Sockeye Salmon

For most of its life a sockeye is a plain silvery-blue ocean fish, but as it prepares to spawn it undergoes a striking transformation, with the body turning a deep scarlet or crimson and the head a contrasting olive-green, while males also grow a pronounced hooked upper jaw, enlarged teeth, and a humped back. The red colour comes from carotenoid pigments that the fish accumulates from its zooplankton-rich diet and, at spawning time, moves out of the muscle, which is why spawning sockeye have paler flesh, and into the skin. The vivid colouration and the male's exaggerated body shape function in competition between males and in attracting females on the spawning grounds.

Sockeye complete one of the most precise animal migrations known, returning from years spread across thousands of kilometres of open ocean to the individual lake shore or tributary where they hatched. For the long ocean portion, they are thought to navigate using the Earth's magnetic field, essentially reading their position from the local strength and angle of the field, a magnetic map imprinted early in life. For the final stretch up the correct river and to the correct spawning site, they switch to smell, having memorised the specific chemical signature of their home stream as juveniles and following that scent trail upstream against all the competing odours of the river system.

Sockeye, like all Pacific salmon, put everything into a single reproductive effort. Once they leave the ocean and enter fresh water they stop feeding entirely, drawing on stored fat and muscle to fuel the arduous upstream migration, the physical changes to their bodies, and the digging of nests and defence of territory. By the time spawning is complete their bodies are exhausted and deteriorating, their immune systems have collapsed, and they die within days. This is not a flaw but a strategy, since concentrating all resources into one big breeding event, and then delivering the nutrients of the parents' bodies to the stream, can produce more surviving offspring in these particular environments than surviving to breed again would.

They are the same species, Oncorhynchus nerka, but with different life histories. Sea-run sockeye migrate to the ocean, grow large on marine food, and return to fresh water only to spawn. Kokanee are populations that have become permanently landlocked, completing their entire life cycle within a lake and its tributaries and never going to sea. Kokanee are much smaller than sockeye, often only 25 to 30 centimetres, because a lake offers less food than the ocean, but they turn the same brilliant spawning red and the males develop the same hooked jaw and humped back, and some kokanee populations arose naturally while others were established by stocking landlocked lakes.