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The life cycle of the Atlantic Salmon begins high in the upper reaches of the river of its parents birth in the gravel bed of a relatively shallow, well oxygenated and fast flowing stream or river tributary.

During spawning time - usually November through to January - the hen salmon excavates a shallow depression in the river bed gravel by violently fanning her caudal fin, displacing the gravel which is then carried downstream by the current leaving a hollow into which she lays her eggs, called ova.

This “nest” is known as a redd and offers the ova shelter and protection from predation by trout, eels, low temperatures and flooding.

With the last autumnal leaves now nothing more than a fading memory, the once majestic and fin perfect salmon that first entered the river system of its birth is now a mere shadow of its former self and no longer in its prime.

The splendid coat of metallic chrome it once wore now faded, hens with bellies full of roe darkened, close to black, whilst the cock fish now wear a tarnished tartan cloak of russet brown and sport a large upwards curving lower jaw, known as a kype.

Once the hen salmon has found a suitable stretch of shallow well oxygenated river bed she sets about preparing the spawning redd, meanwhile the cock salmon continues his courtship whilst patrolling the area keeping rivals at bay.

The depth of this redd is typically less than 30 centimetres with the actual depth being dependant on both the composition of the river bed and the size of the hen fish.

To attract the cock salmon onto the redds the hen signals to the male that she is ready to lay by releasing pheromones into the water. These chemical signals let the male know that she is ready to mate, which, in turn, alerts the male and prepares his reproductive system to fertilise the eggs.

With the salmons ability to detect, and respond accordingly, to these water borne pheromones playing such a crucial part in the reproductive process it is easy to understand how environmental factors such as pesticides, agricultural fertiliser run-off, acid rain and industrial pollutants etc., can have such a very serious and detrimental affect on the salmons chances of successfully reproducing.

Furthermore, the small streams and river tributaries favoured by salmon are particularly vulnerable to environmental fluctuations with agricultural and forestry drainage potentially having serious consequences.

Spawning itself is a fairly synchronised affair, once the female is ready she begins pushing down firmly into the redd repeatedly, inviting her mate to join her and whilst he quivers with excitement beside her the eggs and milt are released simultaneously and the eggs fertilised during this intermingling of gametes.

Then, whilst the cock salmon withdraws a yard or so downstream, the hen salmon swims slightly upstream and with her tail begins agitating gravel and silt so as to covers the eggs, roughly to a depth of between 12cms and 24cms which offer them at least some protection.

Once again, as is so often the case, nature has taken out an “insurance policy” to try and ensure the survival of the species.

With around three quarters of male salmon parr reaching sexual maturity, these cocky, precocious and opportunistic young salmon will frequently seize the opportunity to nip in and fertilise a proportion of the eggs whilst the mature fish are pre-occupied.

Though the frequency varies, it has been observed that up to ten individual parr may attend and assist in the fertilisation of each females eggs.

Swimming closer to the river bed the milt from these smaller fish is deposited directly into the gravel where some of the eggs will have settled, whilst a proportion of the larger salmons milt is swept away and wasted in the waters current.

This mating process may now be repeated several times until all the hen salmons eggs are laid. A hen fish may lay in the region of 700 to 800 eggs for every pound of her own weight, this equates to in excess of 11,000 eggs for a 15 lb fish.

The actual hatch rate of these eggs can be as high as 90% however various external forces such as flash floods, temperature fluctuations and predation greatly reduce their survival rate.

This high fecundity – ova per female - is essential as the survival rate in the wild is extremely low.

The spherical eggs are quite large, about 5mm in diameter and somewhat adhesive for a short while with a pale translucent orangey-red appearance, this colour coming from astaxanthin, a carotenoid, (antioxidant pigment) which the salmon get from their diet.

The actual time schedule of spawning, egg development and incubation and subsequent hatching has evolved largely in response to local conditions and to try to ensure the maximum survival rates of eggs, alevin and fry. These times all vary considerably from river to river and rely greatly on quite a number of factors.

The speed at which the ova develops is largely dependant on water temperature and it is this factor which may provide us with the best forecast of when the eggs may hatch.

Fluctuations in water temperature (climate change) plays a large part as do oxygen levels and any reduction and/or increase in water flow. The depth at which the eggs settle within the redd can also affect development, influenced once again, by the water temperature and oxygen content.

In general this overall incubation period is somewhere in the region of three to four months. During the early development stage the eyes, of the yet to hatch alevin, are clearly visible and as time passes a greatly increased rate of movement within the pea sized egg can be seen.

Nature here is attempting to align various events to coincide with the optimum time - when food is in plentiful supply - for the young alevin to emerge. If this can be achieved these young salmon have the greatest chance of survival and of growing to maturity. However, if, for whatever reason, these conditions are not met then the alevin and fry will suffer a much higher mortality rate.

The eggs, in early spring, having overwintered these past three to four months in the relative safety of the redd, now hatch into tiny translucent creatures with yolk sacs suspended under their throat containing all the necessary nutrients to sustain life, this survival ration pack has the capability of lasting several weeks. These young fish are weak, cannot swim and are extremely vulnerable to predation. At this stage they must rely entirely on stealth to avoid detection and lie low to avoid being eaten.

Remaining safely in the redd until the yolk sac has been absorbed their young gills develop and they now become active hunters and begin feeding on microscopic food particles. At this point they become increasingly active and, wriggling free from the gravel, swim up to the surface where upon taking a gulp of air they achieve neutral buoyancy by filling their swim bladder allowing them to now swim freely and hold their position in the current. This “swim up” as you might imagine is an extremely dangerous stage of their young lives exposing them to many predators.

They are now known as fry, around 3cms in length, and begin to actively hunt for food and establish their own territory whilst hiding behind rocks and boulders or whatever cover they can find to avoid detection and predation by trout, eels, cormorants, kingfishers, heron, goosanders, mergansers and many more.

