Early marine distribution of out-migrating juvenile sockeye salmon (Oncorhynchus nerka)

The early marine phase is a critical period for out-migrating juvenile sockeye salmon (Oncorhynchus nerka). They undergo physiological changes while entering into a new environment, and this is thought to be a period of high mortality. This study examined factors affecting swimming depth as juveniles migrated through Rivers Inlet, and compared swimming depth at the point of ocean entry and life history strategies in sockeye salmon from Rivers Inlet and nearby populations on the Central Coast of British Columbia. Most juvenile sockeye salmon swam within two meters of the ocean’s surface during their out-migration through Rivers Inlet, and were slightly deeper later in the out-migration. In addition, in a mesocosm experiment, Rivers Inlet juvenile sockeye salmon did not alter their swimming depth in response to increasing salinity at their preferred swimming depth, despite experiencing negative physiological effects when swimming in highly saline waters. Similar data on other nearby sockeye salmon populations were collected. Juvenile sockeye salmon from populations that out-migrate through brackish waters tended to be smaller than individuals from populations that leave fresh water and enter directly into highly saline marine environments, and they too tended swim within the top two to four meters of the surface. This shows that conditions in the top 4 m of the water’s surface represent actual conditions experienced by out-migrating juvenile sockeye salmon in the early marine environment and is a critically important observation in terms of understanding such issues as prey availability, the potential role of brackish surface layers in coastal fjords, lagoons and estuaries, and susceptibility to predators. These findings will inform future sampling efforts on these populations, and also suggest that these populations maintain varying life history characteristics which enable them to survive early marine conditions within the top 4 m of the ocean.