|
Typically, salmon are anadromous: they are born in fresh water,
migrate to the ocean, then return to fresh water to reproduce. Folklore
has it that the fish return to the exact spot where they were born
to spawn and modern research shows that usually at least 90% of
the fish spawning in a stream were born there. In Alaska, the crossing
over to other streams allows salmon to populate new streams, such
as those that emerge as a glacier retreats. The precise method salmon
use to navigate has not been entirely established, though their
keen sense of smell is certainly involved. In all species of Pacific
salmon, the mature individuals die within a few days or weeks of
spawning, a trait known as semelparity. Even in those species of
salmon that may survive to spawn more than once (iteroparity), however,
post-spawning mortality is quite high (perhaps as high as 40 to
50%.) Those species average about two or, perhaps, three spawning
events per individual.
Coastal dwellers have long respected the salmon. Most peoples of
the Northern Pacific shores had a ceremony to honor the first return
of the year. For many centuries, people caught salmon as they swam
upriver to spawn. A famous spearfishing site on the Columbia River
at Celilo Falls was inundated after great dams were built on the
river. The Ainu, of northern Japan, taught dogs how to catch salmon
as they returned to their breeding grounds en masse. Now, salmon
are caught in bays and near shore. Long drift net fisheries have
been banned on the high seas except off the coast of Ireland.
Freshwater streams and estuaries provide important habitat for
many salmon species. They feed on terrestrial and aquatic insects,
amphipods, and other crustaceans while young, and primarily on other
fish when older. Eggs are laid in deeper water with larger gravel,
and need cool water and good water flow (to supply oxygen) to the
developing embryos. Mortality of salmon in the early life stages
is usually high due to natural predation and human induced changes
in habitat, such as siltation, high water temperatures, low oxygen
conditions, loss of stream cover and reductions in river flow. Estuaries
and their associated wetlands provide vital nursery areas for the
salmon prior to their departure to the open ocean. Wetlands not
only help buffer the estuary from silt and pollutants, but also
provide important feeding and hiding areas.
When breeding, the female salmon excavates a shallow depression,
called a redd, in the gravel of the streambed wherein she lays her
eggs. The eggs usually range from orange to red in color. One or
more males will approach the female as she deposits the eggs, fertilising
them. The female then covers the eggs by disturbing the gravel at
the upstream edge of the depression. 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. 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.
At sea, before their first return to freshwater, the juveniles
are called grilse. They spend two to eight years (depending on the
species) in the open ocean where they will become sexually mature.
The adult salmon returns to its natal stream to spawn. When fish
return for the first time they are called whitling in the UK. Prior
to spawning, depending on the species, the salmon undergoes changes.
They may grow a hump, develop canine teeth, develop a kype (a pronounced
curvature of the jaws in male salmon). All will change from the
silvery blue of a fresh run fish from the sea to a darker color.
Condition tends to deteriorate the longer the fish remain in freshwater
and they then deteriorate further after they spawn becoming known
as kelts. Salmon can make amazing journeys, sometimes moving hundreds
of miles upstream against strong currents and rapids, to reproduce.
All text is available under the terms
of the GNU Free Documentation License
|
