When it comes to trout and salmon, the needs of fish are quite similar to our needs. They need food, clean water, oxygen and shelter to survive.

However, in order to reproduce, trout and salmon eggs need loose, clean gravel with a good flow of oxygenated water. Clean gravel is gravel which has space between the pebbles, not gravel that is clogged with mud or silt on top of or between the pebbles.

Imagine a pile of marbles in a colander – if you pour water into it, it will flow straight through. Now imagine adding sand. The sand will plug the gaps in the marbles and if you keep adding sand and silt the water will slow right down and become a very slow drip out of the colander. 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, a hollow in the gravel where they lay their eggs).

clean river gravel
Loose, clean gravel is good habitat for trout and salmon eggs

Many people talk about habitat but this word is often misunderstood. It just means the right conditions for good survival.

Trout fry and parr (the next stages in their life after hatching from eggs) need shallow, fast-flowing ‘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 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 (newly hatched juveniles) and fry can drift a short distance downstream and find the right habitat for their next stages of life.

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 very 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 important factor for a thriving trout and salmon population. Shelter has many ways of saving the lives of fish.

  1. A place to hide from predators
  2. A way of providing shade and keeping the water cooler in hot weather (this can save the lives of both fish and fish eggs)
  3. A source of food (lots of insects will live on and around woody debris)
  4. A place to avoid the dangers of fast moving water and debris during a heavy flood
  5. 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 when agriculture or 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 insects (ie fish food) it holds.

Woody debris is a top source of shelter and insect food

[continued in the next post]