768 



Fishery Bulletin 92(4), 1994 



after hatching. River kilometer 45 is located in the 

 upper end of the Columbia River estuary (Fig. 1); 

 however, this section of the estuary is a freshwater 

 environment. Larvae were collected from early May 

 through late July, reflecting a protracted spawning 

 period (Table 2). All white sturgeon larvae in 1988 

 and 1989, and 71% or more in 1990 and 1991, were 

 collected in plankton nets. In 1988 and 1989, larvae 

 were not collected as far downstream as in 1990 and 

 1991. Undoubtedly, smaller areas of capture in 1988 

 and 1989 were due to lack of sampling with the 3.0- 

 m beam trawl in these years. All white sturgeon lar- 

 vae collected in the upper estuary in 1990 and 1991 

 were collected in the beam trawl. Larvae were col- 

 lected at depths ranging from 4 to 29 m. When the 

 larvae were collected in plankton nets, they were 

 most likely being transported by water currents, be- 

 cause the nets were fished from an anchored boat. 



Catches of white sturgeon larvae during the 12- 

 hour collection at the index site fluctuated with catches 

 ranging from 0.0 to 8.8 larvae/1,000 m 3 (Table 4). 



Young of the year 



Annual catches of YOY white sturgeon varied con- 

 siderably, ranging from 11 in 1988 to 273 in 1990 

 (Table 5). Annual catches shown in Table 5 are not 

 necessarily indicative of YOY abundance in respec- 

 tive years, because sampling gears and schemes were 

 not the same each year. In 1988 and 1989, the 3.0-m 

 beam trawl was not used, whereas in 1990 and 1991 

 it was used. The beam trawl was more effective at 



capturing small YOY white sturgeon than was the 

 7.9-m semiballoon shrimp trawl. Also, in 1990 and 

 1991, more sampling was conducted in the lower 120 

 km of the river than in 1988 and 1989. 



On the basis of sampling from 1988 through 1991, 

 it appears that YOY white sturgeon are primarily 

 using the section of river extending from rkm 45 to 

 166 (Table 5). Relatively few YOY white sturgeon 

 were collected in the 68 km of river between 

 Bonneville Dam (rkm 234) and rkm 166; small 

 catches were made at rkm 211 in July 1990 and Sep- 

 tember 1991. 



In 1990 and 1991, YOY white sturgeon were first 

 captured in late June, less than two months after 

 spawning was estimated to have begun. In all four 

 years, YOY white sturgeon appeared to grow well 

 during their first summer; however, monthly mean 

 lengths and weights varied among years (Table 5). 

 During all years, YOY white sturgeon reached a mini- 

 mum mean total length of 176 mm and a minimum 

 mean weight of 30 g by the end of September. No 

 statistical comparisons among years were done be- 

 cause of small sample sizes, the protracted spawn- 

 ing period of white sturgeon, and the fact that YOY 

 white sturgeon were collected throughout the month. 



The YOY white sturgeon were more abundant in 

 deeper areas of the lower Columbia River, at least 

 during daylight; mean minimum depths during 

 trawling efforts in which YOY were captured were 

 >12.5 m in all years. Mean maximum depths at which 

 YOY white sturgeon were captured were > 15.8 m in 

 all years. Bottom substrate over which YOY white 



