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Why do salmon turn red when they spawn?

A Greater Flamingo (Phoenicopterus roseus).

How Is a Salmon Like a Flamingo?

The pink feathers of a flamingo and the pink flesh of a salmon are both caused by the food they eat.

Both animals eat tiny crustaceans, such as shrimp. The crustaceans contain colour pigments in their body tissues.

When flamingos and salmon eat the crustaceans, the pigments stay in their body and tint it.

Besides affecting the colour of the salmon's flesh, the pigments from the crustaceans provide vitamins and other nutrients that the salmon need to grow.

A pile of harvested shrimp.

Head detail of a Chinook Salmon (Oncorhynchus tshawytscha).

Camouflage Colours

When salmon are in the ocean, they are generally darker on their back and lighter on their belly.

This colour pattern helps protect the fish against predators. Seen from above, the salmon blends into the dark water. From below, a light belly is hard to see against the light sky.

Salmon Facts (Fisheries and Oceans Canada)
http://www.pac.dfo-mpo.gc.ca/
fm-gp/species-especes/
salmon-saumon/facts-infos/
index-eng.htm
Atlantic Salmon Conservation
http://www.ascf-fcsa.ca/
english_site/
index-en.html
Grizzly Bears Catching Salmon
(1 min. 26 sec.)
http://www.youtube.com/
watch?v=0NcJ_63z-mA

Salmon turn red when they are ready to spawn as a signal to potential mates.

The colour indicates to the mate

that they are of the same species
that they are ready to spawn.

Each species of salmon changes in slightly different ways. Males change the most dramatically.

The colour and shape of these Sockeye Salmon indicate that they are ready to spawn.

Salmon live a life full of changes. (Learn more about their life cycle). Many of those changes happen when salmon prepare to spawn (produce young).

At that stage of their lives, their bodies change in shape and colour.

The most striking change occurs in salmon of the Pacific Ocean, which turn a dramatic shade of red. It's the colour of their skin that we see, because their scales are transparent and colourless. The rest of the time they look silvery.

Their Atlantic cousins turn bronze-purple with red spots on the head and body. They start out blue-black on their back with silvery sides.

Sockeye Salmon (Oncorhynchus nerka) swimming near the surface of a river.

Sockeye Salmon in a river in Alaska, U.S.A.

These physical changes require a lot of energy. Pacific salmon use up so much energy on their dramatic changes that they die after spawning. Atlantic Salmon do not normally die after spawning. Because they make less-dramatic changes, Atlantic Salmon have enough energy to return to the ocean after having spawned up-river.