Entrainment of Rainbow Smelt Through Oahe Dam, South Dakota

The recreational fishery in Lake Oahe is presently supported almost entirely by a single prey species, rainbow smelt Osmerus mordax. Population size of rainbow smelt has been variable, confounding management decisions that require an understanding of prey availability. Biologists are aware that rainbow smelt and other fishes pass through Oahe Dam; however, no attempt has been made to quantify the number of smelt lost. The objectives of this study were 1) to assess the effectiveness of a modified kodiak trawl and 0.5 mm and 3. 2 mm (bar measure) mesh meter nets to collect rainbow smelt in Oahe Dam tailrace, 2) to determine diet and seasonal trends in smelt entrainment, and 3) to determine meter-net efficiency and formulate entrainment estimates. In 1996, rainbow smelt were collected biweekly with a modified kodiak trawl and paired, 0.5 mm and 3. 2 mm (bar measure) meter nets to develop a safe, yet effective sampling protocol to study rainbow smelt entrainment. Abundance (i.e., relative abundance) of rainbow smelt (RBS/m3) collected was compared among diel periods (morning, afternoon, night), sampling depths (surface, subsurface), tailrace locations (west, middle, east) and mesh sizes (0. 5 mm, 3. 2 mm). Data from the kodiak trawl could not be quantified; therefore, no statistical comparisons between gears were possible. Abundance of rainbow smelt from meter net samples did not differ between mesh sizes (P=0.9568) nor among tailrace locations (P=0.9796). In addition to abundance, there were no differences (N=l 8, P=0.12-0.99) in the length distribution of rainbow smelt collected from the 0.5 and 3.2 mm mesh nets. Moreover, there were no differences in abundance between morning and afternoon periods; however, abundance was greater during the night period. Do to small sample size there was not a stistical difference in abundance of rainbow smelt collected from surface and subsurface samples within a period; however, 70.5 % of the total rainbow smelt were collected on the surface. In 1997, rainbow smelt were collected weekly, from May through October with paired 1 mm (bar measure) mesh meter nets. Overall, 18, 798 fishes representing seven taxa and an unknown category were collected, of which rainbow smelt and yellow perch composed 73.8 % and 25.8 % of the total catch, respectively. A total of 13,864 rainbow smelt were collected, of which 14 % were adult and 86 % were age-0. Therefore, die) and seasonal trends in entrainment of adult and age-0 rainbow smelt were analyzed separately. The size structure of rainbow smelt collected was significantly different among most weeks. Abundance of adult rainbow smelt varied seasonally from 18.3 RBS/I 000m3 on May 20 to 71.2 RBS/1000m3 on June 24. No adult rainbow smelt were collected after September 18. Adult rainbow smelt were more abundant during the night than the day, and on the surface than the subsurface of the tailrace. Abundance during the day was low and highly variable. Age-0 rainbow smelt did not consistently appear in samples until June 24. Abundance peaked on August 8 and September 12 at 458.2 and 482.0 RBS/IOOOm3, respectively. Abundance of age-0 rainbow smelt dropped to 71. 9 RBS/1000m3 by September 18 and decreased through October 4. Age-0 rainbow smelt were more abundant at night than during the day. Prior to July 15, abundance of age-0 rainbow smelt was greater in subsurface samples than in surface samples. However, after July 15, abundance of age-0 rainbow smelt in surface samples was greater than in subsurface samples. Fathead minnows Pimephales promelas were intentionally entrained in varying amounts between 3,000 and 15,000 fish to determine meter net efficiency. Net efficiency ranged from 0-0.34 %. Between May 20 and October 4, 1997, an estimated 547,398, 161 (432,635 kg) rainbow smelt were entrained through Oahe Dam. Entrainment estimates ranged from 27,158 {± 5,540 [95 % confidence interval (CI)]) to 1,193,436 (± 198,980 CI) rainbow smelt per hour during the night and from 601 (± 254 CI) to 176,790 {± 19,543 Cl) smelt per hour during the day