Scientific Investigations Report 2006–5225
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5225
White sturgeon populations vary considerably in abundance and age structure throughout the Columbia River Basin (Beamesderfer and others, 1995). The construction of a series of hydroelectric dams on the Columbia and Snake rivers has created functionally discreet populations of white sturgeon within the basin (North and others, 1993). The abundance and density of white sturgeon is greatest in the unimpounded river downstream of Bonneville Dam. This area supports one of the largest and most productive sturgeon populations in the world (DeVore and others, 1995). Conversely, the geologically isolated white sturgeon population from the Kootenai River has been listed as endangered since 1994. White sturgeon populations in the Snake River downstream from Hells Canyon Dam appear to be persisting, but at a lower abundance than prior to impoundment.
Prior to Coutant’s (2004) promotion of the Riparian Habitat Hypothesis, variations in population status among areas had been attributed to a number of factors. In particular, construction and operation of hydroelectric dams on the Columbia and Snake rivers have directly affected white sturgeon populations in several ways. For example, fish cannot migrate up and down the river as they historically did (North and others, 1993), and areas of spawning habitat have been reduced (Parsley and Beckman 1994). Variations in population characteristics also have been attributed to differences in exploitation rates and recruitment success, access to marine food resources, and suitability of hydrologic conditions and available habitats (Beamesderfer and others, 1995; DeVore and others, 1995).
Coutant (2004) developed a hypothesis to explain variability in spawning success among white sturgeon populations. A critical assumption of the hypothesis is that white sturgeon embryos are either deposited in or transported to shallow seasonally flooded riparian habitat and adhere to newly wetted rocks and vegetation during incubation. He proposed that newly hatched free embryos (eleutheroembryos) remain in these shallow waters, hiding in crevices for protection from predation. When free embryos transition to exogenous feeding larvae, this riparian habitat would theoretically offer plentiful food. When water recedes with decreasing discharge, juvenile sturgeon would also retreat to deeper channels. This hypothesis is supported by findings from the unregulated lower Fraser River where white sturgeon embryos and larvae were collected in island complexes and side channels but not in the main channel (Perrin and others, 2003).
It is generally accepted that white sturgeon embryos and larvae can be found on the riverbed substrate in various water depths and habitats downstream of dams (Parsley and others, 1993; McCabe and Tracy 1994). Previous work in the Columbia River primarily investigated the timing and general location of white sturgeon spawning and did not focus on the occurrence of embryos or larvae in shallow riparian habitats. Parsley and others, (1993) and McCabe and Tracy (1994) collected embryos in depths of 4–27 m and 3–23 m, respectively. Accurate information concerning the spatial distribution of embryos and larvae downstream of hydroelectric dams is critical in assessing how construction and operation of dams may affect survival of early life stages of sturgeon.
If white sturgeon early life stages do occur in shallow riparian habitats, construction and operation of the federal hydropower system may affect survival and subsequent recruitment to age-1. In shallow areas, they could be susceptible to desiccation due to short-term fluctuations in water elevations resulting from daily dam operations. These short-term fluctuations are known to strand outmigrating anadromous salmonids (Tiffan and others, 2002) during the same time that white sturgeon are known to spawn, incubate, and hatch. The dampening of the seasonal river discharge by dams to reduce flooding and increase power generation in winter has also reduced the seasonal water level fluctuations in areas where backwater effects are minimal, and thus reduced availability of newly inundated riparian habitats. Further, white sturgeon populations as a whole may suffer from a loss of productivity due to the extensive loss of riparian habitat caused by impoundment and stabilization of water levels within reservoirs.
Objectives of this study were to (1) determine if white sturgeon embryos, free embryos, and larvae occur in seasonally inundated riparian habitats, and (2) characterize surface substrates at sites where embryos, free embryos, and larvae were found.
U.S.
Department of the Interior | U.S. Geological
Survey
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