The Life Cycle of Salmon
Salmon in The Castlecomer Area
From around October to the end of January the Salmon make their way up to the Rivers and Streams of Castlecomer to spawn. On their way they enter the River Nore at Waterford, pass up by Inistogue, Thomastown, Bennetsbridge,and Kilkenny. They enter the Dinan at the Dinan Bridge and then head up every stream and River including the Deen, Clohogue, Black River, Eskerty River and many more. They face many hazards on their journey up and if they survive, it is no picnic getting back. Following is an account of the difficult Life Cycle of that King of Fish.
Nest – Building and spawning:
The life of a salmon begins in streams – often head streams or streams close to the source of a river – where clear water flows over a bed of gravel. The female – or hen – salmon, finding a place where the current flows through the gravel, uses her tail to scoop out a hollow in the river bed, Meanwhile, while the hen works the “cock” fish stays close, driving off any other males which may come too close. The hollow is about 20 centimetres deep and between 50 and 100 centimetres long and is called a redd. When it is ready the hen fish ejects a stream of eggs into it and these are quickly fertilised by the male which releases millions of sperm cells or “milt” into the water beside the eggs. The hen salmon lays her eggs in several batches in the redd, covering each lot with gravel. Finally a single mound is formed over all the eggs and the parent fish now exhausted, leave the redd and the eggs to fend for themselves. The parents are now known as “kelts”, “slats” or “spent fish”. They have become very thin and their flesh is pale red in colour and has very little food value. Most male kelts die soon after spawning but some of the females survive and may return to spawn again.
Spawning usually takes place in December. A hen salmon lays about 2,500 eggs for each kilo of her body weight. Therefore a two kilo salmon will lay about 5,000 eggs while a fish of four kilos will lay about 10,000. The eggs themselves are orange and are about the size of a green pea with a soft, gelatinous skin. The temperature of the water where they are laid is low, not much above freezing for which there is a very sound and practical reason. While the eggs develop slowly in such cold water they have the advantage that the cold inhibits many of the insects which might otherwise eat them at this time.
The eggs hatch into little fish after about two months. These baby fish are called “alevins” and have soft, delicate skin. Except for their large black eyes they, too, are orange coloured. Attached to the alevin’s slender fish-like body is a balloon-shaped “yolk sack”, full of rich food which the alevin digests while it grows. The alevin is a rather helpless creature, capable only of wriggling, and it remains in the comparative safety of the pebbles on the gravel bottom.
After a month the yolk sack has been consumed and the alevin, now nearly two centimetres long, has taken on a light brown, speckled colouring. It can now make its way out of the gravel to swim in the open waters. These small fish are called “fry”.
Growth of fry and parr
By now it is February or March. the water is getting warmer and the water fleas and the other small animals are beginning to multiply. When the fry leave the gravel they feed on these at first. Later they will eat larger creatures, especially the larvae of insects. Now, the first battle for survival begins. There is not enough room in the nursery ground for the many thousands of the little fish which hatch from the eggs. Only the strongest can survive and on average only four or five fish for every hundred eggs survive until the following winter. It is a hard time for the young and the great majority of them die within weeks of leaving the redd. Most are eaten by larger fish, especially trout and older salmon, as well as by large insects and birds.
The survivors grow rather slowly and during their first summer they reach a length between five and ten centimetres. As they grow the colour changes a little too and these larger, differently coloured fish also acquire a new name. A row of about nine blackish marks appears along each side of the body. These are called “parr” marks and the young fish, at this stage, are called “parr”. As parr the majority of them remain in the rivers for two years, at the end of which time they measure between ten and twenty centimetres long.
In most rivers of the west and south west the water is pure with a very small dissolved lime content in the absence of which, beside other chemicals, few insects can live and those that do are very small. Such rivers are described as “poor” and the parr in them will be rather small. On the other hand, many rivers which drain the midlands have an abundance of lime and minerals in solution. They are rich in insect life and so there is ample food for young fish. In such conditions the salmon parr grow large and fast. From a salmon’s point of view a disadvantage if these rivers is that many other kinds of fish live there as well and so there is much more competition for the available food.
