764 



Fishery Bulletin 92(4), 1994 



Data analyses 



The developmental stages of white sturgeon eggs 

 were determined from descriptions by Beer (1981). 

 Timing of spawning was estimated from developmen- 

 tal stages of eggs and temperature-egg developmen- 

 tal data from Wang et al. (1985 ). Water temperature 

 at the time of egg collection was used in making the 

 estimates, and a daily index of spawning activity was 

 calculated from these estimated spawning dates. The 

 index of spawning activity was treated as a dichoto- 

 mous variable: spawning occurred or did not occur 

 on a particular day. 



Stepwise regression (Ryan et al., 1985) was used 

 to determine relationships between the abundance 

 of freshly fertilized ( stage 2) white sturgeon eggs col- 

 lected in plankton nets at the index site and physi- 

 cal parameters, including water temperature, tur- 

 bidity, mean water column velocity, near-bottom 

 water velocity, and Bonneville Dam discharge. Stage- 

 2 eggs were assumed to be approximately three hours 

 old or less (Beer, 1981). Bonneville Dam discharge 

 for these comparisons was estimated by averaging 

 hourly discharge at the time of sampling with dis- 

 charges during the three hours prior to sampling. 

 White sturgeon egg abundance and all physical pa- 

 rameters were tested for normality. Egg abundance 

 (eggs/1,000 m 3 ) and turbidity failed the test for nor- 

 mality; both were normalized by using a logj trans- 

 formation. Data collected just prior to, during, and 

 just after the spawning period were used for the re- 

 gression analyses. Transformed egg abundance was 

 also plotted against each of the above physical pa- 

 rameters to investigate the possibility of nonlinear 

 relationships. The plots suggested that the relation- 

 ship between egg abundance and water temperature 

 may be nonlinear; therefore, we used second-degree 

 polynomial (quadratic) regression (Ryan et al., 1985) 

 to examine this relationship. 



For data analysis, YOY white sturgeon were sepa- 

 rated from older juvenile sturgeon by length. A YOY 

 was defined as being between 25 and 325 mm total 

 length and less than one year old. Sturgeon shorter 

 than 25 mm were considered larvae. A white 

 sturgeon's birth date was assumed to be 1 January, 

 although in reality the birth date was generally later 

 in the year. 



Results 



Eggs 



The number of white sturgeon eggs collected from 

 1988 through 1991 ranged from 1,404 in 1988 to 2,785 

 in 1990 (Table 2); however, sampling effort was not 



equal each year. The percent of white sturgeon eggs 

 collected in plankton nets, as opposed to artificial 

 substrates, also varied annually, ranging from 37% 

 in 1991 to 87% in 1989. Virtually all white sturgeon 

 eggs were collected in the 11-km section of river ex- 

 tending from rkm 223 to 234, immediately down- 

 stream from Bonneville Dam. In both 1990 and 1991, 

 four white sturgeon eggs were collected at rkm 193. 

 In all years, 4% or less of white sturgeon eggs col- 

 lected in plankton nets were infected with fungus, 

 indicating infertile or dead eggs. 



From the spawning index, which was derived from 

 back calculations by using the developmental stages 

 of all eggs, we estimated that spawning occurred on 

 38 days in 1988, from 22 April to 22 June, and that 

 58% of the spawning days were in May (Fig. 2). In 

 1989, spawning occurred on an estimated 43 days, 

 from 22 April to 2 July, and 53% of the spawning 

 days were in May. In 1990, spawning was estimated 

 to have occurred on at least 48 days, from 23 April to 

 14 July, and 46% of the spawning days were in May. 

 Finally, for 1991, we estimated that spawning oc- 

 curred on 39 days, from 5 May to 14 July, and 56% of 

 the spawning days were in May. 



Water temperatures measured at Bonneville Dam 

 and at sampling sites during the spawning period 

 varied annually (Fig. 2). Water temperatures at 

 Bonneville Dam sometimes differed by about 1°C 

 from those at egg collection sites. From 1988 to 1991, 

 white sturgeon spawned at water temperatures rang- 

 ing from 10 to 19°C (Bonneville Dam or sampling 

 site temperatures). 



Bonneville Dam discharge (mean hourly discharge 

 by day) also varied annually (Fig. 2). The highest 

 daily flows through Bonneville Dam during the sam- 

 pling periods occurred during the spawning periods 

 in 1990 and 1991. Combining data from all years, 

 we concluded that spawning occurred on days with 

 mean discharges ranging from 3,399 to 10,505 mVs. 

 During the 4-year study, stage-2 eggs were col- 

 lected at temperatures from 10 to 18°C, turbidities 

 from 2.2 to 11.5 ntu, near-bottom velocities from 0.6 

 to 2.4 m/s, mean water column velocities from 1.0 to 

 2.8 m/s, and depths from 3 to 23 m. 



White sturgeon spawned primarily in the area 

 upstream from rkm 222. Virtually all stage-2 eggs 

 were collected between rkm 223 and rkm 234 (about 

 600 m downstream from the spillways at Bonneville 

 Dam). Small numbers of stage-2 eggs were collected 

 at rkm 193— three in 1990 and one in 1991. Exact 

 spawning locations could not be determined because 

 it was not possible to measure the distance that white 

 sturgeon eggs were carried by the river current im- 

 mediately after spawning. In addition, at least some 

 white sturgeon eggs, which adhere to bottom sub- 



