Sacramento-San Joaquin River subpopulations The California Department of Fish and Game periodically monitors the status of white sturgeon in the Sacramento-San Joaquin River system to estimate abundance and mortality rates. The most recent survey completed in 1997 estimated nearly 140,000 white sturgeon (> 40 inches) in the Sacramento River (Shaffter and Kohlhorst 1999). The number of adult white sturgeon fluctuates annually and is largely the result highly variable year classes affecting adult recruitment. For example, strong year classes from the early 1980s recruited large numbers of adult white sturgeon to the fishery beginning around 1994; conversely subsequent severe drought conditions across California from 1987 to 1992 are expected to affect the size of the adult white sturgeon population since reproduction was generally poor during those years (Schaffter and Kohlhorst 1999). As recruitment basically ceases and growth and general mortality reduce the number of legal-sized fish, the adult population should decline the next few years. However, the subsequent series of wet years beginning in 1993 should contribute to another cycle of strong recruitment producing fish that will enter the fishery late in the next decade. The California Department of Fish and Game believes the current low exploitation rates, the ability for white sturgeon to successfully reproduce and recover rapidly from reduced adult populations during wet cycles, and fishing size limits that protect the most productive (i.e., fecund) female white sturgeon suggest that current fishing regulations are adequate to conserve white sturgeon in the Sacramento-San Joaquin River basin.
Columbia-Snake River subpopulations
Today, the white sturgeon population residing in the lower, free-flowing reach of the Columbia River is considered the most productive in the species’ range (Devore et al. 1999a). The recreational and commercial harvest in the lower Columbia river has recently averaged over 42,000 annually through 1997. Estimated abundance for legal-sized (42-60 inches) white sturgeon in the lower Columbia River in 1997 was nearly 157,000 fish, a decline from the estimated 227,700 fish in 1995. It appears that the decline was primarily a result of a decrease in the recruitment to the legal-sized population and emigration from the Columbia River system, and not over-harvest.
Upstream in the first three impounded reaches of the Columbia River the estimated abundance of white sturgeon in 1997 increased from a general population crash in the mid 1980's (North et al. 1999). Generally, white sturgeon increased in all size classes sampled. Researchers suspect that more restrictive harvest guidelines along with some relatively strong year classes in recent years are contributing to increasing populations in recent years. These landlocked populations continue to sustain limited harvest and consumption fisheries.
Historically within the Snake River, white sturgeon could range from its confluence with the Columbia River upstream nearly 615 river miles to Shoshone Falls, a natural barrier for sturgeon. Twelve dams were constructed along the Snake River between 1901 and 1975 that fragmented white sturgeon populations from the confluence to Shoshone Falls. The abundance of land-locked white sturgeon in the Snake River varies considerably from river reaches with reproducing populations to other reaches containing few individuals and no detectable recruitment. Reaches with reproducing populations currently provide catch and release sport fisheries only.
Upper Columbia River subpopulation
The abundance of white sturgeon from the Columbia River mainstem between Keenleyside Dam and the Canada-U.S. border was most recently assessed in 1995, when a population of 1,120 individuals (95% CI = 980 to 1,300) was estimated (RL&L 1996a). This figure does not include sampling in Lake Roosevelt in Washington State, and thus represents a minimum estimate of the total subpopulation. A smaller group of white sturgeon, considered part of the same subpopulation, has been identified at the upper end of Arrow Lakes Reservoir, located upstream from Keenleyside Dam. Estimates place the size of this group at 38 individuals (95% CI = 23 to 78) based on the results of 2000 studies (RL&L 2001).
Kootenai River subpopulation
The Kootenai River subpopulation has been in general decline since the mid-1960's (Apperson and Anders 1991, Duke et al. 1999). The remaining wild population is comprised primarily of adult sturgeon older than 25 years, with very little recruitment observed in the wild population since the mid-1970's (Duke et al. 1999, Ireland et al. 2000). In 1997, there were an estimated 1,468 adults (95% CI: 740 to 2,197) and 17 juvenile white sturgeon (Paragamian et al. 1997).
Fraser River and Nechako River subpopulations
Mark recapture estimates for subpopulations in the Nechako and lower Fraser Rivers (R.L. & L Environmental Services Ltd. 2000), plus the preliminary estimate from work being done by the Lheidli T’enneh First Nation (Yarmish and Toth 2001) for subpopulation in the Upper Fraser River show the following population sizes:
Fraser Regional subpopulation: 22,000 (95% CI = 9,800–70,700)
Upper Fraser subpopulation: 255 (95% CI = 158–352)
Nechako River subpopulation: 571 (95% CI = 421–890)
The preliminary population estimate for the Upper Fraser River may be skewed by the high recapture rate of individuals found in known areas of congregation. The fish tend to congregate in areas of preferred habitat but can also be found at very low densities in other areas of the mainstem. However, after extensive sampling, investigators do not believe the population can be larger than a few hundred individuals >50 cm in length (Yarmish, pers. comm).
