Juvenile salmonids traverse coastal meta-nurseries that connect rivers via the sea

Abstract

Conventionally, juvenile salmonids are thought to migrate unidirectionally from freshwater systems to marine systems and therefore only inhabit natal drainages. Although scattered evidence suggests juveniles can move bidirectionally between freshwater rivers and the ocean, including into non-natal drainages, such movements have never been documented with high replication. Here, we detected hundreds of movements of juvenile salmonids between drainages that involved 0–22% of cohort emigrants in Washington State and California. Individuals moved up to nine times and between drainages up to 70 km apart. These findings reveal a life-history type of salmonids whose remarkably complex migrations have gone unnoticed. Implicitly, juveniles may use any coastal freshwater habitat accessible from the sea and may not descend from spawning populations of drainages they inhabit. Consequently, typical conservation focused on natal drainages may overlook freshwater habitat elsewhere. A concept of coastal areas as meta-nurseries formed by multiple watersheds connected by the sea may accurately describe anadromous species’ habitat options and better inform management.