What’s does a good river for fish look like?

The kinds of rivers that are able to produce lots of food for fish are rivers which have some or all of the following things:

  1. The right geology to create water with a high mineral content. High pH water (also called alkaline) is what insects like to live in. The English chalk streams are a good example of this. In a healthy chalk stream a young salmon can go from egg to smolt (big enough to go to sea) in one year. The fish in chalk stream put on weight fast because there is lots of food.

This is the reason Iceland has lots of fish in its rivers despite having few trees – the lava flows weather quickly in the wet climate, producing run-off with a high mineral content.

On the other hand, having low pH (or acidic) water isn’t necessarily bad thing. It does mean recruitment rates are slower, but it also means more trout will migrate to the sea to look for food. As there isn’t enough food to sustain a large population of brown trout in the river, many will migrate. This means the river will have more sea trout.

  1. Lots of trees on the banks. These provide more insects for the fish to feed on. Spiders, ants, earwigs, slaters, caterpillars, beetles and many more insects will drop from the branches into the water, where fish eat them. The roots of trees also strengthen the banks which encourages the river to scour (dig into) the bed of the river, this maintains water depth and pools.
  2. Wood in the water. Slowly rotting wood is the perfect place for aquatic invertebrates (water-living insects), providing them with food and shelter. Therefore, the more woody debris there is in the river the more food the fish will have.
  3. A variety of different depths and speeds of flow. In deeper pools you’ll find water boatmen, non-biting midge, damsel flies and dragon fly nymphs. In fast shallow water you’ll find some upwing (mayfly) nymphs, stonefly nymphs and caddis. Each of these insects is a good foodstuff for trout and salmon. When the fish are very small they’ll eat the smaller insects and as they get bigger they’ll eat a greater variety. Each insect requires its own suitable habitat – the insects’ and trout’s needs are the same.
  4. Lots of river. The straighter a river is, the less variety of speed and depth it will have, and it will also be shorter than a wiggly river. Assuming the river starts and ends in the same place a straight line is the shortest the river can be. The more bends and changes in direction it has the more suitable habitat it’s likely to have, and the more food and fish it can support.

Another positive effect of a river being sinuous (wiggly) is that it mitigates flooding events. Bad floods are usually the result of lots of tributaries to a river being too straight and all dumping their water into the main river after heavy rainfall, and that water comes together at one place which is susceptible to flooding – usually an urban area downstream.

If, however, the tributaries are more sinuous, then the arrival of the water at the critical area is staggered. Delivering the same amount of water over a longer time period reduces the peak of the flood and the height the flood water will reach.

This is also a benefit during droughts. If the river is longer, and flows slower the water has more time to soak into the surrounding ground. That water is then slowly released over time, meaning the river system can hold more water for longer.

Good habitat – riffles and pools, lots of trees overhanging the water to provide insects and shade

What kills trout and salmon? 

If something causes lots of mud and silt to enter the water, that can affect the fish. Forestry operations where they put in roads to extract timber and then heavy vehicles using those roads through wet weather can result in large volumes of mud and silt getting into the river. This can adversely affect suitable habitat for insects and fish spawning areas. In extreme cases the fish’s gills (the things they use to breathe) become blocked and they die.

The same can happen if hydro-power generating systems or wind turbines are installed without proper planning or at a time of year when rainfall, and soil run off is heavy, or of poor ploughing practices are used in agricultural areas, leading to erosion and runoff of topsoil (which ends up in the rivers).

Predators will also have a greater impact on trout and salmon numbers when the fish do not have sufficient shelter in which to hide.

Pollution incidents kills fish. Angers are the first line of defence when it comes to pollution. If the river looks or smells funny or you see more than one dead fish, then report it to SEPA (Tel. 0800 80 70 60).

Poor habitat – straightened channel, no riffles, bankside trees and vegetation cleared, muddy runoff from fields

What can I do if I want to help the fish in my river?

  1. Don’t pull fallen trees out of the water.
  2. Think of gravel as a farmer thinks of his topsoil – as a valuable resource. It’s where many insects and small fish live and it’s here the fish spawn. If you remove it you reduce the productivity of the river and the quality of the habitat that is available. You may also destabilise the river, leading to bank erosion.
  3. Don’t cut trees down on the bank – unless you’ve taken advice from the Wild Trout Trust or similar and are putting in a tree kicker. Gareth Pedley of the Wild Trout Trust knows his stuff and can be emailed at gpedley@wildtrout.org
  4. Don’t get heavy machinery in the river (again unless you’re following advice from an expert). Messing with rivers, without taking professional advice first, is almost always bad for the fish. Rivers might seem simple but their flows are complex. If you move gravel you will create a draw, the river will suck gravel from upstream to replace what has been removed. If you put in any kind of structure you will change the flow and there will be consequences both upstream and downstream. Many people have lost large areas of valuable farm land, buildings and fences by messing around with the river, or by not excluding live stock from its banks.
  5. Report any pollution incidents to SEPA in Scotland or the EA in England
  6. Help fight for more beavers in the UK. The rumours that their dams block fish passage isn’t true. Beaver dams are leaky structures and fish can swim through them. Salmon can jump them.
  7. Keep herbivores off the banks. If sheep and deer are eating the trees on the banks then put up a fence to keep them away. Trees on the banks, especially those with overhanging branches, are vital fish habitat. The best trees are Oak, Birch, Alder, Rowan, Hazel, Ash, Willow and Cherry. Less good are Beech and Sycamore, which give blanket shade (too dense) and any non native species which have few associated insects.
  8. Donate to the Wild Trout Trust who do lots of excellent work in this realm, including giving free advice.
  9. Never dredge and campaign to stop dredging. Dredging de-stabilises the river and creates a need for more future dredging.
  10. Plant trees wherever fish spawn. Dappled shade is very good for eggs, fry and parr.