Scientific Investigations Report 2006–5225
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5225
Three of the eight sampling gears collected white sturgeon embryos, free embryos, or larvae (table 1). Artificial substrates were successful in collecting white sturgeon embryos. They were fished for a total of 1,273.45 hours. Kick nets were used for 22.42 hours and were effective in capturing white sturgeon free embryos. One larva was collected during the 46.53 hours that staked D-shaped plankton nets were fished.
The bilge pump, slurp gun, and the view scope were all used less than one hour and did not collect early life stages of sturgeon. D-shaped plankton nets deployed from a boat were fished 2.75 hours and did not capture embryos or larvae except for one fungus-covered sturgeon embryo that was not counted in the total. Square-shaped plankton nets that were staked into the substrate were fished for 25.60 hours and did not capture embryos or larvae.
All white sturgeon embryos detected during field sampling were collected on artificial substrates. A total of 254 viable embryos were collected. An additional 23 dead or damaged embryos were collected, but were not included in the total.
Samples were collected with artificial substrates during June 6-10 and 13-17, 2005. Artificial substrates were not set during the weekend of June 11-12, 2005, due to increased recreational boat traffic. A total of 33 sets were completed accounting for 1,273.45 hours of effort. Artificial substrates had a mean soak time of 38 hours and 35 minutes (range 22:20–48:30). White sturgeon embryos were collected in 17 of 33 artificial substrate sets (fig. 5), whereas 15 sets resulted in no catch of sturgeon embryos and one set captured only two dead embryos. Appendix 1 lists all embryo capture sites.
The average set depth of the 17 artificial substrates that captured embryos was 2.15 m (range = 0.61–3.35), however variations in river discharge continuously altered depths. The mean water temperature during the sampling period of artificial substrates, as measured downstream of Bonneville Dam at Cascade Island (Columbia River Data Access in Real Time, 2005), was 15.9°C (range = 15.4–16.4°C).
Based on embryological development stages outlined in Beer (1981) we collected embryos that were from stage two (changing pigmentation) to stage 13 (early neurulation). Stage one (unfertilized) eggs were not captured during sampling. An additional four stages occur after early neurulation and prior to hatching, but we did not detect any embryos from these stages. The most prevalent stage collected was stage six (fourth through sixth cleavage), with 78 of the 254 embryos from this stage. Table 2 presents a complete outline of embryos collected and staged during this study.
The presence of white sturgeon free embryos and larvae in shallow wadeable habitat was confirmed near Ives Island downstream of Bonneville Dam. Six 0-1 day post-hatch free embryos were captured on June 8-9, 2005, using kick nets at sites 22, 31, and 32 at the head of Ives Island (fig. 6). Free embryos at this stage are in “hiding phase” (Brannon and others, 1985) exemplified by low mobility due to the presence of a large yolk sac and the lack of developed fins. The free embryos were detected at water depths of 0.31–0.70 m within the interstitial spaces among the cobble/gravel substrate that is the dominant riverbed matrix of the nearshore environment of Ives Island. The temperature range was 15.9 to 16.0°C at the time of capture for free embryos. Composite velocities of 1.38, 2.50, and 1.50 m/s were measured at free embryo capture sites 22, 31, and 32, respectively.
One white sturgeon larva was captured (site 47; fig. 6) in a water depth of 0.59 m on June 14, 2005, in a D-shaped plankton net anchored to the substrate. This larva was estimated to be 10 days post-hatch and would have been exogenously feeding. The temperature at the capture site was 16.7°C. A composite velocity of 0.88 m/s was measured at the capture site. Appendix 1 lists all free embryo and larval capture locations.
In August 2005, substrate composition characterization was completed for some of the sites where white sturgeon early life stages were captured. Substrate sampling in August, after water levels dropped, allowed easy access to many sites that would have been difficult to sample during the high flows of June. However 12 sites were still too deep to be effectively sampled by wading with a sampling frame. Grab samplers were considered, but cobbles and boulders would make such sampling devices ineffective.
Substrates were characterized at 9 sites, including 5 of the 17 sites where at least one white sturgeon embryo was collected and all 4 sites where free embryos and larvae were captured. The combined percentage frequency averages for all 9 sites characterized were: 0.35 percent fines, 53.56 percent gravel, 45.69 percent cobble, and 0.41 percent boulder. The combined percentage frequency averages for the five embryo sites characterized were: 0.00 percent fines, 52.88 percent gravel, 46.39 percent cobble, and 0.73 percent boulder. The combined percentage frequency averages for the three free embryo and one larva sites characterized were: 0.78 percent fines, 54.40 percent gravel, 44.82 percent cobble, and 0.00 percent boulder. Detailed information about each site is presented in Appendix 2. Data and plots for substrates are based on methods outlined in Potyondy and Bunte (2002).
U.S.
Department of the Interior | U.S. Geological
Survey
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