Having lived in fresh water for about two years a great change comes over the parr. Their skins begin to assume a silvery colour and changes take place in their bodies to prepare them for a journey to the sea. In April they swim down stream in very large numbers. These small, silvery salmon are called “smolts”. As a rule they swim by night, keeping hidden in the daytime from birds and fish which might otherwise eat them. The departure of these fish from inland waters is known as “the smolt run” or “smolt migration”. The size of smolts varies from river to river. They are large in the rivers Liffey and Lee, with an average length of 16 centimetres: middle sized on the Shannon and Corrib at about 14 centimetres and as small as 12 centimetres on average in the Owenea in Donegal. Not all parr change to smolts at two years old nor do they all leave in April. In all rivers the majority are two years old but some will go early at one year, while others stay in fresh water as long as three years and a very small number take four years to grow up. One year old smolts are more plentiful in the rivers where there is rich feeding and three year olds where food is scarce. Size seems to be an important factor and parr which grow quickly migrate earlier.
Life in the sea
After leaving its fresh water nursery the salmon will spend at least a year in the sea. In that time it may travel great distances; some Irish salmon go to the west coast of Greenland while salmon from Sweden have been caught off the west coast of Ireland. The outward journey is slow and salmon feed as they travel. No one is quite sure what smolts eat since they are very seldom captured at sea, but well grown salmon feed mainly on shrimp and fish. Salmon caught off our coast have been feeding on sand-eel, sprat and young herring.
In the sea, where the salmon hunt, food is much more plentiful than in the rivers where they were reared and they grow much faster. A fish which took two years to reach a weight of 50 grams in the river increases to about 2,500 grams at sea in the next year. After a year or so at sea most Irish salmon head for home again. As they approach the coast they are at the peak of condition. Not only have they grown big but they have also stored away enough food to last them for more than six months. This food is kept in the form of fat in the flesh and this is what makes the salmon such a very rich and good food to eat. Some stay a sea for more than one year. These travel further and they include the ones which go to the feeding grounds off Greenland. These fish stay away for just under two years, giving them two summers of feeding on plentiful foods. A few remain away even longer and there are records of salmon which didn’t return for four years. Such fish become giants, weighing as much as 27 kilos. Why some salmon return after a year while some others stay away for longer is a mystery. In some rivers older fish are never seen while in others they are quite plentiful.
The return journey
Salmon always try to return to the rivers in which they were born. How they achieve this is not known. All that can be said for certain is that it is a migratory instinct governing the way of life of this magnificent fish, but how it works is a mystery. One theory, similar to the one about migrating wildfowl, is that , in some way they are sensitive to and respond to the earth’s magnetic field.
Great numbers of salmon swim close to the coast in some places. Many approach the north coasts of Donegal and Antrim and from there some swim to rivers in Cork and Kerry. Some are heading for Scotland. Salmon which pass along the coast of Mayo are mostly heading for the Moy and other nearby rivers, but some are returning to countries as far away as Sweden. On these journeys they can travel at least 50 kilometres a day, but they spend a lot of time searching for their own river. Many enter wrong estuaries before finding the right one. How they find their way home eventually is not known for sure. They may recognise a particular smell from the water of the river they were reared in.
The instinct to return to the river in which it was born is so strong that if salmon eggs from one river are planted in another there is the risk that many of the young will not return. However, there are cases where the substitute river is recognised by the fish, and the salmon which have spent a year or two developing in a particular river, not that in which they were born, do succeed in returning to it. The need to return to the river of birth was discovered only a few years ago. Until that time eggs from various rivers were freely distributed to others. This is no longer considered good practice.
