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How fish climb

There are many different styles of fish passes created on our canals and rivers to help fish 'climb' past locks, weirs and other man-made structures on their migration routes.

Salmon jumping at a weir on the River Tyne at Hexham

There is nothing more spectacular than watching salmon make their way upstream to spawn by jumping at a local weir, when suitable river conditions allow. The spectacle certainly draws the crowds and the cheers and ‘aahhs' are clearly audible to show success or failure in an attempt.

Unsuitable conditions

But what happens when river conditions are not suitable, for example when river flows are low? Many fish will drop down the river to look for suitable spawning places.

Others will keep looking to find a certain ‘attraction' flow that might be coming over the weir in one corner or sometimes in the centre of a weir. This special flow can be from a fish pass, a structure constructed in the weir or obstruction that takes water at a set flow, allowing the fish to swim up it and over the weir without having to jump.

Why fish passes are built

Many of our rivers have some sort of obstruction, usually a man-made weir that was built as part of the development of navigation on our waterways, or to supply a head of water for some industrial activity.

From Norman times onwards obstruction to passage became an issue, but the industrial revolution saw massive changes. This resulted in a reduction or total loss of some species in some rivers, including European sturgeon, allis and twaite shad, sea lampreys and river lampreys, plus Atlantic salmon.

Weir removal

In some places the industrial activity has stopped and the weir is no longer required, so it can be dismantled and passage for all fish can be restored. But where the weir has to remain, there is now a focus and momentum to allow fish to swim where they need to go, usually for spawning. To do this a fish pass is required to be constructed.

It may be that a river needs a number of fish passes to allow fish to swim all the way upstream to the spawning sites. At this site on the River Tyne (pictured) a new Larinier-type fish pass, including a resting area, was installed in 2016.

Larinier type fish pass

Types of fish passes

There are two main categories of fish pass: those based on a series of pools and those that are 'baffle' based. A baffle is an object that slows the flow of water, similar to traffic calming measures on a busy road.

You will often hear a reference to a fish ladder rather than a fish pass but in truth they can be called either. The reference to a ladder probably comes from the fact that a pass can be constructed in a series of small steps allowing fish to move easily from one pool to the next.

Fish pass choices

The choice of fish pass is dependent on a number of factors, including what the bed of the waterway is like, the swimming speeds of fish present, the slope to be overcome, how the pass copes with debris, and whether the pass needs to allow all species of fish or just members of the migratory salmon family, known as the salmonids.

It is only relatively recently that the extent to which coarse (freshwater) fish need to move upstream and downstream has been recognised, and many fish passes are now designed to take them into account.

In addition, it is now necessary to make special provision for elvers, the young of eels. Elvers need to swim upstream to find suitable habitat to grow and live, so again there has now been the development of techniques to allow this.

Larinier passes

One of the most common types of fish pass is called a Larinier, named after the French engineer who first designed them. One of the features of these passes is the baffles that slow the flow and reduce turbulence which would otherwise put the fish off from ascending them.

Lariniers are suitable for coarse fish as well as salmonids. They tend not to block easily and if the height of the obstruction is too high a resting pool can be incorporated into the structure to allow a staged movement, giving fish time to rest. They can also be built large enough to allow canoes to move through them as well as fish.

Denil fish passes

Another type of fish pass using baffles is the Denil fish pass, first designed by a Belgian scientist, G Denil. In contrast to the Larinier, the baffles are on both the bottom and sides of the pass which is much narrower. This allows Denils to have a steep slope, relative to other types of fish pass, to overcome small to medium height differences over a relatively short distance.

Steep rise Alaskan fish pass

Steep Alaskan fish pass

A development from the Denil is the steep rise Alaskan fish pass, which is more suited to salmonids than coarse fish and can accommodate a steep rise .

Deep vertical slot fish passes

Apart from removing the impounding obstruction, these are the most technically perfect solution for fish passage. As part of the Unlocking the Severn project we have installed deep vertical slot fish passe at Diglis, Holt Fleet and Lincombe on the River Severn. Fish such as Allis shad and twaite shad that cannot leap weirs and are relatively weak swimmers are able to successfully use these hi tech passes.

You can find out more information about deep vertical slot fish passes here.

Pool passes

Pool passes are perhaps the oldest type of fish pass in use. They are generally applicable for most fish species, are extensively used throughout the world and in most cases require low maintenance.

The pass consists of a number of pools, arranged in series of steps separated by cross walls. These pools dissipate the energy of the falling water and providing resting areas for ascending fish.

Perhaps the best known example of a pool pass is Pitlochry fish ladder or pass near Perth, which is made up of 34 pools spread over 300 metres. The fish ladder was built at the Pitlochry Dam to allow salmon to travel up and over the 86-metre-high dam to get upstream to their spawning grounds.

Fish ladder

A variation on these are the deep notch and orifice fish passes. These are formed from rectangular channels separated into pools with walls, which allow water to flow over them via notch or slot, plus an opening at the base on the other side of the wall.

Rock ramp fish passes

Where the drop or rise from an obstruction is relatively low, it is possible for the water flow to be gradually reduced through the development of what is called a rock chute (ramp) fish pass.

Large boulders and stones are placed below the obstruction to mimic a rapid river with refuge, reduced flows and resting pools provided by the stonework. This is clearly a more natural looking structure than the concrete of the other passes mentioned, but it is not suitable for all locations.

Rock ramp fish pass

Elver passes

The European eel is classified as critically endangered with numbers having declined by more than 90% over the last 40 years. Young eels travel from the Sargasso Sea with the currents to ascend rivers in Europe and parts of North Africa, where they may spend 20 years before making the return journey to spawn.

River obstructions have been identified as a key barrier preventing eels accessing suitable habitat. Works that makes this part of their journey easier should help in the long-term recovery of the species. Elver passes come in a variety of sizes, but are often a box-like construction filled with material, such as astroturf or bristles, which aids the elvers' movement through the pass.

Elver passes on our waterways

On the River Trent the eel population has been given a helping hand with the construction of an elver pass at Hazelford Weir, near Nottingham. The photograph below shows the eel pass at Hazelford with a different internal structure: studs of varying sizes made from a robust co-polymer. The elvers use the small ones and the more mature eels use the larger ones.

Boxes for the new eel pass at Hazelford
Elver pass

More work to be done

We own a number of sites where further work may be needed to improve fish passage. Work of this type is expensive and is typically accomplished by organisations working in partnership, bringing together different experiences, knowledge and specialist skills, and allowing different funding streams to be .


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Last Edited: 10 March 2022

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