Eel Pass Design
The plight of the European eel (Anguilla anguilla) has garnered much press in the past few years, as the species is reported to have declined in number by as much as 95%.
Improve eel passage
Restoration of eel populations
As a result of the decline of eel populations, there is currently a European strategy and associated legislation to restore eel populations, with a key element being the establishment of targets for silver eel escapement from catchments. A major step towards achieving such targets will be increasing the number of barriers which eels are able to pass, as a huge number of barriers (both natural and anthropogenic) currently restrict the upstream (and downstream) migration of eels.
Increasingly, specific eel passes are being installed on such barriers, in order to enable eels to continue upstream. These can take the form of a pass in a more traditional sense, alterations in the morphology of the structure or easements to reduce the degree to which a structure is a barrier to migration.
Eel pass and elver passage innovation
Fishtek uses innovative approaches to improving eel and elver passage and we are able to provide eel pass design for virtually any site, regardless of the size or location of the barrier. Passes can be gravity or pump fed, with a wide range of designs and substrate types available. A full CAD facility (including 3D solid-model engineering drawings) is available in house for the design. To date the Fishtek team have designed over 100 eel passes.
These passes are typically small, with a width <500mm and comprised of a channel filled with suitable material such as bristle media or eel tiles, with a trickling flow passing through. Eels then crawl upstream through this material in order to move up the pass.
Gravity fed passes are the most common type of eel pass
We have designed a very large number of gravity fed eel passes, including individual passes up to 40 m long and 1 m wide. These have been at weirs / barriers anywhere in height from less than 0.5 m to over 5 m and on rivers with flows from a few dozen litres per second up to over 100 cubic metres per second. The actual form (both in terms of the dimensions, slope and internal shape) of eel passes is highly variable and as such the solution designed at each site is unique to that site.
Gravity fed eel passes can be designed and constructed in a wide variety of ways and can be formed from concrete, steel, aluminium, GRP or plastics such as HDPE. Within the pass, a crawling media has to be provided and this can be either eel ’tiles’ or eel bristle media (a substrate that looks like bristles from a broom embeded into a plastic base).
Up-and-Over (pumped) eel passes consist of an aluminium, steel or GRP boxed channel with anti-predation lids.
The pass is lined with eel crawling substrate. The entrance to the upslope is submerged just below the invert level of the structure and rises above the obstruction, discharging eels through a downslope to the upstream side. A pump fed spray bar at the top most point of the pass sprays a trickle flow down the upslope and downslope. Up-and-Over eel passes provide an effective option for improving eel passage that is independent of upstream water levels.
An essential element of the European strategy will be assessing the extent to which eel populations are recovering.
We have developed several unique designs of eel traps which can be designed as part of an eel pass, or alternately retrofitted to an existing pass.
These can be incorporated into both gravity-fed and pumped eel passes and we have designed and supplied these at several sites previously, where they are being used to monitor the number and timing of eels using the eel pass.