Key features of salmonid lifecycles
Migratory salmonids, Atlantic salmon (Salmo salar) and
sea trout (Salmo trutta) have complex life-cycles,
spending parts of the lives in freshwater and parts in the sea.
Salmonids require near-pristine fresh waters in which to breed and
flourish. They are also near the top of many fresh water food
chains, placed below certain fish-eating birds (e.g. sawbill ducks
and cormorants) and mammals (e.g. otters). Salmonid populations are
therefore regarded as good indicators of the state of health of
many river and lake ecosystems. The sustainability of salmonid
stocks thus depends upon maintaining water and habitat quality as
well as on judicious management of their fisheries.
Atlantic salmon (Salmo salar) originate in
rivers around the North Atlantic from Spain to Russia, in Iceland,
and on the eastern coast of North America. They are found in the
majority of rivers in England and Wales, ranging in character from
steep spate rivers in Wales to lowland chalk streams in southern
England. They spawn in freshwater but spend about half of their
life in the sea, where they achieve most of their growth (a
strategy known as 'anadromy'). Salmon spawn in the autumn and
winter in excavations in gravel beds known as redds. After hatching
in the spring, the 'alevins' remain in the gravel drawing
nourishment from their yolk-sacs. When the yolk sac is depleted the
fish emerge from the gravel as ' fry', disperse and begin to feed.
At this stage, they set up territories, which they defend against
competitors. In their first autumn and winter, the young fish, now
known as 'parr', may redistribute themselves, possibly moving into
deeper water or sheltering under stones. After one to three years
residence in freshwater, the parr develop into 'smolts' and migrate
to sea in the spring. Little is known about the movements of our
salmon in the sea, although they probably move into the Norwegian
Sea in the summer and autumn of their first sea year, and some
migrate as far as western Greenland during the following summer.
The fish grow rapidly, returning to their home rivers as mature
adults after one to three years at sea; different age groups may
return at different times of year. Most salmon spawn only once,
although a small proportion may survive to spawn again after a
second sojourn in the ocean.
Our native trout (Salmo trutta)
co-exist with salmon and have similar life-cycles. Nearly all
trout undertake seasonal migrations, but variable components of
local populations adopt different migratory life styles. Thus, some
individuals are fully resident in their natal stream or only
migrate within their river ('classic' brown trout), while others
undertake a smolt transformation and emigrate to sea to grow.
However, all return when sexually mature to spawn in their natal
areas. Those that migrate to the sea are distinguished as sea trout
and are mainly females; residents are predominantly males. Genetic
population structure is sustained by the accurate homing of
migratory trout before they spawn.