The Atlantic salmon is an iconic species in the north Atlantic
Ocean and historically supported large fisheries throughout its
range. There are three distinct populations of Atlantic salmon
North American, European, and Baltic. Like all salmons, this species
is noted for undergoing long migrations and significant physiological
changes during a transition in habitat from freshwater rivers, to
coastal seas, and back to freshwater rivers.
Adult Atlantic salmon live in coastal seas and feed on pelagic
invertebrates and some fishes. During the oceanic portion of their
life cycle, these fish are primarily concerned with growing and
storing energy that they will require for successful reproduction.
This period typically lasts for 2-3 years. Once they reach reproductive
size, they begin a long migration to their preferred spawning ground,
fair inland, in freshwater rivers. Interestingly, though the three
populations mix at sea, they divide into their respective groups
to reproduce. In fact, each individual Atlantic salmon returns to
spawn in the river where it hatched, so rivers around the north
Atlantic are home to distinct subpopulations of this salmon. Thousands
of individuals migrate to and reach the spawning grounds at the
same time. Once they arrive, females dig nests in light gravel and
lay their eggs on the river bottom. Males fertilize the eggs externally,
and then the females bury the nests. Unlike the pacific salmons
(such as the Chinook Salmon [link]), Atlantic salmon do not die
after reproducing just once. They can repeat this cycle several
times. After they hatch, baby Atlantic salmon spend approximately
2-3 years living in different riverine habitats as they slowly make
their way to the ocean, where they stay until they reach maturity
and begin the cycle again. Though this lifecycle is typical of the
species, it is not required for survival. Some subpopulations are
landlocked and replace the oceanic portion of their lifecycle with
large, inland lakes.
Atlantic salmon are important oceanic prey for
several species. This salmon was historically also an important
fishery species, and Atlantic salmon fisheries have been regulated
for at least 800 years in Europe. In the ocean, large boats historically
targeted this species in very large numbers. While they migrate
toward their spawning grounds, Atlantic salmon are targeted by fishers
using traps and other semi-permanent structures installed in rivers.
Unfortunately, overfishing, climate change, and competition from
nonnative species all threaten Atlantic salmon, and several subpopulations
are critically endangered (very highly vulnerable to extinction)
or even extinct. Because individuals return to spawn in the river
where they hatched, climate change is one of the most significant
future threats to this species. As rivers in the southern part of
their range become too warm for eggs to survive, subpopulations
that spawn in those rivers will almost certainly go extinct.
Atlantic salmon are one of the most aquacultured marine fishes
and are farmed in many places around the world, including outside
of their native range. Now, essentially all of the Atlantic salmon
sold in the seafood industry is from farms rather than from wild
populations. Aquaculture of this magnitude presents its own problems,
however, and escaped fish may threaten natural populations in the
north Atlantic or may lead to establishment of invasive populations
in other parts of the world where it is farmed.
|Salmon emerge from eggs in freshwater rivers.
Until they absord their egg sacs their known as alveolin, then
they're called fry. If they escape predation and grow from fry
to about 2inches long, they are called Parr, or Fingerlings.
A Salmon parr needs to escape predation, and develops camaflaging
"parr marks" (like bands of colour) across their
backs. These break up their shape and help the fish avoid
being eaten by heron, mink, goosander, and other fish such
There is often confusion within the angling community when
it comes to distinguishing between Salmon and Brown Trout
(Salmo trutta) as the two species closely resemble each other.
With the young Parr, the main differences are that the salmon
has a more streamlined shape, a deeper fork to its tail, a
longer pectoral fin, a sharper snout and smaller mouth than
a young Trout, and has no orange on the adipose fin (at the
rear, to the back)
A Salmon Parr will only sport a couple of spots on its gill
cover (often just one large spot), and its parr or fingerling
markings are quite crisply defined.
After a couple of years of living in freshwater, the young
salmon undergoes some major alterations to its body and chemistry
which enable it to migrate to the salty waters of the Atlantic
Ocean where the fish will feed amd grow. These changes are
called "smoltification" and occur in esturine waters
where the young fish gather in shoals before setting out to
the open sea.
Visually, the change from a Parr to a Smolt is primarily
a loss of the stripes seen on a parr, and a change of colour
as the fish becomes silvery. This is caused by guanine crystals
which form a layer in the skin and obscure the spots and fingerling
markings (although these are still visible on the gill covers).
Their bodies elongate and their fins darken. The silvery
scales can be rubbed off easily. It's hard to distinguish
between the sexes until the males became ready to breed.
As adults, the Salmon will return to the freshwater of their
birth to breed. When they first appear, as "Fresh Run
Salmon" both sexes are bright silver and pretty similar,
with easily detachable scales. You can often tell how long
an adult salmon has been in fresh water by the birghtness
of its silver colouring, and from the sea lice which die when
their hosts switch from salty to fresh water. A silvery adult
with near complete sea lice in situ will have only just left
the marine environment; one with less of a gleam and more
degraded (or without) sea lice is likely to have been back
in the river for longer.
The adult female fish will retain more of this silver colour
than the male, although being in fresh water will cause her
scales to darken, and she will develop a tiny kype or bump
on her lower jaw.
Fresh run Hen Salmon
A female adult salmon is known as a Hen, a male is known
as a Cock.
Once in breeding colours it's easy to recognise a Cock salmon.
The two main features are the change in colour and of the
lower jaw. A breeding male's colouration is often described
as a tartan pattern, and means the fish is patchy with different
shades of oranges, silver, and red markings. Overall the fish
is brownish-orange, pinkish-red, or yellowish in colour. There's
plenty of variation between individuals. The lower jaw develops
a prominent hook or "kype" which is used to fight
rivals who threated its teritory. It also makes eating almost
impossible; adult salmon mostly die after mating.
Cock salmon with kype
The Cock salmon will also have an enlarged adipose fin.
Again, there's occasional confusion between an adult salmon
and a large adult brown trout. The Salmon have a more streamlined
shape, a concave tail with a thinner neck than the trout tail,
far fewer (or no) black spots below the lateral line (about
half way down the fish body), and its upper jaw tends not
to reach any further back than the rear of the eye. Obviously,
wiht an adult cock salmon with kype and tartan pattern, the
difference us clear and immediate.
Adult Brown trout Salmo trutta
One more stage in the life cycle of a salmon is that on the
kelt (not illustrated). Kelts are salmon which have spawned
but still remain in the river. They regain a more silvery
colour but are noticably thin, with a distended vent and frequently
have red "maggots" on their gills.
A hen salmon will use her tail to scrape a nest or "redd"
in the pebbles and lay eggs which are fertilized by the male,
cover them over, deposit more eggs on top, cover them, and
so on. In time these will hatch into tiny salmon fry and so
the cycle begins again.