extracted water affected by these new environmental regulations have expressed concerns over the efficiency of using another screen filter within their intake systems. Fine filtration requires additional pumping power; this increases operational costs for those using the water stream. A finer filter also restricts the flow rate of their intake systems, possibly leading to a drop in productivity for the plant.

This means, solutions that both protect the eels during in their migration while having minimal impact on industrial plants and their productivity are needed. Due to the high number of possible permutations for screen configuration, it is almost impossible for anyone operating an inlet pipe to buy an eel-friendly screening solution off the shelf. Engineers at Croft Filters, however, have the manufacturing equipment and technical ability to create custom screens that protect eels while still allowing for optimum flow rates for industrial processes.

Within the UK and throughout Europe, governments are seeking innovative means of protecting the eel and elver population as they go through their migration stages in their lifecycles. Screen filters, if used correctly, are the most cost-efficient and effective means of giving these eels a fighting chance of survival.

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August 2021

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