Life in the river
Mature female salmon are extremely fertile fish, depositing thousands of eggs in a redd. The number of eggs each female salmon produces depends on her size at the time of spawning, but a female often produces around 1,100 eggs per kilogram of body weight. The eggs develop in the redd and hatch during March or April. The very young salmon ('alevins') have a large external yolk sac on which they feed for their first few weeks, remaining buried in the gravel until late spring. Then they emerge as young salmon ('fry') and disperse to feed. Salmon fry compete for the best feeding areas with the strongest individuals displacing the weaker ones to poorer feeding areas. During their first year the vast majority of fry will die from starvation and predation.
After one year living in the river 'fry' become 'parr' and they usually remain feeding there for a further one to three years until they are ready to migrate into the sea. Fry and parr feed on a variety of aquatic insects that live in the river, but also feed on other insects that accidentally land on the water surface or fall in from overhanging trees.
As the parr get ready for their passage to the sea they begin moving down the river and become 'smolts'. A number of changes take place as a parr turns into a smolt in preparation for life at sea. The fish becomes more streamlined and silvery in appearance and loses the dark markings along its side. Smolting usually occurs during late spring and soon smolts gradually experience a change from fresh to salt water as they swim out into the estuary. When smolts leave the river they are about 10- 20cm in length and once they begin feeding in the sea growth is extremely fast. A salmon smolt that typically weighs about 100g when it leaves the river will, after only one winter at sea, often weigh around 3.5kg.