Eel Pass Design
Eel populations throughout Europe have suffered a dramatic decline in recent years, with current populations estimated at <5% of those 25 years ago. As a result, there is currently 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 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.
Traditional Eel Passes
Fishtek uses innovative approaches to improving eel and elver passage and we are able to design eel passes 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.
These passes are typically small, with a width <300mm and comprised of a channel filled with suitable material with a trickling flow passing through. Eels then crawl upstream through this material in order to move up the pass.
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 passes provide an effective option for improving eel passage that is independent of upstream water levels, or at tidal sites the tide state.
Tidal gates are often a significant barrier to the upstream migration of elvers. One solution to improve their 'passability' is the damper, which delays closing of the tidal flap until after level equalisation. They consist of a compressible material that increases the resistance of the flap and hence the pressure required for them to close. There are a number of compressible damper products available and it is possible to fabricate a fairly low cost adjustable solution for a particular gate.
Retarders are essentially a spring or tension device that holds the tidal gate open above level equalisation, allowing more saline ingress and elver migration. The size of retarder for each site must be calculated and will depend on the weight and size of door and the degree of saline ingress deemed acceptable.
The retarder is relatively easy to fit to installations that have a steel winch cable and pulley wheel. Larger gates generally have a steel cable/winch installed for maintenance. Retarders are a low cost, adjustable option to improve eel and fish passage at tidal gates. They are relatively easy to fit, and require minimal maintenance.
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.
Eel Pass Fabrication and Installation
We provide a full fabrication service for all eel passes and we are able to fabricate passes based on existing drawings as well as those designed in-house. Passes can be constructed from a range of materials, including GRP, ABS plastic, HDPE, aluminium, galvanised or stainless steel. Alternately, a suitable structure can be built on site using concrete or stone.
We have recently developed a low-cost eel pass, utilising ABS plastic pipes and a flexible eel pass substrate material, which is easily fitted to virtually any structure. We are also able to fabricate the specific 'bristle-board' pass design required for gauging weirs, in order to maintain the telemetric integrity of the structure.
The eel traps that we have developed in house can be constructed as part of a pass and can be fabricated from GRP, aluminium, galvanised or stainless steel. These can be installed as part of the pass installation, or alternatively retrofitted to a pass.
We offer a full installation service for all eel passes, both pumped and gravity fed as well as easements such as the damper and retarder. In circumstances where a structure is of an appropriate form, a fabricated pass may not be required and eel passage can be improved by fixing a suitable crawling substrate directly to the face of the structure.