The Fate of Coho Salmon Nomads: The Story of an Estuarine-Rearing Strategy Promoting Resilience

Abstract

The downstream movement of coho salmon nomads (age 0), conventionally considered surplus fry, has been an accepted characteristic of juvenile coho salmon for the past 40 to 50 yr. The fate of these nomads, however, was not known and they were assumed to perish in the ocean. Several studies and observations have recently provided new insights into the fate of nomads and the role of the streamestuary ecotone and estuary in developing this life history strategy that promotes coho resilience. Chinook and sockeye salmon have developed the ocean-type life-history strategy to exploit the higher productivity of the estuarine environment and migrate to the ocean at age 0. Nomad coho can acclimate to brackish water, and survive and grow well in the stream-estuary ecotone and estuary, but instead of migrating to the ocean they return upstream into freshwater to overwinter before migrating to the ocean as smolts. Nomads may enter the estuarine environment from natal or non-natal streams, rear there throughout the summer, and then emigrate to a non-natal stream for overwintering and smolting in the spring. These estuarine and overwintering habitats have enabled coho to develop this unique nomad life history strategy that may help to ensure their resilience. Restoring estuarine habitats may be essential to the recovery of depressed populations of coho.