Population Viability Analysis of Snake River White Sturgeon

1. Background -- Status of white sturgeon populations in the Snake River

The white sturgeon (Acipenser transmontanus) is the largest freshwater fish in North America, with a lifespan that can exceed 100 years. Females mature after 15 to 30 years, and reproduce at irregular intervals (>3 y). A semi-anadromous species, the white sturgeon uses large rivers as spawning and rearing habitat, but older lifestages can also survive under the right conditions.

White sturgeon populations in the Columbia and Snake Rivers have been divided into separate populations by a series of dams. Our modeling is part of a larger research effort to evaluate the various factors that influence populations of white sturgeon in the Snake River, in preparation for the relicensing of hydropower facilities. The status of white sturgeon in the Snake River was summarized by a Collaborative Relicensing Team consisting of government agencies, Native American tribes, utilities and other stakeholders:

"White sturgeon populations are currently depressed and the probability of long-term persistence is in question. Causal factors may include: fragmentation of habitat, genetic isolation, food availability, modification of the hydrograph, load following, modification of water chemistry and quality, over harvest, sediment transport, channel morphology and entrainment."

Photograph of a large white sturgeon caught on the Snake River many years ago (above).

White sturgeon appear to be at risk of extinction in some sections of the Snake River. The life history "strategy" of slow growth and late maturation (Rochard et al. 1990) makes it difficult for them to recover from declines in population size. Anadromous white sturgeon stocks in the lower Columbia River and the Sacramento-San Joaquin rivers and estuaries are in better condition than those farther inland, suggesting that landlocked populations are less successful than those with access to the ocean (DeVore et al. 1995). One reason for this pattern probably involves loss of access to abundant marine prey that allows faster ocean growth. Another reason may be that small, fragmented populations are relatively isolated within river reaches and reservoirs between dams. Habitat fragmentation can impose a variety of problems ranging from chance extinctions because populations are smaller to lack of access to critical habitat.

In the Snake River, landlocked white sturgeon populations are a State of Idaho Species of Special Concern. Population estimates vary widely among sections of the river (Cochnauer 1983; Lepla and Chandler 1994; Lepla and Chandler 1995a; Lepla and Chandler 1995b). For some of the river segments between dams, low numbers can be explained by short segment length. However, the longest river segment (174 miles between Brownlee Dam and Swan Falls Dam) also has low abundance.

The Desired Future Condition for Snake River white sturgeon is to ensure long-term persistence of a self-sustaining population of white sturgeon, not to exclude the optimization of a fishery.

2. Modeling Studies of Snake River White Sturgeon

This project provides modeling support for efforts to understand factors influencing these populations. We used the PVA model for a variety of purposes, including:

• To evaluate the relative risk of extinction posed by each of several factors for Snake River reaches with small population sizes. Click here for more details

  
  

• To evaluate alternative management policies, including reconnection options, with regard to improving the status of white sturgeon populations in the Snake River.Click here for more details

  
  

Contact: Yetta Jager E-mail:jagerhi@ornl.gov
Oak Ridge National Laboratory