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Salmon Lifecycle

Salmon are really cool! They are anadromous which means they hatch in freshwater, swim to the sea to grow and mature, then return to freshwater to reproduce. Their ability to find their way home when it is time for them to reproduce is one of the most remarkable things about salmon. They battle strong currents and waterfalls to reach their hatching place. Here they release eggs and sperm, (spawn) and die shortly after. Their bodies float down the river filling the water with nutrients for other fish. In this way, salmon keep the freshwater lifecycle going.

Salmon have been around for centuries. Long ago, they provided food for the Indians living along the coast and interior of British Columbia. But with colonization came dams, pollution and the destruction of forests, and these have drastically reduced salmon populations in some rivers. There are five species of Pacific Salmon. They are:

• Chinook
• Pink
• Sockeye
• Chum
• Coho

In addition to having different spawning habits, each species has a unique appearance and life history.

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Pacific Salmon Life Cycle

1. Egg

Pacific Salmon begin their lives in freshwater. Each female salmon digs a nest or redd and deposits thousands of tiny pink eggs under the gravel of the streambed. This depositing of eggs occurs during the fall months. The spawning process is repeated in hundreds of small streams.

Over the winter months embryos develop within the eggs. The duration of the incubation period depends on water temperature and on the species. At first the head and body regions begin to form. The eggs are very fragile at this stage. Any movement may prove fatal to the little creatures, which are covered only by a thin shell-like membrane. About one month after they have been deposited in the gravel, eyes begin to appear. This eyed stage means that the embryo is developing normally and is now able to withstand considerable movement.

There is a great deal going on within this sheltered nursery. The embryo is receiving its nourishment from a yolk sac, which is attached to its underside. The yolk is made up of a mixture of water, fats, protein and salts. This yolk sac supplies the young salmon with all the food it needs for its development. Besides food, this tiny being needs oxygen as part of its growing process. As the young incubating salmon grows, it needs more and more oxygen from surrounding water. Many biologists believe that at some point the eggshell becomes limiting. The animal may not be able to extract enough oxygen from surrounding water. It is at this point that the hatching process takes place.

This hatching process is a very well timed event. When the organism within the egg has grown and developed to the point that its transparent capsule is too confining, it is ready to break out. Enzymes are released which dissolve the eggshell. When the shell is broken a wiggling little alevin (a-le-vin) emerges. Once the alevin has discarded the membrane of the egg, it can absorb oxygen from the flowing water directly through its gills.

Assuming that there is an adequate supply of oxygen, the alevin's growth rate will be determined by temperature. Alevins are wholly dependent on their yolk sac for nourishment. This fixed food supply must last for two to three months. Changes in the environment can affect the rate of development in young salmon. Higher water temperatures will make the alevin develop faster. However, faster development may be coupled with a reduced total growth. This happens because in warmer waters metabolic processes such as digestion and respiration are much less efficient.

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2.Emergence

Once the yolk sac has been absorbed the alevin, now called a swim-up fry, must leave the gravel in search of new food sources. The fry leaves the depths of the sheltering gravel guided by two systems: gravity and stream flow.

The fry emerges from the streambed by swimming straight up, against gravity. These tiny free-swimming fish maintain a nearly vertical position in the water on their journey upwards. They gain altitude in short stages. A steady, vibrating, tail motion is the force which enables them to reach the surface of the water. At this point the young fry is still heavier than water. Its main aim is to reach the surface and inflate its swim bladder.

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This upward journey under ideal conditions presents no real problems. However, if the gravel is covered with silt or heavy debris, the young salmon will actually attempt to tunnel out. The fry is fairly well equipped for this since its skin is tough, scaleless and covered with mucous. Its gravelly surroundings have made it adaptable to being fairly mobile in very cramped quarters. The fry is capable of:

• backing out of an unsuitable passage
• butting against barriers such as sand
• burrowing through the ground for considerable distances.
• curving its body and moving snake-like through narrow passages

On breaking the surface, the fry snatches air with a sideways snapping motion of its head. Then it drops back, keeping its mouth and gill covers tightly closed. Several gulps of air may be necessary before the tiny creature achieves its goal - neutral buoyancy. When it gulps, some of the air is transferred through a duct in its upper gut, into the swim bladder. Any fry that takes in too much air may be seen swimming head down to avoid floating to the surface. Tiny bubbles trail from their mouths as they get rid of the extra air.

The typical daily fry run begins shortly after dusk. It continues until around midnight. Sometimes there is a second peak of emerging swim-up fry shortly before dawn. A few hours after emergence all the fry will have achieved neutral buoyancy. Hundreds and hundreds of fry will be swimming in a normal horizontal position before dawn in their home stream.

3. Survival (Imprinting)

Every stream is unique. Water temperature, rate of flow, size and porosity of the gravel are all obvious factors which influence a stream's uniqueness. All the users of the water affect the quality of the stream. The quality of the water, therefore, differs from stream to stream and from one area of a stream to another. Young salmon are able to detect immeasurable traces of elements present in their environment. Odours from the rocks, plant life, other aquatic organisms, all have an everlasting influence on the young salmon.

During evolution each animal has adapted to the symphony or combination of all these stimuli. There are critical stages in all animals' lives. At a very early stage salmon imprint on the odour of their home stream. Young fry absorb the key elements of its stream. They retain this sense of having-been-there-before so that after years in the ocean most salmon will be able to navigate successfully back to the stream of its origin.

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A very complex interaction occurs between the tiny salmon and its environment. Stimuli from the water actually program the salmon to return to the home stream.

