What do our river fish need?
When it comes to the needs of trout and salmon, it’s quite similar to our needs. They need food, clean water, oxygen and shelter to survive.
Trout and salmon eggs need loose clean gravel with a good flow of water and oxygen though. Clean gravel is gravel which has space between the pebbles, not gravel that has a layer of mud or silt on top or between the pebbles. Imagine having marbles in a collander, if you pour water in it will flow through. Now imagine adding sand. The sand will plug the gaps in the marbles and if you keep adding sand the water will slow right down and become a very slow drip out of the collander. If there isn’t enough flow of water through the pebbles the eggs don’t get enough oxygen and die. This is called ‘choking a redd’, a redd is a fish nest.
Many people talk about habitat but this word is often misunderstood. It just means the right conditions for good survival.
Loose clean gravel is good egg habitat.
Trout fry and parr (the next stages in their life after being eggs) need shallow fast ‘riffle’ water. This is water that big fish can’t live in. So the fry and parr are safe from being eaten by other bigger fish and don’t compete with them for food. These areas in a river are highly oxygenated and insects and algae (on which the insects feed) like this. The young trout can access food on the surface, in mid-stream and on the bottom without having to move far. If a river is canal like and doesn’t have bobbly riffle water (usually found at the necks and tails of pools) then the amount of fish the river can produce each year is reduced. How many trout survive their first life stages is dependent on having enough of the right habitat.
Riffle water is good fry and parr habitat.
Usually adult trout and salmon will spawn in an area where there is good riffle habitat just downstream. So when the eggs hatch the alevins and fry can move a little downstream and find the right habitat for their next stages.
The size of the gravel dictates the size of the fish that will live there. The small spaces between small gravel is the right size for small fry. As the fish get bigger they need to find new areas where the spaces between the stones are big enough for them to fit in, but not so big that bigger fish already live there.
The more ‘hidey holes’ fish have the less likely they are to be washed away after a heavy rain.
For these reasons having lots of gravel, in lots of sizes is so important for ‘recruitment’.
A river’s ability to produce new generations of fish is called ‘recruitment’. The number of fish a river can hold is called its ‘carrying capacity’.
Shelter is the next very important thing for trout and salmon. Shelter has many ways of saving the lives of fish.
- A place to hide from predators
- A way of providing shade and keeping the water cooler, this can save the lives of fish and fish eggs
- A source of food (lots of insects will live on and around the wood)
- A place to avoid the dangers of fast moving water and debris during a heavy flood
- The best kind of shelter will also filter the water and help to keep it clean. A big tree traps smaller bits of wood, these in turn trap still smaller pieces and the end result behaves like a water filter.
Examples of good shelter might be boulders in a river, a beaver dam, or a fallen tree, deep water is also a form of shelter. Beaver dams and fallen trees are particularly good at acting like a sieve and filtering the water. This improves water quality and oxygen levels. It means fish are less likely to be killed by mud and silt in their gills if there’s an erosion event or heavy industry is churning up the nearby soil.
A stable river also helps fish survive. Trees on the banks help to stabilise them and prevent the banks from collapsing into the river. The trees’ roots bind the soil together. The longer the soil is there, the more complex it is and the more worms and other insects (fish food) it holds.
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 all of the following things.
- The right geology for high conductivity – this means that the water has a high mineral content. High Ph water (also called alkaline or base rich) is what insects like to live in. The chalk streams are an 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.
However having low Ph water isn’t necessarily bad thing. It does mean recruitment rates are slower, but it also means more trout will go to sea. As there isn’t enough food to sustain many trout in the river, more will migrate. This means the river will have more sea trout.
- Lots of trees on the banks. Trees on the banks 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.
- Wood in the water. Slowly rotting wood is the perfect place for water living insects (aquatic invertebrates), providing them with food and shelter. Therefore, the more wood there is in the river the more food the fish have.
- 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 food stuff 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 insect’s and trout’s needs are the same.
- Lots of river. The straighter a river is, the less variety of speed and depths 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 therefore more food and more fish it can support. Another positive effect of a river being wiggly (sinuous) is that it mitigates bad floods. Bad floods are usually the result of lots of tributaries to a river being too straight and all dumping out 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 means that 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.
What kills trout and salmon?
If something causes lots of mud and silt to enter the water that can affect fish. Forestry operations where they put in roads to extract timber and then heavy vehicles use those roads through wet weather can result in large volumes of mud and silt to get into the river. This can affect the 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.
Predators will also have a greater impact upon trout and salmon numbers, when the fish do not have sufficient shelter to hide in.
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).
What can I do if I want to help the trout/salmon in my river?
- Don’t pull trees out of the water
- Think of gravel as a farmer thinks of his topsoil. It’s 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.
- 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 email@example.com
- Don’t get heavy machinery in the river (again unless you’re following advice from a geomorphologist). 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 up 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.
- Report any pollution incidents to SEPA in Scotland or the EA in England
- 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.
- 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.
- Donate to the Wild Trout Trust who do lots of excellent work in this realm, including giving free advice.
- Never dredge and campaign to stop dredging. Dredging de-stabilises the river and creates a need for more future dredging.
- Plant trees wherever fish spawn. Dappled shade is very good for eggs, fry and parr.