This period of fry emergence and subsequent dispersal takes place mainly during the hours of darkness with this tactic thought to be so as to minimise predation and maximise the chances of survival. Nevertheless it is at this stage that the highest rate of mortality occurs with around 90% of the fry failing to thrive.

The vast majority of fry remain relatively close to the redd, depending on the strength of the waters current. Those that disperse early seeking new territory generally find and take up station in the better lies were food is plentiful and thereafter protect their new found territory against late arrivals who must make do with whatever remains unoccupied upon their arrival.

As they grow in size the fry now become opportunist feeders and move on from consuming microscopic plankton to larger food with items such as aquatic invertebrates, midge, nymphs, caddis flies, stoneflies, mayfly, small fish and even salmon eggs being taken with relish. Their numbers are regulated by such things as predation, temperature, pollution and competition from other fish.

Over the autumn the fry slowly acquire camouflage with dark vertical fingerprint bars appearing along their flanks intermingled with black and red spots lying predominantly along the lateral line whilst sporting a predominantly forked tail. By their second spring they will have reached a length of 5cms or thereabout and now closely resemble small brown trout, they have now become parr.

Parr will remain in the river of their birth for a further two to three years depending on water temperature and food availability before undertaking yet another dramatic physiological transformations in preparation for their marine migration to the rich Atlantic feeding grounds.

This process is known as smoltification and occurs once the parr have reached a length of circa 10 to 20 cms. Once smoltification begins the young salmons appearance slowly begins to take on a silvery sheen andd internal changes begin to control salt regulation as the salmon begins to prepare and adapt for life at sea. They are now more elongated in shape and can swim not just against the river current but critically - also against it.

Having spent at least one year in freshwater - with river temperature and food availability having a significant influence on both their development and how quickly this stage is reached - the young salmon, now roughly 10 to 20 cms in length swim at a crossroads in their young lives as they begin to prepare to undertake a dramatic transformation in preparation for their marine migration to the rich Atlantic feeding grounds.

Those fortunate enough to have survived the rigours of their early river life and having somehow managed to avoid the savage tooth, claw and beak onslaught of their many predators, now face yet another critical period of development as changes to their shape, colouration, physiology and behaviour begin to occur as they prepare for their adaptation to the next stage of their life in the salty ocean environs.

During this process, known as “smoltification”, internal changes occur which begin to control salt regulation as they begins to prepare and adapt for life at sea.

Their fins gradually darken and the previous spots and fingerprint markings of the parr recede, apart from a few black spots on the gill covers.

Now, with a layer of guanine crystals forming on the skin a quite incredible transformation takes place and the young Salmon throws off its parr livery and now finds itself reborn, resplendent in a coat of the finest metallic silver.

This morphological ability of the Atlantic Salmon to survive and thrive in freshwater and saltwater in this manner is highly unusual requiring clever adaptations of their body chemistry.

This ability is known as being “euryhaline”, of which there are two types of fish, “anadromous” - encompassing those who mature at sea and spawn in rivers such as the salmon and the other “catadromous” - with eels (Sargasso Sea) being the most obvious example – who mature in freshwater yet spawn in the sea.

Evolution has guided generations of these young salmon to only head for the estuary when the sea temperatures are most likely to support the necessary food required to not only keep them alive at sea but also allow them to gain weight and strengthen quickly.

The timing of this initial migration, usually in March, April and May, with most fish departed by June, is of critical importance to avoid the many predators - old and new - that lie in wait, not only in the estuary downstream, but also in the wider ocean beyond.

These “post smolts” - as they are known - are now more elongated in shape and capable of swimming not just against the river current but critically - also with it.

During this stage shoals of these juvenile salmon congregate in the lower reaches of their rivers where they find themselves highly susceptible to predation by seals, birds and other fish.

In an attempt to minimise the inevitable casualties through predation the majority of this seaward migration takes place during the hours of darkness - on an ebbing tide.

Travelling under cover of darkness in this way and assisted by the tidal flow allows these smolts to travel at pace through the estuary and out to sea whilst being much less visible and at risk of predation than would normally be the case if this precarious journey occurred during the hours of daylight.

Although little is known with any certainty regarding the actual routes taken by the post-smolts or returning adults during their epic migratory journeys some research has indicated that these young schooling salmon now head for the cold deep sea waters off the Faroe Isles, Greenland, the Norwegian Sea and the vast expanse of the Atlantic Ocean where they grow quickly feeding voraciously on small fish, sand eels, capelin, herring and a great variety of small crustaceans.

At this stage their very survival depends to a large degree on their growth rate as the faster they can grow in size it naturally follows that predation upon them will be significantly reduced.

The actual survival rates of salmon whilst at sea is determined by many factors including the ocean currents and the availability of prey items, both of which can vary greatly from year to year.

Whilst the young fish may stay at sea for just over one year returning as a “one-sea-winter” salmon now called a “Grilse” - weighing somewhere between two and eight pounds - or it may remain at sea for a further year, or perhaps even longer, returning to its birth river then as a “multi-sea-winter” salmon.

It is worth mentioning here that the majority of the salmon that feed in the Norwegian Sea are those that return after just the one year whereas those that head to Greenland generally remain there for multiple years before making their return to their native rivers.

These multi-winter-salmon are now one of a very small number of fish species at the very pinnacle of the pelagic food chain and will return to their home river considerably earlier in the year, usually in the spring and can often weigh in excess of thirty pounds although if sufficient food is available whilst at sea they may attain weights of double that.

Atlantic Salmon are an emblematic species for both Scotland and Ireland and much work has already been undertaken to try and understand what can be done in an effort to preserve this magnificent creature for future generations.