The Effects of Hatcheries on Estuarine Life History Expression by Wild Juvenile Salmon

Physical habitat connectivity in a watershed can be defined as the opportunity for water, materials, energy, organisms, etc. to flow freely through an ecosystem, supporting key functions such as food webs, sediment transport, nutrient cycling, and the life cycles of native fishes. In the case of Pacific salmon, the connectivity of stream and estuarine habitats affords a variety of alternative pathways in time and space that individuals in a population can utilize to complete their anadromous life cycles. Whereas physical connectivity creates the potential for diverse life history expression, realization of that potential—the act of connecting one habitat to another—depends on the movements of individuals in a salmon population. Here we present evidence that salmon hatchery programs can establish phenotypic barriers to the realization of estuarine habitat opportunities independent of the physical continuity of estuarine habitat.  In hatchery-dominated systems artificial propagation programs have re-allocated salmon production in time and space, replacing the dispersed distributions and life histories of naturally-spawned populations with fewer point sources of similarly-sized juveniles released in concentrated pulses. Most hatchery programs select for a few freshwater phenotypes (i.e., large subyearling or yearling riverine smolts) that narrowly define the subsequent times of arrival, size distribution, and temporal abundance patterns of juveniles in the estuary. These attributes, in turn, constrain the phenotypic behavior of juveniles within the estuary. In the Columbia and Salmon River estuaries, for example, hatchery-released subyearlings tend to enter the estuary at a larger mean size, occupy deeper habitats further from shore, and migrate through the estuary more quickly than their naturally-produced cohorts. Hatchery programs also can concentrate spawner distribution and timing if the hatchery adults spawn in the wild, narrowing the periods of emergence and estuary entry among naturally-produced juveniles. Estuarine rehabilitation efforts have expanded considerably in recent years to promote salmon recovery by restoring the connectivity of tidal wetlands and floodplains.  In some cases, hatchery and other fish management programs also must be adjusted to accommodate a greater variety of salmon phenotypes that can fully realize the estuary’s diverse habitat opportunities, including those that are restored.