San Francisco Estuary and Watershed Science

Many native fishes in the San Francisco Estuary and its watersheds have reached all-time low abundances. Some of these declining species (e.g., Chinook salmon Oncorhynchus tschawytscha) have been under artificial propagation for decades. For others (e.g., delta smelt, Hypomesus transpacificus, and green sturgeon, Acipenser medirostris), this management option is just beginning to be discussed and implemented. Propagation strategies, in which organisms spend some portion of their lives in captivity, pose well-documented genetic and ecological threats to natural populations. Negative impacts of propagation have been documented for all Central Valley Chinook salmon runs, but limited efforts have been made to adapt hatchery operations to minimize the genetic and ecological threats caused by propagated fishes. A delta smelt propagation program is undergoing intensive design and review for operations and monitoring. However, if limiting factors facing this species in its estuarine habitat are not effectively addressed, captive propagation may not be a useful conservation approach, regardless of how carefully the propagation activity is designed or monitored. Scientifically defensible, ecologically based restoration programs that include monitoring and research aimed at quantifying natural population vital rates should be fully implemented before there is any attempt to supplement natural populations of delta smelt. Green sturgeon are also likely to face risks from artificial propagation if a large–scale program is implemented before this species’ limiting factors are better understood. In each of these cases, restoring habitats, and reducing loss from human actions, are likely to be the best strategy for rebuilding and supporting self–sustaining populations.