A gene has been identified that influences the size to which adult salmon grow.
Findings from the study, involving researchers from Norway, Finland and Edinburgh, could help inform fishing management strategies.
Researchers studied dozens of populations of wild Atlantic salmon in rivers across Norway.
They were interested to understand why adult salmon coming back from sea to spawn show large differences in size, ranging from 1 to 20 kg in weight.
The variation in size among salmon has long puzzled scientists.
It was previously known that female salmon are typically bigger, as they stay at sea for about two or three years, while males stay about one year at sea and tend to be smaller than females.
Maturing at their respective ages carries benefits for each gender.
However, it was little understood what controls this difference between the sexes, and why there are exceptions to both rules.
Scientists found a single gene, known as VGLL3, which controls how long males and females spend at sea, and how large they are able to grow, before returning to spawn.
Each fish carries two copies of this gene, which influences whether they return to rivers either early or late.
A fish’s size as an adult depends on which combination of early and late genes it has.
Fish with two early copies of the gene mature soonest, when they are small, and those with two late copies of the gene spend more time at sea and mature when they are bigger.
Fish with one of each copy will mature at different times according to their gender - with males becoming adults early, and females maturing late.
The various combinations of genes cause variation in size throughout the population.
This finding could help maintain stocks of late maturing salmon, which have been in serious decline in some rivers, and aid aquaculture breeding programmes.
The study, led by researchers at the University of Turku in Finland and the Norwegian University of Life Sciences, involved researchers from Edinburgh’s School of Biological Sciences.
It was published in Nature.
The discovery of this gene will help us to understand why large fish are in decline and how we can manage wild salmon populations more effectively.