Back to the spawning ground
The youngest salmon to return do so in May after spending little more than a year away. As the summer advances more and more arrive, the greatest numbers in July. These salmon are all quite small, mostly between two and three kilos, and are called “grilse” or “peal”. Generally speaking the later they come the bigger they will be, as the late comers have had more time to feed at sea. Grilse may continue to arrive until about October. Besides grilse small numbers of “large summer fish” which have spent rather more than two full years at sea arrive in the same season. They are much bigger and generally weigh between three and five kilos.
Towards the end of December, “spring fish” begin to appear. These are about the same size as the large-summer fish. Although they have spent about six months longer at sea than the summer fish they have not had very much more time for feeding since the rate at which they feed in the sea decreases in winter. Through January and on to May the spring fish continue to arrive, but they are never so numerous as grilse and in some rivers there are no spring fish at all. Moreover the number of spring fish varies greatly from year to year. For example on the river Shannon, over a period of 20 years, spring fish accounted for between 1.6% and 13.3% of total annual catches.
No matter what time of year they come most salmon wait in the river estuaries until the river floods. Therefore few salmon move up river in dry weather, but do so very quickly when it rains. They don’t eat anything after they return to their rivers, but will often snap at fish or insects or at lures which look like their natural food. If they didn’t have this habit there would be little reward for the angler.
When they enter fresh water salmon retain the silvery colour they acquired at sea. But they soon begin to lose this and acquire an irregular pattern of reddish brown markings, which help to camouflage them against a background of gravel or mud. Also back in the river the cock salmon’s lower jaw develops a hook which is called the “kype”. It becomes so big that he can’t fully close his mouth. But the female keeps her neat streamlined form. When the fish arrive from the sea the sex organs are very small and usually show no trace of development. But inside their bodies the milt and the eggs are growing. By the time of spawning the ovary, containing the eggs, fills nearly all the space in the body cavity of the hen fish. While in the cock, the male organ, the testis, also enlarges but not, of course, to the same extent as the ovary in the hen.
Grilse, which enter rivers late in the year, have only a few months left before spawning. An early spring fish, which appears at the beginning of January, has nearly a year to wait. Whatever the length of time they spend in fresh water before spawning, adult salmon use the reserves of fat stored in their flesh for food, so that when they do spawn most of them, especially the males, are quite exhausted. Furthermore their is little food for them in the rivers at spawning time and the water also carries the spores of fungus and other organisms which can cause fatal diseases in weakened fish. However some salmon do manage to survive long enough to journey back to the sea and abundant food, recover from their efforts, grow even larger and return in a year or so to spawn again. In some rivers as many a s one salmon in ten survives to spawn again, but the numbers can be down to one in a hundred or so in others.
This, then, is the life cycle of the salmon. It is an adventurous life for a fish, divided, as it is, into phases; feeding and growing slowly in the nursery river; the journey to the sea; feeding, fast growth and long journeys at sea, the return to the original river, and finally a period of fasting there during the journey upstream to the spawning beds in which the eggs, which ensure the next generation are produced.
It is an efficient life cycle. The average salmon lays 5,000 eggs of which only about two complete the cycle to return as adults and restart the process by laying and fertilising another 5,000 eggs. It seems like a low rate or survival. In fact it is quite high in fish terms, many times higher than the survival rate for cod and other fish which lay as many millions of eggs as the salmon does thousands and from which an average of only one pair of adults survive.
This has been the way of life of salmon for countless thousands years, long before human beings began to fish for them. Until now the salmon has survived and multiplied in the face of natural dangers and of men who hunted them everywhere but now, at last, they are threatened with extinction. The changes brought by man on the salmon’s environment through pollution, for example and the enormous technical advances made in commercial fishing gear have combined to bring the salmon to the brink of disappearance. The question we must all ask ourselves is; will salmon become merely a memory, like the bittern of Ireland and the dodo of Mauritius. Some people already refer to it as the Irish Dodo.