Male salmon are maturing earlier and becoming smaller, and it shows in their genes. This was the discovery of a study that examined scale samples from salmon in the River Teno in Northern Finland over a 40-year period, and looked at the population genetic profile of a gene that determines salmon’s age of maturity and size. The results show that the ‘big salmon gene version’ has become rarer in the population over time, and has been replaced by the ‘small salmon gene version’.