Trout and chips (of the micro variety)

We have lots of weirs in rivers which are used to provide us with flow information which we use to issue flood warnings and control abstractions in times of dry weather. However, these structures can prove difficult obstacles for fish to get over, reducing the lengths of river available to them. We assist fish passage wherever we can and on many structures have built fish passes.

This week we have been carrying out some research into how easy it is for fish to use one type of fish pass. The location was a gauging weir at Bishop’s Frome on the River Frome in Herefordshire.

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The weir has a Hurn baffle fish pass installed. This has slots which are placed in a row straight up the weir face to encourage fish to jump straight up and over the weir. This is a common installation on our gauging weirs as they are gently sloping.
On longer, steeper slopes, the baffle slots go diagonally up the slope so the fish can expend energy, then pause, before going again, all the way to the top.

We commissioned our consultants, APEM, to investigate whether such a design is suitable as an ‘all species’ (chub, dace, roach, trout) fish pass or whether it is trout specific. We have started the investigation by looking at how easily trout can negotiate the pass.

APEM staff electro-fished the River Frome and caught 14 brown trout (all over 150mm). The fish were anaesthetised, and a PIT tag (micro chip) was inserted into the fish. The fish were measured, the PIT Tag serial number recorded against the length and they were released downstream of the weir.

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When a fish passes through antenna loops placed at the top and bottom of the weir, a camera and sensors record the bar code and capture an image of the fish which is recorded on a hard drive.

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Five days after the fish were released 5 have gone over the weir (they were detected by the upper antenna). Strangely those 5 fish include the smallest (188mm) and the largest fish (338mm) that we tagged. The other 9 fish appear to still be nearby as they have been picked up by the lower antenna loop but have yet to make it to the top.

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The antenna loops will remain in place for at least another week to help monitor the movements and behaviours of the fish when confronted with this type of baffle configuration.

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Giving fish a helping hand on the River Isbourne

The River Isbourne is a tributary of the River Avon. Only thirteen miles long, it rises on the Cotswold escarpment, joining the Avon at Evesham. It’s reputed to be one of only two rivers in the world that flows entirely south to north (the other being the Nile)!

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Our monitoring of the Isbourne has revealed that the river fails to meet the requirements for good ecological status under the Water Framework Directive. A major reason is the absence of some fish species we’d expect to find in a river of this type. Roach, Chub, Stone Loach and Minnows are missing from the upstream sections, while Roach, Stone Loach and Gudgeon are missing from the downstream part.

The problem

Like many English lowland rivers, the Isbourne has a number of redundant mills along its length, many of which have weirs and water management structures associated with them. These often represent significant obstacles to fish trying to swim upstream to breed or colonise upper parts of the catchment.

Because of its proximity to the Avon, the Isbourne should provide important extra habitat for fish normally resident in the Avon, as well as supporting its own resident populations. This means that barriers at the downstream end of the river have a disproportionately large impact.

Hampton Mill Weir

The first upstream obstruction on the Isbourne is at Hampton Mill, approximately 750m above the confluence with the river Avon. The legacy of a redundant mill, the two metre high weir was causing a moderate barrier to fish as the water flow was too fast and shallow to allow them to swim over it. During high flows and flood conditions the weir is drowned out allowing fish passage, but not during normal flow conditions. The picture below shows the weir before we got to work.

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Ideally we would like to remove barriers like these completely and return the river profile to its original state. This, however, is very expensive and can also sometimes lead to knock-on impacts to local flood risk and erosion.

A baffling solution

In the case of Hampton Mill weir we’ve attached a series of baffles to the existing structure, as seen in the picture below. The baffles, which are made out of recycled plastic, can be fitted quite quickly and don’t require any major structural work to the weir.

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The design is intended to increase water depth over the weir, and reduce the flow velocity. This will make it easier for fish to swim over the weir to reach additional upstream habitat, including spawning gravels.

We have also installed eel tiles along the right wall of the weir. Again, fabricated from recycled plastic they are covered with lumps and bumps of various sizes and encourage eels and elvers up the weir. They like to stick to the sides of things as they move along!

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The photo below shows water going over the weir after the baffles were installed last week. You can see that the flow is being significantly disrupted and the water velocity is reduced. This will make is much easier for fish of all species to pass. We hope that they will form an orderly queue to explore this lovely catchment.

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Our river monitoring programme will show how successful we’ve been over coming years.

Part of a bigger plan

Hampton Mill on the River Isbourne is just one of over five thousand structures across the Midlands that we believe is having and adverse affect on fish movements. Over the last couple of years we have been working to identify and rank them according to the impact that they’re having. This is part of our ongoing work to secure compliance with the Water Framework Directive.

We have a programme of projects across the region which address some of these barriers. We currrently have about a hundred different schemes being designed or built. Clearly there is a long way to go but by identifying and prioritising we can ensure what resources are available are used to best effect.