Entry to the river
Once they reach the coast the salmon locate the river of the birth by the river's characteristic odour which was imprinted in the fish on the way out. On finding the estuary the salmon may quickly swim up the estuary into freshwater. However, the ease by which this is done does depend on river flows and the size of the tide, although with its relatively high flows this is less of a consideration on the Tay than smaller rivers. During droughts for example, salmon may remain in estuaries waiting for rain, or even perhaps remaining in the open sea.
Once in the river the salmon usually try to make good progress upstream towards whichever part of the river they were born. The speed at which they travel depends on several factors. One again in river flow, with high flows encouraging fish to migrate while droughts cause them to congregate in deeper pools. Another consideration is water temperature. Salmon are cold blooded and this means that their ability to swim is related to the temperature of the water. If it is cold and the water near freezing, salmon are not able to swim as quickly as they can when the water is warm in summer. This means that in cold weather it is more difficult for salmon to swim through rapids or it may be impossible for them to jump over waterfalls, while in summer they could pass the same obstacles with ease.
Therefore, cold weather in the winter or dry weather in the summer will tend to hinder the salmon's migration but spates help them to "run".
While salmon seem to make continuous progress for a period after they enter the river, radio-tracking studies have revealed that after a time they tend to settle down in some part of the river and may remain in a semi-dormant state, sometimes for months, doing little but conserving their energy for when spawning time eventually comes.
The approach of spawning
When spawning time approaches the salmon once again become active. In the days leading up to spawning the fish will make their way from the secure areas they have been resting to seek out spawning sites. The fish may have been resting up close to their eventual spawning destination but others may still travel some distance in this, the final migration.
During this period the salmon are inclined to throw
caution to the wind and can be found in shallow water
area which they would normally avoid and are particularly
vulnerable to predation.
When salmon arrive from the sea they are strong powerful fish packed with energy gained during their marine feasting. At that point they are bright silver in colour and look in the peak of condition.
However, in freshwater, the salmon do not feed. Their entire upstream migration and eventual spawning, a process which might take months, is dependent on accumulated fats in their bodies. Not surprisingly over time the fish do change. They do gradually lose condition and reserves are converted into eggs or milt. At the same time the appearance also changes. They silver colour disappears and they develop darker colours. Male salmon become quite red and females usually a dull brown. Males also develop a hooked lower jaw, known as a kype which is used in fighting other males over mates.
After spawning has been completed the salmon are a poor shadow of the pristine fish which left the sea months earlier. They are now known as "kelts" and are emaciated and sometimes battered and infected by fungus. Unlike most of the Pacific salmon, Atlantic salmon are not all doomed to die and can potentially live to spawn again, though most do not.
After spawning, female salmon quickly shy away from the spawning areas and move downstream trying to conserve their energy. On the other hand males remain active, racing about the stream hoping to find other mates. There is therefore a high death rate of males in the aftermath of spawning but a much higher survival rate for females.
Many of those fish which do survive may be swept seawards by large floods which they are powerless to resist. The stronger ones will instead remain in the river often for several months during which time the colouring up process goes in reverse. By the early spring the kelts lose their dirty appearance and are restored with a gleaming silver coat, just as they did when the were smolts. Some "mended" kelts can appear in surprisingly good condition and can sometimes be confused with spring salmon.
For those which make it back to sea and recommence feeding, they may regain weight again and return once more to spawn again. However, very few of the fish entering Scottish rivers today are fish coming back for their second attempt.