Scientific Investigations Report 2006–5225
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5225
Through field sampling we proved that white sturgeon embryos, free embryos, and larvae are present in shallow water adjacent to spawning areas, confirming a critical assumption of the Coutant (2004) Riparian Habitat Hypothesis. While we did not attempt to test the hypothesis itself—that the presence of riparian habitat is critical to the success of white sturgeon populations—we did prove that some proportion of the eggs, free embryos, and larvae can be found in inundated riparian habitats. The high water velocities characteristic of this white sturgeon spawning area are likely to disperse embryos and free embryos throughout the river channel, with some ultimately settling near the shoreline in shallow water. The free embryos detected by kick netting at the head of Ives Island were able to enter interstitial spaces among the gravels and cobbles to keep from being washed further downstream.
These findings raise concerns that water level fluctuations caused by load-following operations at Bonneville Dam or other hydropower system related operations could dewater these early life stages. Water-surface elevation gages at Bonneville Dam and near Ives Island (Fish Passage Center, 2005; Columbia River Data Access in Real-Time, 2005) reveal that water elevations in this area can decrease more than 1 m during a 24-hour period (fig. 7).
Embryos that adhere to the substrate in shallow areas will incubate about 4–10 days before hatching. Thus, the potential for dewatering is related to the original depth at which the embryos attach as well as seasonal and short-term cyclical fluctuations in water levels. If embryos happen to attach to the riverbed during a period of daily cyclical high water elevation, the risk for dewatering increases, particularly if the seasonal hydrograph is declining. However, if embryos adhere to the riverbed during the ascending limb of a seasonal hydrograph, the water depth over the incubation site may continue to increase despite daily fluctuations in water surface elevation caused by tides or hydropower operations.
In this study, embryos were collected on five artificial substrates that were set in less than 1.5 m of water. The potential for dewatering at these capture locations was evaluated by assessing the difference in water elevations between the set time of the artificial substrates and an estimate of when the embryos would have hatched. According to Deng and others (2002), white sturgeon embryos hatch in about 7 days at water temperatures of 16°C. Table 3 shows that although none of the locations sampled were dewatered during the embryo incubation period, water depth at two sites dropped to less than 0.17 m. Embryos in very shallow water may be exposed to fluctuating water temperatures and reduced water velocities may deprive eggs of oxygen, increase the chance of fungal growth, and increase risk of predation. After hatch, white sturgeon free embryos are dispersed by the river currents before settling to the river bottom. After absorbing their yolk sac, they emerge from the interstitial spaces of the substrate and disperse downstream as larvae. In this study, free embryos and the single larva captured were found in areas where the water depths were 0.31–0.70 m. These areas were shallower than areas where embryos were collected. Thus, these older stages may be more susceptible to dewatering. Table 4 shows that the site where the single larva was captured was dewatered within 24 hours and that the water depth at another site where a free embryo was captured dropped to 0.06 m. Figure 8 shows that all sites where free embryos and the larva were captured were dewatered for short periods within days. Since these early life stages are more mobile than the adhesive embryos, they may be able to avoid dewatering by moving to deeper areas or by remaining within interstitial spaces among gravels and cobbles that retain water.
The sampling we conducted specifically targeted near shore habitats. Research has not been undertaken to systematically sample all depths downstream of a white sturgeon spawning area to describe the spatial distribution of spawned embryos within the river channel. Further research should be done to determine the distribution of white sturgeon early life stages throughout the river channel to characterize the role of permanently wetted versus seasonally inundated habitats.
Laboratory studies should be performed to test the ability of white sturgeon embryos, free embryos, and larvae to withstand dewatering. If white sturgeon embryos, free embryos, and larvae are regularly present in shallow water habitat, it can be reasoned that water level fluctuations due to load following at hydroelectric dams may have an adverse effect on the growth and survival of these early life stages. Experiments should investigate the influence of duration of dewatering and effects of fluctuating water temperatures on survival and growth of young fish.
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: https://pubs.water.usgs.gov/sir20065225
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 19:27:03 EST
