Salmon Run - Background

Background

Most salmon are anadromous, a term which comes from the Greek anadromos, meaning "running upward". Anadromous fish grow up mostly in the saltwater in oceans. When they have matured they migrate or "run up" freshwater rivers to spawn in what is called the salmon run.

Anadromous salmon are Northern Hemisphere fish that spend their ocean phase in either the Atlantic Ocean or the Pacific Ocean. They do not thrive in warm water. There is only one species of salmon found in the Atlantic, commonly called the Atlantic salmon. These salmon run up rivers on both sides of the ocean. Seven different species of salmon inhabit the Pacific (see table), and these are collectively referred to as Pacific salmon. Five of these species run up rivers on both sides of the Pacific, but two species are found only on the Asian side. In the early 19th century, Chinook salmon were successfully established in the Southern Hemisphere, far from their native range, in New Zealand rivers. Attempts to establish anadromous salmon elsewhere have not succeeded.

Species of anadromous salmon
Oceans	Coasts	Species	Maximum			Comment
			length	weight	life span
North Atlantic	Both sides	Atlantic salmon	150 cm	46.8 kg	13 years
North Pacific	Both sides	Chinook salmon	150 cm	61.4 kg	9 years	Also established in New Zealand
		Chum salmon	100 cm	15.9 kg	7 years
		Coho salmon	108 cm	15.2 kg	5 years
		Pink salmon	76 cm	6.8 kg	3 years
		Sockeye salmon	84 cm	7.7 kg	8 years
		Steelhead	120 cm	25.4 kg	11 years	Anadromous form of rainbow trout
	Asian side	Masu salmon	79 cm	10.0 kg
		Biwa salmon	44 cm	1.3 kg

The life cycle of an anadromous salmon begins, and if it survives the full course of its natural life usually ends in a gravel bed in the upper reaches of a stream or river. These are the salmon spawning grounds, where salmon eggs are deposited, for safety, in the gravel. The salmon spawning grounds are also the salmon nurseries, providing a more protected environment than the ocean usually offers. After 2 to 6 months, the eggs hatch into tiny larvae, called sac fry or alevin. The alevin have a sac containing the remainder of the yolk, and they stay hidden in the gravel while they feed on the yolk. When the yolk has gone, they must find food for themselves, so they leave the protection of the gravel and start feeding on plankton. At this point, the baby salmon are called fry. As the end of the summer, the fry develop into juvenile fish called parr. Parr feed on small invertebrates and are camouflaged with a pattern of spots and vertical bars. They remain in this stage for up to three years.

As they approach the time when they are ready to migrate out to the sea, the parr lose their camouflage bars and undergo a process of physiological changes, which allows them to survive the shift from freshwater to saltwater. At this point, salmon are called smolt. Smolt spend time in the brackish waters of the river estuary while their body chemistry adjusts their osmoregulation to cope with the higher salt levels they will encounter in the ocean. Smolt also grow the silvery scales which visually confuse ocean predators. When they have matured sufficiently in late spring, and are about 15 to 20 centimetres long, the smolt swim out of the rivers and into the sea. There they spend their first year as a post-smolt. Post-smolt form schools with other post-smolt, and set off to find deep-sea feeding grounds. They then spend up to four more years as adult ocean salmon while their full swimming and reproductive capacity develops.

Then, in one of the animal kingdom's more extreme migrations, the salmon return from the saltwater ocean back to a freshwater river to spawn afresh.