A baby salmon first sees the light of day as a ‘fry’. It goes to sea when one to three years old. Returning as a young adult, it runs the gauntlet of weirs nets and fishermen’s hooks, swimming upstream to its childhood haunts on an empty stomach. Salmon don’t feed when they return to the river.
Scientists at the University of Helsinki took tissue samples from 5,000 salmon and genetically finger-printed them. Captives were tagged and returned to the wild, their ages known from the fish equivalent of tree-rings on their scales.
‘Great care was taken not to harm the fish’, lead researcher Kenyon Mobley told Daily Finland, ‘we have recaptured adults returning to spawn several years later and juveniles returning as adults’.
The great challenge for a fish is to breed successfully. Salmon aren’t into partner-fidelity. According to the Helsinki researchers, those in the Teno, a famous Nordic salmon river, can have up to eight lovers.
A female will have at least two. She will be two or three years old, on average, when spawning. For males, becoming a father is an up-hill, or up-stream, battle.
Some return to the river after only one year at sea. They are lucky to mate at all, losing out to bigger rivals in the watery Ballroom of Romance.
The eggs are laid in a gravel-bed nest. The male fertilises them, after which the female covers the nest to protect it. Their work done, the exhausted parents drift downstream and back to the sea, hoping to return one day. Few succeed in doing so.
Spawning is a lottery. The scientists monitored the years spent by 246 sexually mature adults freshwater and at sea, comparing them with the numbers of offspring they produced.
The older and larger a fish, they found, the younger it parented. A female from the Teno produces 60% more offspring for every year she spends at sea.
A male’s breeding success also increases with age; the number of young he fathers triples, on average, for each extra year he is away.
It might seem wise, therefore, for a fish to remain at sea for as long as possible, but there’s a problem. Doing so would be risky; it’s a dangerous environment out there. A fish could be eaten by a predator, or trapped in fishing nets.
Although a 36kg specimen was caught there in 1929, the largest known Atlantic salmon, very few big fish return to the Teno nowadays.
Finnish salmon may have the edge on their Irish cousins; stocks are declining here. Although normal numbers of our young fish go to sea, far fewer are returning to spawn than in the past. The culprit may be global warming. With ocean temperatures rising, the salmon’s prey has moved northwards to colder well-oxygenated waters. Irish fish have to travel greater distances to reach their feeding-grounds, with corresponding exposure to danger. Are they less able to compete for food against Nordic fish after such a journey? Teno salmon may not have so far to travel.