Salmon life cycle: click to enlarge
The early development of the salmon. 1, The fertilised egg; 2, the egg just before hatching; 3, the newly hatched salmon; 4 and 5, the larval salmon nourished from yolk-sac (y.s.) which is diminishing while the fish is increasing in size; 6, the salmon with yolk absorbed (about six weeks old).
Reproduction: Salmon live most of their life in the sea, but when they are mature and ready to breed, they enter fresh water to spawn (reproduce), traveling to a stream or pond high in oxygen. The female digs a nest in the gravel (called a redd) with her tail. She then pushes her thousands of eggs into the nest and the male milks the eggs, fertilizing them. Most salmon die after spawning.
The Eggs Hatch and Grow: The newly emerged salmon (called alevins) still have a food sac attached to them. When the food sac is used up, the salmon fry emerges from the nest and must find food (like insects) for the first time. As the fry matures, it becomes camouflaged (with parr marks) and is called parr. When it becomes silver-colored, it will be called a smolt. After growing for a while, the smolts swim downstream to the sea.
Adapting to Salt Water: When a smolt reaches the estuary (where the river meets the sea), a process begins in which their body changes, allowing them to soon live in salt water (this is called smoltification).
Maturing at Sea, then Returning Home: The salmon lives in the sea until maturity (1 to 7 years, depending on the species); some migrate thousands of miles in the sea. They then return to the place where they hatched and continue the cycle. No one knows how salmon return home -perhaps they remember the distinctive set of smells along the way. On their journey home, they do not eat at all, they often change color, their muscles soften, and they will die soon after spawning.
Predators: Salmon are preyed upon by many animals, including bears, wolves, people, many birds (like wading birds and kingfishers), and other fish. For every 8000 eggs produced, 4500 alevin survive, from which 650 fry survive, from which 200 parr survive, from which 50 smolt survive, from which only 2 spawning adults survive (who produce thousands of eggs).
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The clipart above is taken from Clipart ETC, which is a part of the Educational Technology Clearinghouse.
