Aquaculture

Salmon aquaculture is the major economic contributor to the world production of farmed fin-fish, representing over $1 billion US annually. Other commonly cultured fish species include: tilapia, catfish, sea bass, carp, bream, and trout. Salmon farming is very big in Norway, Sweden, Scotland, Canada, and Chile and is the source for most salmon consumed in America and Europe. Atlantic salmon are also farmed in Russia, Tasmania, Australia and the UK.

Salmon are carnivorous and are currently fed a meal produced from catching other wild fish and other marine organisms. Consequently, as the number of farmed salmon increase, so does the demand for other fish to feed the salmon. Work continues on substituting vegetable proteins for animal proteins in the salmon diet. Unfortunately though, this substitution results in lower levels of the highly valued Omega-3 content in the farmed product. Intensive salmon farming now uses open net cages which have low production costs but have the drawback of allowing disease and sea lice to spread to local wild salmon stocks.

Another form of salmon production, which is safer but less controllable, is to raise salmon in hatcheries until they are old enough to become independent.

They are then released into rivers, often in an attempt to increase the salmon population.

Life History of Salmon

Eggs in different stages of development. In some only a few cells grow on top of the yolk, in the lower right the blood vessels surround the yolk and in the upper left the black eyes are visible, even the little lens.




Salmon fry hatching - the larva has grown around the remains of the yolk - visible are the arteries spinning around the yolk and little oildrops, also the gut, the spine, the main caudal blood vessel, the bladder and the arcs of the gills.


In order to lay her roe, the female salmon uses her tail fin to excavate a shallow depression, called a redd. The redd may sometimes contain 5,000 eggs covering 30 square feet. The eggs usually range from orange to red in color. One or more males will approach the female in her redd, depositing his sperm, or milt, over the roe. The female then covers the eggs by disturbing the gravel at the upstream edge of the depression before moving on to make another redd. The female will make as many as 7 redds before her supply of eggs is exhausted. The salmon then die within a few days of spawning.


The eggs will hatch into alevin or sac fry. The fry quickly develop into parr with camouflaging vertical stripes. The parr stay for one to three years in their natal stream before becoming smolts which are distinguished by their bright silvery colour with scales that are easily rubbed off. It is estimated that only 10% of all salmon eggs survive long enough to reach this stage. The smolt body chemistry changes, allowing them to live in salt water. Smolts spend a portion of their out-migration time in brackish water, where their body chemistry becomes accustomed to osmoregulation in the ocean.

Salmon Ocean Habitat

Anadromous salmonids spend part of their lives in salt water. Chinook salmon, coho salmon, steelhead trout and coastal cutthroat trout are all anadromous. These fish leave their streams and migrate out to the ocean, where they grow much larger than salmonids that stay in the stream all the time. Chinook usually move into the estuary when they are several months old. The other anadromous fish all spend at least one year in the stream before migrating to sea.

Salmon and steelhead spend from two to five years in the ocean, depending on the species. The waters just off the coast of northern California and Oregon experience an upwelling of nutrients, which supports a rich ecosystem. Food is plentiful and the salmon grow large quickly. Schools of smelt, like the ones pictured above, are just one source of foods.

Salmon Freshwater Habitat

This section of the Shasta River provides good salmon and steelhead habitat. Trees shade the water and keep banks stable. It may also provide suitable spawning gravel.

Trout and salmon need cold water to survive and grow. Rain, and snow melting from mountain peaks feed their stream and lake habitats. Healthy salmonid streams are usually shaded by trees. The tree roots make the stream banks stable and provide hiding places for the fish. Leaves from the trees fall into the stream and become food for insects, which are in turn eaten by salmon and trout.

Insects such as stoneflies, mayflies and caddisflies spend their early life stages as nymphs in the gravel riverbed. When mature, they swim to the water surface and then fly away to reproduce. Fish eat both the nymphs and adults.

As young salmon and trout grow larger, they move from shallow areas into deep pools. Pools are scoured when water plunges over or around boulders and logs. The "bubble curtain" is a favorite place for salmon and trout. They can't be seen by predators above and there is plenty of oxygen. The current brings insects and other small food items. At the end of pools, where the stream narrows, the current picks up and washes the gravels clean, making them ideal for nests.

Salmon Life Cycle: Spawning

When a female salmon arrives at her home stream, she chooses a nesting site with just the right combination of clean gravel, adequate depth, and good flow to provide oxygen for her eggs. She digs her nest by rolling onto her side and pumping her tail against the gravel. Stones are dislodged and carried downstream by the current. Every so often, she checks the depth of the nest by "crouching": lowering herself into the nest and inserting her anal fin into the spaces between the stones.

Males fight for access to nest-building females. The dominant male courts the female by quivering and crossing over her back.

When she is ready to lay, he moves alongside her and together they release eggs and milt. At the last moment subordinate males rush in and may manage to fertilize some of the eggs. The eggs settle into the spaces between the stones. The nest is covered with loose gravel as the female builds another nest upstream. Both male and female soon die after spawning, but females will defend the nest until they are too weak to do so.

Salmon Life Cycle: Adult

Salmon live in the ocean for 1 to 7 years. Pacific salmon range as far south as Monterey, California and as far east as the coast of Siberia. When conditions are right, an unknown signal tells them to begin the migration home. Somehow they find their way across thousands of miles of ocean, even in overcast weather, when sun and stars cannot help their navigation. Reaching the coast, they pick up the scent of their home river with noses so sensitive that they can detect dissolved substances in parts per 3,000,000,000,000,000,000!

Once they enter fresh water, the salmon stop feeding. Their stored fat and muscle must last long enough to take them past numerous obstacles, and sustain them while they build nests, fight for dominance, and spawn. They undergo many physical changes: bright spawning colors appear, the males of some species get humped backs, hooked jaws, and sharp canine teeth. The digestive tract degenerates, and the ability to fight disease and heal injuries declines.

Salmon Life Cycle: Early Stages

The eggs lie in the gravel through the winter as the embryos within develop. IN early spring, yolk-sac fry, or alevins, hatch. The tiny fish carry a food supply(a sac of egg yolk) attached to their bellies. They will now leave the protection of the gravel until the yolk is used up, 12 weeks or more. At that time, the young salmon, now called fry, swim up to the surface, gulp air to fill their swim bladders, and begin to feed.

Fry spend a year or more in their home stream in the case of some species, feeding on insects and other tiny animals. For these species, high quality stream habitat is particularly important. Streambed vegetation created shade and supports many of the insects the young fish will eat. Snags, roots, and boulders provide hiding places and act as "breaks" that keep flood waters from sweeping the fry downstream. Chinook salmon head for the sea soon after they emerge from the gravel.