Barriers in River
Discover how barriers in the river cause a problem for the Tweed's migratory fish species and how the Tweed Foundation work to remove them.
Why Are Barriers a Problem?
Many of the Tweed's fish are migratory species. Some, such as the Atlantic Salmon, are anadromous, meaning they spend their juvenile stages in river before migrating out to sea. Others, such as the European Eel, are catadromous, meaning that they spawn out at sea but spend most of their time in the river. Some fish, such as the resident Brown trout, even undertake migrations within the river system, travelling up to 80 miles in order to spawn. The river can therefore be thought of as a highway for fish, allowing them to reach their respective feeding and spawning grounds.
Just some of the Tweed's migratory fish species. Left to right: Salmon, Sea Trout, European Eel and Brown Trout
Often, however, migrations of fish can be restricted or prevented due to in-river barriers. Obstacles in the river can leave areas of the catchment with reduced numbers, or even (in extreme cases) completely void of juvenile fish, particularly when adult fish have been prevented from reaching their spawning grounds.
Edington Cauld, an obstacle on the Whiteadder
The Tweed Foundation are therefore constantly on the lookout for in-river barriers to fish migration with the aim to open up as much of the catchment as possible to migratory fish.
Types of Barrier
In-river barriers are generally split into two categories: natural barriers and man made barriers.
Natural Barriers
Natural barriers are those that occur naturally in the river, such as waterfalls. Though they can restrict upstream access for fish, they are often not viewed as a problem and are generally left alone. This is because they can actually give rise to some interesting population dynamics within a water course, sometimes leading to genetically distinct populations of trout and, in some cases of partial barriers, they can limit the sizes of fish that can migrate upstream.
An example of this can be seen on the College Burn in the Till catchment, where the waterfalls at Hethpool Linn act as a partial barrier to Sea trout migration, only allowing Sea trout of approximately 2lb to migrate upstream.
Man Made Barriers
Man made barriers are often the biggest concern in terms of access for migratory fish. This is because they tend to prevent fish from reaching parts of the catchment that would ordinarily be within their natural range.
The main forms of man made barriers found throughout the Tweed system are caulds (weirs), culverts and fords.
Many of the caulds throughout the Tweed catchment are reminiscent of the textiles industry throughout the 19th and 20th centuries. Though many of them have since been removed by both the River Tweed Commission and Tweed Foundation, several caulds still remain.
The picture opposite is of Gala Skinworks Cauld.
Culverts tend to occur where roads or railway lines cross over a small water course. These can cause problems for fish migration in one of two ways: if there is not enough depth of water for fish to swim up through, or if the force of the water flow is so strong that they are not able to make it past.
Fords are locations at which people cross through the river. Most fords do not cause too many problems for fish access as they generally have no or a very little step for fish to get over. However, one of the last remaining barriers completely impassable to Salmon in the Tweed system is Haugh Head Ford on the Wooler Water.
Haugh Head Ford (shown opposite) blocks off 17km of potential spawning habitat for Salmon. Though a fish pass was installed in the ford, fish are still unable to make it up and over. Therefore, a project is now underway to remove Haugh Head Ford and its associated check weirs in order to improve access for Atlantic Salmon.
Barriers in Tweed
Navigate the map to see some of the locations of potential barriers to fish migration throughout the Tweed system.
Solutions
One of the Tweed Foundations jobs throughout the year is to ensure there are no obstacles in the river that could act as barriers to fish migration. Many obstacles can simply be removed from the water, but some require fish passes or easements to allow the fish to get over the obstacle and continue upstream.
Fish passes tend to be the desired solution on larger obstacles, such as caulds and weirs. They tend to fall into one of two categories: stepped fish passes or baffled fish passes.
Stepped Fish Passes
Stepped fish passes consist of a series of connect pools that fish can swim through to get up and over barriers. Stepped fish passes are also sometimes referred to as fish ladders.
The Gala fish pass, located in the Gala Skinworks Cauld, is an example of a stepped fish pass.
Baffled Fish Pass
Baffled fish passes generally consist of a narrow channel of water in which beams (baffles) are placed to slow the flow of water while also creating deeper pools for fish to swim through. The baffles can be placed either vertically (these known as denil fish passes) or horizontally (these being known as larinier fish passes).
The Ettrick cauld has an example of a baffled fish pass which are most often used on caulds with shallow faces and in culverts.
Fish Easements
Fish easements tend to be created on obstacles in smaller water courses where a conventional fish pass would not suffice or not be economical. Using the concept behind baffled fish passes, the Tweed Foundation have created several bespoke fish easements to allow fish passage (particularly for trout) over obstacles throughout the catchment, such as this one on the Harden Burn.
A Success Story
With both the Tweed Foundation and River Tweed Commission removing many of the obstacles in the river over the past century, the River Tweed is now relatively clear of barriers to fish migration.
Obstacles in the river 120 years ago. Light red shows the areas partially blocked off to Salmon while dark red shows areas completely blocked off to Salmon.
Just 120 years ago though, this was a different story altogether. Records show that many of the Tweed's main tributaries had barriers to fish migration, with 78% of the entire catchment being either partially or completely blocked off to Salmon. As a result, much of the Salmon spawning was restricted to just the main stem of the Tweed.
However, as the river was gradually opened up, Salmon started to spread throughout the catchment. Now, much of the catchment is open to Salmon, with only a tiny proportion of their natural range still being blocked off.
With projects currently underway on both the Slitrig and Wooler Water to remove barriers, the Tweed Foundation are working to continue this good news story.
Haugh Head Ford which is being removed from the Wooler Water to open up 17km of spawning ground upstream for Salmon