From the time the young fry emerges it is constantly receiving imprinting cues. The cues received from the water in one part of the stream are slightly different from those absorbed a little farther down or upstream. Growing fry establish territories in the stream. Less aggressive or smaller fry will be displaced and forced into areas where the food supply is limited. Some fry are even displaced from the stream.

Throughout the freshwater rearing stage, which varies from species to species, the fry receive a succession of cues which may assist them in returning to the natal spawning grounds. Young salmon may undergo a series of imprinting processes. Recent research reveals that imprinting may take place within a very short period of time (24 hours).

Biologists are still not certain exactly when the imprinting process begins. Some studies have shown that the salmon are most affected by their surroundings after they have emerged from the gravel. It could be important that the sequence in which imprinting occurs may correspond exactly to the reverse sequence of stimuli that the returning spawner receives on the way home. In other words, the fry are influenced in freshwater by the natal stream first and lastly by the estuary just before they enter the ocean. On the return migration they enter the estuary first then follow their nose back to the natal stream.

4. Fry Migrants

Simple explanations of salmon fry behaviour are not possible. Some species or stocks within species, migrate downstream to reach their rearing areas. Some others must go upstream. There are five species of Pacific salmon: pink, chum, sockeye, coho, and chinook. They are all anadromous (an-ad-ro-mous). This means that they hatch in freshwater, migrate as fry or smolts to the ocean, spend some time in saltwater and return as adults to freshwater to spawn.

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The five species of Pacific salmon spend varying amounts of time in freshwater. Sockeye spend the longest (1 - 3 years) whereas, pink and chum salmon depend the least on freshwater. Along with chum fry the pink immediately set out for the sea. Coho remain in freshwater on the average for one year. Chinook usually have a freshwater residence time of between three months to a year.

All young salmon face the same biological hazards: starvation, predation, disease. All young salmon have two defence mechanisms: the panic response and seeking cover.

5. Smoltification/Ocean Migration/Ocean Distribution

Salmon fry need food. Salmon fry need to avoid predators. Many animals in the estuarine area are predators of the salmon (e.g. birds, snakes, and larger fish). The ideal spot to accommodate both these needs is found in the estuary (es-tu-ar-y). Estuaries are the zones where fresh and saltwater mix. When freshwater flows into seawater the outflow creates a circulatory effect. Nutrients from the ocean are brought to the river mouth area.

This wealth of nutrients supports teaming populations of microscopic organisms. The peak of plankton production occurs during the late spring when salmon juveniles are migrating through this food abundant area. The length of time spent in the estuary is very species - specific.

Some species such as the pink salmon appear to pass through very quickly. Chinook salmon may spend months in the area. A lot depends on the estuary itself. For the species which spend any time in estuarine waters their growth rate is astonishing. When the salmon is in the estuary it is known as a smolt.

The distance that salmon will travel in the ocean varies as much as the direction. The extent and direction of the salmon's travel depends on time and size when they enter the ocean and the marine conditions they encounter. The growth rate in the ocean is very rapid.

6. Homing

Each of the Pacific Salmon species has its own life history and habits. Each species divides itself into separate runs in different river systems. There is an even further division into stocks, or races, within the watersheds. The stocks intermingle while they are in the ocean. As the salmon approach maturity they begin a movement to coastal waters. As they leave the ocean pastures and head closer to land the stocks disperse.

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Each salmon is seeking its home or parent stream. The ability of most salmon to navigate accurately from the ocean to a particular spawning ground is astonishing. Some salmon stray, that is, they migrate to and spawn in a stream other than the stream in which they were born. This occurrence may have some very positive effects on the survival of a particular stock of salmon. Straying ensures that the stock may survive even if some environmental or man-made disaster has threatened the natural spawning site. For some stocks of salmon the stream of origin lies thousands of kilometres from the sea. Other species spawn in coastal waters very near the ocean.

The environmental cues used by adult salmon to migrate from the ocean feeding grounds to the nearshore areas are not well understood. It is known that it's not a random occurrence. It is also known that the event is well timed and well directed.

It is apparent that once migrating salmon reach the coast they congregate in the shallow waters near the mouth of their river of origin. Distinctive home stream odours assume great importance. Studies show that the sense of smell is the most significant factor in homing. Visual cues are of secondary importance.

7. Spawning

When the salmon enter the mouths of their rivers in order to begin their upstream journey they cease feeding. They swim more or less as a group. Some species have long treks to make; others do not need to travel so far inland to reach their destination. The species that must cover 30 - 50 km. per day have to swim at incredible rates. The long journey to their ancestral spawning grounds is a remarkable feat. The trip upstream can present many obstacles. The fish faces both natural and man-made hazards. Swimming against the current, the salmon may encounter waterfalls, dams, fallen trees, adverse water conditions and predators.

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Since the salmon have been programmed to return to the same tributary of the river and even the same stream and gravel shallow in which their parents spawned and they themselves were hatched, they swim on.

At the spawning grounds the females swim slowly along the bottom, touching the gravel with their extended lower fins. They are searching out suitable places for their nests (redds). It is not certain exactly what influences the female's choice of sites. The males are occupied watching for unattended nest-building females. The female is the dominant partner. The male responds to her actions. The male also aggressively guards their territory against other male intruders. Each female salmon digs a nest or redd and deposits thousands of tiny pink eggs under the gravel of the streambed. And the cycle begins all over again.

Reference

Federal Department of Fisheries and Oceans / BC Provincial Ministry of the Environment. 1988. Salmonids In The Classroom: Primary Curriculum Resource Materials For The Study Of Pacific Salmon In British Columbia.