How did anadromous migration help salmon and trout spread into colder freshwater regions?

The development of an anadromous life cycle—spawning in fresh water while migrating to the sea for feeding and maturation—has been a critical factor in the expansion of salmon and trout into colder freshwater regions.1

By utilizing marine environments for growth and maturation, these fish can bypass the limitations of food availability in restricted freshwater habitats. This strategy allows them to rapidly colonize new areas, such as lakes and streams left behind by receding glaciers, which might otherwise be inaccessible to species that are entirely confined to a freshwater life cycle. Essentially, the use of marine invasion routes provides a mechanism for these species to extend their range significantly, enabling them to become the dominant fish in many recently glaciated freshwater environments across the Northern Hemisphere.1