Semelparity, breeding once and dying, is the rule in the genus Oncorhynchus. Salmo salar is iteroparous. After spawning, a surviving fish becomes a kelt: emaciated, its stomach lining atrophied from months of not feeding in fresh water. Some kelts drift back to the estuary, resume feeding, and rebuild condition at sea.
Repeat spawners are a minority and a skewed one. Where the trait has been measured, the great majority of returning repeat spawners are female, and the proportion of a run made up of repeat spawners varies enormously between rivers, from effectively nil to a substantial share in some Norwegian and Canadian systems. The larger a repeat spawner grows, the more eggs it carries, so a small number of these fish can contribute disproportionately to a river's next generation.
What is less settled is how much of the variation is genetic and how much reflects migration distance, river temperature, and fishing pressure removing the large fish that would otherwise return. Kelt survival is difficult to measure because the fish are hard to track once they re-enter the sea.