Poor Progeny to Parent Survival Rates in an ESA Listed Spring Chinook Population - Hatchery or Density-Dependent Effects?

The Tucannon River spring Chinook salmon (Oncorhynchus tshawytscha) population has steadily declined since the construction and operation of the Federal Columbia and Snake River Hydropower System.  Legislation under the Water Resources Act of 1976 authorized the establishment of the Lower Snake River Compensation Plan to help mitigate for the losses of salmon and steelhead due to construction and operation of the Snake River dams.  As part of this legislation it was determined that 2,400 spring Chinook annually escaped into the Tucannon River.  The Tucannon River spring Chinook salmon hatchery supplementation program was designed to escape 1,152 adults back to the Tucannon River, with the expectation that the remaining 1,248 (52%) would come from natural production.  However, progeny-to-parent ratios of naturally produced Tucannon River spring Chinook salmon are typically below replacement and the population is listed as “threatened” under the U.S. Endangered Species Act.  Natural productivity of this population was analyzed using stock-recruit analysis, smolts/redd, and smolts/spawner to examine the effects of hatchery origin fish spawning in the river.  The data showed no relationship between natural production and hatchery composition on the spawning grounds.  Density-dependent effects on replacement are also examined and appear to play a larger role in low natural origin progeny to parent survival rates than hatchery fish abundance on the spawning grounds.