As we approach November the spawning season for Atlantic Salmon (Salmo salar) begins. Spawning occurs throughout November and December; in larger rivers this may begin and finish a month early or later.
Salmon are anadromous spending 2 to 3 years as juveniles in freshwater streams before migrating to the ocean for 1 to 4 years travelling over 6000 miles then returning back upriver to spawn. Many individuals die after spawning only the surviving population are able to spawn again. 1000 to 17,000 eggs can be laid by a single fish; however, an extremely small amount of eggs survive to maturity due to predation, ecological factors and interference during other life stages. Three returning fish per parent is considered a successful outcome.
Populations of Atlantic Salmon exist in rivers due to adult’s ability to return to their natal river using olfactory cues to great accuracy. Marine nutrients are accumulated and are stored within their bodies which eventually release during the death and decay of the fish in its natal river. These nutrients provide for aquatic plant life and developing salmon; this is also known as ‘the great nutrient cycle’.
Redds are gravel depressions created by the upstroke of a female Salmon’s body and tail, various redds are dug with hundreds of eggs deposited in each one during the 2-day spawning period. As soon as the eggs are placed, they are very quickly fertilised by the male; the fertilised eggs are then covered in gravel by the female. The incubation time of the eggs are influenced by an array of factors including water temperature as well as O2 and CO2 levels.
Predominantly eggs hatch during spring at this life stage they are known as alevins, having absorbed their attached yolk sack they leave the gravel as fry at about 3cm in length. Fish are known as Par after developing side markings, spending around 2 years in freshwater. Their bodies physically and morphologically change – this is known as smoltification; developing a dark back, becoming more silver and adapting gills and kidneys to aid in survival at sea. These Smolts begin to leave rivers in May with all gone by June gradually moving from estuarine mouths to deeper ocean waters.
There are various names given to returning Salmon. Grilse are fish returning back up our Scottish rivers after only spending a year at sea, reaching around 3kg. Multi-sea-winter-salmon make up the majority of returning adults, these fish have spent a few years at sea and tend to be larger than Grilse due to longer feeding periods in the marine environment. Finally, Spring Salmon return in Spring! On return physical and morphological changes occur again to adapt to freshwater, feeding ceases as adults survive on stored body energy.
And so, the cycle begins again!
Studies have shown Salmon have returned to Scottish rivers throughout the year which is unusual and not regularly seen by Atlantic Salmon in other areas. This is why Scotland’s Atlantic Salmon stock is incredibly significant for both marine and freshwater ecosystems. It is especially important to conserve the spawning stock which is vital to the species future.
Words by Christopher Wenham