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The diagram below summarises the main trout lifecycle stages. For more detail on each stage, see the relevant section in the text and photographs below.
Sea trout and brown trout are the same species (Salmo trutta) but sea trout adults go to sea to feed and return to freshwater to spawn. For more information on sea trout, click here.
Trout lay their eggs in nests in the river gravels, known as redds. The female (hen) builds the nest, usually between November and January when the water is cold and carrying lots of oxygen, because that is what the eggs need to hatch.
She looks for gravel with a good flow of water passing through, so the gravels need to be loose and largely free from silt and between 5 and 50mm in diameter.
The hen fish will start by testing the gravels with her anal fin. If the gravels are good, she will dig a hole, turning on her side and flexing her body. She will then release some of her eggs. The male (cock) fish will release his sperm or milt over the eggs to fertilise them. The hen then moves forward and digs again to throw up gravel to cover the fertilised eggs.
A trout redd can be very obvious in clear water – it looks like a patch of very clean gravel, heaped into a mound often with a hollow upstream of the mound. Redds can vary enormously in size, from 50cm2 to over 150cm2. Small trout generally create smaller redds in finer gravel, and big sea trout can create redds the size of a kitchen table top with much bigger stones. (photo of redds). How many eggs are laid also depends on the size of the hen trout – a 500g trout will typically deposit around 800 eggs.
As the eggs develop, you can see trout ova develop within them. This is called the ‘eyed ova’ stage. How quickly the eggs will hatch depends on water temperature – colder water means slower development in the egg.
At 7.80C the eggs will hatch in 60 days but at 4.70C they will take 97 days to hatch. Generally, it is assumed that most eggs hatch in February and the age of a trout is measured from this ‘birthday’. How many eggs hatch varies enormously depending on quality of the water and gravel – it can be as low as 4%, or, at the top end, exceed 80% where conditions are really good.
The newly hatched trout are called alevins, and they live in the gravel, feeding off the remaining yolk that is attached to their body for 14-30 days, again temperature influencing their rate of development.
Once the yolk has been eaten, the alevin become fry, emerge from the gravel, move towards the light and start to feed on tiny insects in the water.
Mortality rates at this highly vulnerable stage are very high. The fry are just a few centimetres long and consume a lot of energy, so they need to find food quickly, and plenty of it. They also become territorial – they want to be out of sight of other fry, so need habitat that has plenty of stones and plants to enable them to hide from the neighbours. And they are still very tiny, so they need shallow water (1-40cm) that isn’t too fast flowing.
The picture below shows a tiny trout taking it's first meal. The transition from yolk (i.e. maternal provisioning) to independent feeding is a critical life stage - and the one at which the majority of mortality takes place.
A trout of less than one year old is called a parr. They are recognisably trout now, but have distinctive fingerprints or parr marks along the side (photo of a parr) which they lose as they get older.
Parr have similar habitat needs to fry : plenty of cover to hide from each other and from predators, especially fish eating birds. They can cope with deeper and faster water as they grow. In order to find their own territory, they will gradually drop downstream with the flow rather than fight their way up against the flow.
It is quite difficult to tell whether a parr is salmon or trout. One clue is the habitat - salmon parr tend to prefer faster riffles than trout as they are stronger swimmers. The diagram below, from the Atlantic Salmon Trust, identifies the main differences.
1. More streamlined shape
2. Deeply forked tail
3. Longer pectoral fin
4. Lack of orange on the adipose fin
5. Smaller mouth
6. Sharper snout
7. Only 1-4 spots on gill cover (often one large spot)
8. Well defined parr marks.
9. Shorter maxilla (does not extend beyond the eye)
Adult brown trout that remain in the river retain their territorial behaviour, and will protect their territories or ‘lies’. Most trout will have a feeding lie, typically in an area where the river current acts as a conveyor belt for food so they can simply face upstream and catch invertebrates as they drift past, expending as little energy as possible. They will also have one or more resting lies, where they are safer from predators. Typically this will be under an undercut bank, tree root, rock or log.