A grilse is a salmon which matures after only one winter at sea. One of the main determinants of grilsing is genetics. This means that breeding from grilse parents will increase the grilse percentage in their offspring. Most strains of salmon give rise to some grilse and much effort has been spent through selective breeding to reduce this percentage. The internal and external environment of the fish also play a role in the development of grilsing.
Environment influences the genetic predisposition to grilse.
High summer temperatures and in turn an increased feeding rate may influence the development of grilsing. Similarly, there is a school of thought that low winter temperatures may reduce maturity. In terms of grilsing, fish generally fit one of two categories. There is a group of fish that are genetically programmed to mature as grilse. These fish will mature under most environmental circumstances.